Joule's Early Researches

Joule’s Early Researches

GORDON JONES*

Introduction

Before the 1939-45 war the Joule collection contained several of Joule’s manuscript notebooks. Some of these disappeared during the war and now the collection lacks the important notebooks written in the period before 1843-the period during which Joule performed experiments leading to his announcement of the Principle of Conservation of Energy.

Unless these notebooks miraculously appear again or other material becomes available, it seems that our knowledge of Joule’s intentions in his early work must be gleaned from his published papers. Here, unfortunately, different writers have given different interpretations.

This article compares some of these interpretations and gives another variation, which sees Joule as a gifted experimenter, interested in put- ting the study of current electricity on a firm quantitative basis and in the efficient production of mechanical power from electricity. Joule is, though, not the mere experimenter. He is certainly influenced by theoretical presuppositions, but not the ones we usually associate with his name-the dynamical nature of heat and the conservation of energy. These were acceptable to Joule’s scientific intuition but did not guide him in the planning or interpretation of his experiments before 1843.

To make the article readable for persons not familiar with Joule’s early experiments a brief summary of his work between 1838 and 1844 is given. References to pages from the following works are included in pa- rentheses in the text:

1. The Scientific Papers of James Prescott Joule. Published by the Physical Society, London. Volume 1 (1884): Volume 2 (1887). I have used the reprint by Dawsons, London (1 963).

* History of Science Department, University of Aarhus, Denmark.

Cenraurus 1968: vol. 13, nr. 2: PP. 198-219

Joule’s Early Researches 199

2. Reynolds, Osborne: Memoir of James Prescott Joule. Memoirs and Proceedings of the Manchester Literary and Philosoph- ical Society. Series 4. Volume 6. (1 892).

3. Rosenfeld, Leon: Joule’s Scientific Outlook Bulletin of the British Society for the History of Science. Volume 1 (1952) 169.

4. Kuhn, Thomas S . : Energy Conservation as an example of Simultaneous Discovery. Paper 11 of Critical Problems in the History of Science, edited by Mar- shall Clagett. University of Wisconsin Press, Madison (1 959).

5. Lowery, H.: The Joule Collection in the College of Technology, Manchester. Journal of Scientific Instruments. Volume 1 (1 93 1) 1 .

JOULES WORK 1838-1 844

I. Early experiments.

The very earliest of Joule’s publications were letters to Sturgeon’s An- nals of Electricity. They began in January 1838, when Joule had just passed his nineteenth birthday, and describe attempts to improve electric motors.

Finding the results disappointing, Joule attempts to improve his electromagnets. A quantitative investigation leads him to discovery of thc law of attraction between two electromagnets which is stated in his 4th letter, almost at the same time as Jacobi and Lenz reported their discovery in March 1839. The 6th letter (30 August 1839) describes a newly-con- structed motor, and seven months later Joule reports on experiments performed with it.

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2. Joule’s 7th letter.

This is the first of three letters, each with the title, On Electromagnetic Forces. It is dated 10 March 1840 and announces the factors limiting the power and duty of the electric motor. A modem reader sees its importance in that Joule was now working with transformation from chemical energy to electrical energy and thence to mechanical energy. Joule sum- marised his results in a lecture given in 1841. One of his conclusions was: “That the economical duty at a given velocity, and for a given resistance of the battery, is proportional to the mean of the intensities of the several pairs of the battery.” (JSP 47)

3 . Some publications following the 7th letter.

The 7th letter marks the end of reports on motors for a while. Joule turns to experiments on electrical heating (see next section), but at the same time continues work on electromagnets. In connection with these he defines an absolute unit of current which he thinks “more appropriate as well as generally advantageous” than that proposed by Faraday. A degree of current is just able to decompose nine grains of water in one hour.

Joule’s experience with electric cells is such that he communicates some observations to the London Electrical Society. He also publishes on magnetostriction-the first published investigation, commemorated by the naming of the “Joule effect”. The results of his measurements on magnetostriction were reported on 16 February 184 1 in the first of Jou- le’s rare public lectures. The lecture, at the Royal Victoria Gallery of Practical Science, also contained remarks on electric motors (JSP 47) and on possible explanations of magnetic properties (JSP 52/53). These latter give an early indication of Joule’s views on the nature of matter (Daltonian) and heat (vibratory).

4 . Electrical heating.

At the end of 1840 Joule published his famous paper on the heating effect of an electric current. The experiments are divided between measurements on metallic conductors, on cells producing an electric current, and on electrolytic cells. The correspondence between experimental and

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theoretical results is close enough for Joule to conclude that the 12R law is valid in all these cases (JSP 77). A further conclusion is that the heat generated is proportional to the number of atoms electrolyzed in each cell, multiplied by the virtual intensity of the battery. (JSP 78)

As a finale to the main paper Joule comments on a theory of combustion attributed to Berzelius, that a discharge of electricity occurs between the combustible and the oxygen and that this is the origin of the light and heat produced during combustion. Joule believes that the electricity must pass through some sort of resistance to produce the heat of combustion (JSP 78).

5 . Heat of Combustion.

In pursuing the above analogy between chemical combustion and electrical heating Joule embarks (JSP 81) on experiments to measure the heat of combustion and the affinity for oxygen of various metals. He finds good proportionality between these two quantities. However, on comparing the experimental values of heats of combustion with those obtained by calculation on the basis of his theory, Joule finds that the latter are consistently higher. In this paper he blames the loss of heat during the combustion experiments; but finding that his results here are confirmed by Dulong, he is forced to reconsider his electrolytic experiments. A new series of experiments points out his former errors and leads to a more precise interpretation of heating during electrolysis and adjustment of the results of the electrolytic measurements.

6. On the Heat evolved during the Electrolysis of Water. (JSP 109)

Joule read a paper with the above title before the Manchester Literary and Philosophical Society on 24 January 1843. It reports extensive electrolytic experiments. These had importance, not only in the correction of Joule’s previous errors, but also in leading up to his “general observations” (JSP 119). Here, too, we can see implicit use of equivalence between various forms of energy, ‘ I . . . the intensity of a Daniel1 cell such as I used is equivalent to 6 , O I29 in a pound of water per degree of current . . .” (JSP 114).

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7 . Seven observations from January 1843.

The observations at the end of the paper on heating during electrolysis contain the remarks which have excited readers over many years. In the 4th observation Joule announces the proportionality of heat and mechanical energy. From the experiments on electric motors (1839) and from the heating experiments (started 1840) he knows that both the mechanical and thermal effects of current electricity are proportional to the chemical action in the battery and so are mutually proportional. In the 5th observation Joule goes so far as to postulate the possibility of conversion of mechanical energy into heat.

Because of their great interest we quote the seven observations in their entirety:

1st. In an electrolytic cell there are three distinct obstacles to the flow of a voltaic current. The first of these is the ordinary resistance to conduction; the second is resistance to electrolysis without the necessity of chemi’cal change, arising simply from chemical repulsion; and the third is resistance to electrolysis accompained by chemical changes.

2nd. By the first of these heat is evolved, exactly as it is by a wire, according to the amount of the resistance and the square of the current; and it is thus that a part of the heat belonging to the chemical actions of the battery is evolved. By the second, a reaction in the intensity of the battery occurs; and wherever it exists, heat is evolved exactly equivalent to the loss of heating-power of the battery arising from its diminished intensity. But the third resistance differs from the second inasmuch as the heat due to its reaction is rendered latent, and is thus lost by the circuit.

3rd. Hence it is that, however we arrange the voltaic apparatus, and whatever cells for electrolysis we include in the circuit, the caloric of the whole circuit is exactly accounted for by the whole of the chemical changes.

4th. Faraday has shown that the quantity of current electricity depends upon the number of chemical equivalents which suffer electrolysis in each cell, and that the intensity depends on the sum of chemical affinities. Now both the mechanical and heating powers of a current are, per equivalent of electrolysis in any one of the battery-cells, proportional to its intensity or electromotive force. Therefore the mechan- icaJ and heating powers of a current are proportional to each other.

5th. The magnetic electrical machine enables us to convert mechanical power into heat by means of the electric currents which are induced by it. And I have little doubt that, by interposing an electromagnetic engine in the circuit of a battery, a diminution of the heat evolved per equivalent of chemical change would be the consequence, and this in proportion to the mechanical power obtained*.

6th. Electricity may be regarded as a grand agent for carrying, arranging, and

* I am preparing for experiments to test the accuracy of this proposition.-Note, Feb. 18th, 1843.


converting chemical heat. Suppose two of Daniell’s cells in series to be connected, by thick wires, with platinized plates immersed in dilute sulphuric acid. Owing to the near balance of affinities, very little free heat will be evolved, per equivalent of chemical action, in any part of the circuit; and that little will be equivalent to the difference of the intensity of the battery and the intensity due to the electrolysis of water, or to 2-1.35=0.65, if we do not regard the heat arising from secondary action in the battery. But then a great transfer of latent heat, equal to 8”-27 per equivalent, will take place from the battery to the electrolytic cell; and this by the immediate agency of the current. Again, if a large battery be connected by thick conducting- wires with a coil of very thin wire, nearly the whole of the heat due to the chemical changes taking place in the battery will be evolved by that coil, while the battery itself will remain cool.

7th. Pouillet having deduced from his experiments, repeated with great caution by Becquerel, the general conclusion that, during combustion, the oxygen disengages positive and the combustible negative electricity, and Faraday having proved that a constant quantity of electricity is associated with the combining proportions of all bodies, it only remained to prove that the intensity of the electricity passing from the oxygen to the combustible at the moment of their union is just that which is equivalent to the actual heat of combination. This I have attempted; and in so doing have met, 1 think it will be admitted, with such succes as to put the beautiful electrical theory of chemical heat, first suggested by Davy and Berzelius, beyond all question (JSP 119-121).

The observations are followed by an appendix, dated 20 February 1844, in which Joule takes pains to stress that the terminology of his previous work in no way binds him to the substantial theory of heat. The results of the experiments to support the 5th observation were reported by Joule on 21 August 1843 at the meeting of the British Association at Cork.

8. The mechanical equivalent of heat.

Joule had stated in his 5th observation that the conversion of mechanical power into heat should be possible by means of a coil rotating in a magnetic field. His next paper, On the Calorific effects of Magneto-Elec- tricity and on the Mechanical Value of Heat, records his experiments to verify this.

The results can best be illustrated by quoting Joule himself. In Part I appears the concise but potent conclusion: “We have therefore in magnetoelectricity an agent capable by simple mechanical means of de- stroying or generating heat” (JSP 146).

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In Part I1 we have: “The quantity of heat capable of increasing the temperature of a

pound of water by one degree of Fahrenheit’s scale is equal to, and may be converted into, a mechanical force capable of raising 838 lb. to the perpendicular height of one foot.” (JSP 156)

The paper ends with remarks on the duty of steam engines and on the electric motor. It is dated July 1843. A postscript was added in August 1843 recalling Rumford‘s experiments on production of heat by friction. Joule’s conviction of the validity of the view of heat as equivalent to mechanical energy is expressed in the statement that he is “. . . satisfied that the grand agents of nature are, by the Creator’s fiat, indestructible, and that wherever mechanical force is expended, an exact equivalent of heat is always obtained.” (JSP 158)

THREE VIEWS OF JOULES EARLY DEVELOPMENT

It is strange that so little is known of the formation of Joule’s ideas on energy conservation. Strange, because during the rest of his long and active scientific career he seems not to have given any precise details either in writing or in any recorded conversation. Not until after Joule’s death in 1889 did anyone try to trace the threads which Joule had so expertly knitted together. Osbome Reynolds, writing in 1892, depended almost entirely on Joule’s published papers, in spite of having known Joule personally. Perhaps the priority polemic can be blamed for the fact that Joule was never persuaded to publish his recollections of this important period. There was, admittedly, a considerable amount of print used in the discussion of who first formulated the Principle of Con- servation of Energy, but there is nothing here to tell us how the idea arose in Joule’s mind, nor precisely when the various stages of recognition took place. We do have a comment from Joule (Phi2 Mag 1864) that he had not seen fit to announce his belief before he could support it experimentally. But he does not say whether the belief came late or early.

Reynolds discusses Joule’s investigations in chronological order. On Joule’s recognition (in his 7th letter) of the induced e. m. f. in his motor he says: “Joule is not, however, content with recognizing this internal


resistance, but has made a complete experimental determination of its law-l‘that it is proportional to the product of the velocity of rotation multiplied by the magnetism.” This statement is equivalent to saying that the electric action spent in overcoming the resistance induced in the machine, namely, the product of the current multiplied by the induced resistance, is proportional under all circumstances to the product of the square of the current multiplied by the velocity of the machine; and this is also proportional to the mechanical effect; so that Joule had now proved that there are quantitative equivalents in mechanical effect for the electric action (product of electromotive force multiplied by current), and for the chemical action expended in producing the mechanical effect.” (Re 40)

Joule continues work connected with the electric motor but also starts the experiments on electrical heating: I‘. . . and while yet occupied in clearing up the outstanding questions, such as the limits imposed by the saturation of the magnets, he has already started a new research, and, this time, with a purely philosophical object.” (Re 44)

The line of investigation has been suggested to him by the proportionalities mentioned above: “He does not explicitly say so, but it may be clearly inferred from the subsequent line of his work that the discovery of these proportionalities suggested to him the existence of definite equivalents amongst all the effects, resulting in or produced by a definite amount of electric action, as a consequence of the existence of .quantitative relations between the several effects and the electric action. In order to test these views he first investigates the heat produced by a current. . .”(Re 47)

Reynolds describes how Joule follows up the proposed theory of combustion and obtains the results published in January 1843. The summary of results, says Reynolds, shows that Joule was already in the throes of generalization (Re 55). Commenting on the 5th observation of this summary, he writes: “Had he stopped here, it would have been almost impossible to avoid the conclusion that he had not only realized the equivalence of the mechanical and heating powers of an electric current, but had already realized the probable equivalence of the mechanical and heat effects.” (Re 56)

The 7 th observation shows, however, that Joule still regards electricity as necessary in the conversions.

Reynolds points out that Joule had been silent as to his views on the

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nature of heat, but was prepared in 1844 to admit to having had a strong attachment to “the theory which regards heat as motion amongst the particles of matter.” Reynolds denies any influence of this on the line of Joule’s investigations: “This must have been an instinctive attachment, formed without question or consideration, for a theory of infinite possibility resting on a rational base, as against a dogma opposed to many familiar facts.” (Re 58)

The position after the seven observations is summed up as follows: “Joule had now discovered and described all the equivalences but one on which the conservation of energy is founded; the heat, and the chemical equivalents of electric effect, and the heat equivalents of chemical effects. He has not yet generalized, because he has not realized, that the underlying principle is mechanical effect. . . He looks for further developments, but apparently without, as yet, realizing their true character or importance.” (Re 5 8)

Thus Reynolds introduces the investigation which was to discredit the substantial theory of heat and announce the mechanical equivalent of heat. He describes the experiments and then gives a summary of the steps taken by Joule (Re 66) , ending with the remarks: “Joule had thus, by means of the measure of electric action, traced a definite quantity of physical effect throughout the whole region of physics, recognising it in all the transformations it was capable of undergoing, and discover- ing all its modes, flinching at no experimental difficulties to keep it in view until he had brought it again into full light as work or mechanical energy.”

The electric action soon lost its fundamental place in Joule’s interpretation of his findings: “Having written his paper, he seems at once to have perceived the bearing of his discoveries on phenomena, which had not hitherto attracted his attention. It is at least remarkable that throughout his papers so far he has not made any reference to the heat produced by friction, although he frequently mentions that part or the whole power developed in his engines was expended in overcoming friction. The papers themselves would bear the interpretation that he had not hitherto realized that there was any other agency but that of electricity, by which heat and work were convertible; a view which is to a great extent borne out by the remarkable manner in which he comrnen- ces a most remarkable postscript, which contains a clear and somewhat full statement of his views at the time, showing, in somewhat crude lan-


guage, that he had already, in August, realized the law now called the Conservation of Energy as the result of his discoveries.” (Re 70)

We can regard the last two quotations as a summary of Reynolds’ views. They have been blindly followed by many later Joule biographies. However, at the opening of the Joule Museum at Salford on 3 July 1951, Professor Leon Rosenfeld cast doubts on Reynolds’ views. The paper was published, but because it had to be in the form of an address its author was not able to go into great detail. This is, I feel, unfortunate, because although the opinions expressed are clear enough it is rather difficult to pinpoint those errors of Reynolds’ judgement which are the targets of criticism. “It is often said-and unfortunately Osborne Reyn- olds’ biography of Joule, so excellent in other respects, has added au- thority to that view-that Joule is typical of the experimental physicist who bases his conclusions only on the results of experiments without allowing himself to be biased by preconceived theoretical views.” (Ro 17 1)

The few quotations just given should show that Reynolds did, in fact, credit Joule with experimenting on the basis of a “purely philosophical object”, though not in his very early work.

Rosenfeld gives his own account of Joule’s early motives thus: “. . . the idea at the back of his mind was that of equivalence between the phenomena of heat production and the passage of the current through the resistance. If he were not looking for an exact balance, there would have been no point in asking whether there were one or more causes of heat production. Here the “book-keeping” mentality of Joule’s environment is conspicuous, albeit hardly conscious to Joule himself.” (Ro 173)

In granting to Joule the theoretical view of equivalence between electrical and chemical heating, Rosenfeld says more or less the same as Reynolds. It would be difficult to say otherwise since Joule’s statements at this stage are quite clear. The idea of equivalence between heating by chemical combustion and by passage of electricity against a resistance is explicitly put forward as a principle which Joule follows and stresses. A distinct difference of opinion does appear in Rosenfeld’s next para- graph: “At this stage he appears to be non-commital about the theory of heat, but in a later paper he declares that he was actually convinced that the dynamical theory was the true one. Here, then, we have his own word for stating that he was guided from the start, in the orientation

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which he gave to his researches, by a definite theoretical view about the nature of heat.” (Ro 173)

This contrasts sharply with Reynolds’ opinion that Joule’s belief in the dynamical theory of heat was an “instinctive attachment” and that Jou- le had not considered the character of heat before the time of the seven observations. Concerning the commencement of the experiments on electrical heating Reynolds writes: “His philosophy has been directed to the relation between electricity and heat, just, as in his previous work, it was to the relation between electricity and work. But the fundamental character of heat has engaged his attention no more than has that of “work; the skin over his curiosity, arising from commonplace familiarity with these, not having yet been pricked.” (Re 51) He also holds that from the experiments on the electric motor Joule I ‘ . . . inferred that without being convertible, or being in any way primarily related, the chemical effect and the mechanical effect were each quantitatively related to the electric action.” (Re 47) Rosenfeld, on the other hand, writes: “From such clear and unambiguous hints we are, therefore, justified in conclud- ing that Joule was inspired from the very beginning by theoretical ideas: the idea of the dynamical nature of heat and a perhaps not quite articulate idea that there might be an equivalence between heat and other physical agencies. The experiments that he undertook on the connection between heat and electricity were made with a view to testing those ideas.” (Ro 173)

It would appear that the essence of Rosenfeld‘s criticism lies in (a) Reynolds’ denial of Joule’s early and conscious appreciation that his investigations would cast supporting light on his views on the nature of heat and (b) the insistence that electricity was the fundamental in Jou- le’s investigations and that direct equivalence between heat and other physical agencies came very late into Joule’s thought.

Rosenfeld regards the induction experiments reported in July 1843 as a deliberate attempt by Joule to disprove the substantial theory of heat. Joule is presented as being fully aware of the importance of his work: “The qualitative fact that heat appears in this case in the revolv- ing coil was the decisive proof he was looking for that heat is not simply transferred by the current from one place on the circuit to the other, but is actually generated. This was therefore a crucial experiment which disposed of the view of the caloric theory of heat and definitely confirmed the dynamical theory.” (Ro 173)


Here, then, is the greatest gulf between Rosenfeld and Reynolds. It is probably here that Rosenfeld‘s criticism had its origin.

In 1957 another notable writer gave his impression of Joule’s early development. In examining the trigger factors which led to the Principle of Conservation of Energy, Thomas S . Kuhn had occasion to mention the path taken by Joule (K 326).

Joule’s first letters from 1838 are, Kuhn says, exclusively concerned with the design of improved electric motors and he was simply working on one of the many new problems born from nineteenth century discovery. In 1841/2 discouragement with motor design forced him to seek instead a fundamental improvement in the batteries that drove them. The reintroduction of the motor and the concept of mechanical work in 1843 caused Joule’s papers, for the first time, to read like investigations of energy relations. But even in 1843 the resemblance to energy conservation is incomplete. Joule had still to trace other new “connexions” during the years 1844 to 1847. Starting from an isolated problem, Joule had involuntarily traced much of the connective tissue between the new nineteenth century discoveries.

This appears very similar to the interpretation which Rosenfeld con- demned in no uncertain terms. Kuhn neglects any theoretical presuppositions which Joule may have had. In fact, he would have him the victim of fate, involuntarily tracing the threads connecting the various physical agencies.

What, then, were Joule’s intentions in starting the first investigations on electrical heating? Kuhn admits no other motive than that it showed relation to Joule’s attempts to improve the electric motor: “Joule’s concern with batteries and more particularly with the electrical production of heat by batteries dominates the five major contributions in Papers, pp. 52-123. My remark that Joule was led to batteries by his discouragement with motor design is a conjecture, but it seems extremely probable.” (K 346)

One is forced to agree that no other motive was necessary. That Jouk had formed a theory of chemical combustion by the end of the investigation in no way implies that even this provided his inspiration.

Joule’s support of a dynamical theory of heat also receives sober consideration from Kuhn. He points out (K 356) that perhaps Joule would not have developed his theory of conservation had he not tended to re-

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gard heat as motion, but that his published works indicate no such decisive connections.

That three such eminent scholars should offer such disparate views of Joule’s early work can only indicate one thing-that there is lack of conclusive evidence in his printed papers to decide what thought processes led Joule along that particular trail. In the search for the truth Ro- senfeld has, of course, an advantage. In preparing the JouIe Museum he must have come into close contact with Joule’s thought. He is probably the person best qualified to comment on Joule’s attitude to his scientific work, but it seems that his outburst of 1951 has failed to scotch the “impossible legend and that the idea of Joule “merely experimenting” arises in the papers themselves.

I t has been a matter of interest to me to see how far the papers would bear the interpretation that Joule experimented without the guidance of a grand view of nature and with no conscious intention of verifying a dynamical theory of heat. I offer the comments below, not because I think they provide a uniquely plausible account, but because I feel they can shed light on the growth of Joule’s reputation (or notoriety?) as a mere experimenter. I have given credit for theoretical views when the evidence is reasonably convincing, but have been very wary of reading into Joule’s words the philosophical views which are so implicit in a modem reader’s outlook.

A FOURTH VIEW

Joule’s earliest experiments-his interest in engines.

Joule showed a keen interest in the possibilities and limitations of electromagnetism as a mover of machinery. This was certainly the inspiration for his earliest experiments and there is no suggestion in any of the early letters that he had any other intention than to reduce the cost of working the electric motor ad infiniturn (JSP 14).

It is clear from later papers that he retained this interest in electromagnetic motive power. In fact, one might even say that it was a major interest. In the period immediately following the 7th letter Joule worked on improvement of electromagnets, improvement of electric cells,

Joule’s Early Researches 21 1

and an investigation of magnetostriction. All these indicate that Joule was still searching for efficiency in the motive power from electromagnetism. This is emphasised if we read the last comments in the 7th letter: ‘‘I think my engine might be improved by increasing the conductive power of its coils and the softness of the iron of the electro-magnets; but the augmentation of the intensity of each element of the battery is very important, as it is attended by a proportional increase of duty.” (JSP 26)

In the case of the investigation of magnetostriction the search is mentioned explicitly: “I undertook experiments . . . to ascertain whether the new source of power could be advantageously employed for the movement of machinery.” (JSP 48)

Between 1841 and 1843 there was no publication mentioning experiments with motors, but we have Joule’s word for it that it was during this period that he carried out experiments on motors with Scoresby in Bradford (JSP vol 2 page 1). There is, too, a stunning example of Jou- le’s interest in power production which Reynolds has noted (Re 67) in the paper of July 1843, which announces the mechanical equivalent of heat. It is indeed remarkable that in this momentous paper Joule should point out the practical implications of his discovery, i. e. the importance to engineers, before mentioning the philosophical implications in a postscript. The conclusion to this same paper is also noteworthy in that although Joule’s experiments had indicated that work can be transformed (via electricity) into heat, and that heat fails to appear proportionally as work is obtained from an electric motor, there is no evidence that heat can be made to produce a proportional amount of work. Yet this is what Joule concludes. The attempt to connect his results with the economy of engines is obvious.

It seems fairly certain that Joule’s interest in electromagnetic motive power was an important factor in his researches.

The search for quantitative relationships.

Joule’s expectations of the electric motor were not to last long, but the efforts he made to increase its efficiency indicate his unusal aptitude for experimental work. In examining the electromagnets, wherein he suspect- ed lay the reason for his motor’s poor performance, he discovered ac

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important relation concerning the attractive force of electromagnets (JSP 13). Thus, shortly after his twentieth birthday, Joule had announced his first original discovery.

Not only was he at the forefront of research in electromagnetism; he was also aware of developments in other parts of the world. In the Vic- toria Gallery lecture he acknowledges Jacobi as the discoverer of the Lim- itation set to the performance of the electric motor by the induced e. m. f. and continues: “Jacobi had not, however, given precise details concerning the duty of his apparatus; nor had he then determined the laws of the engine. I was therefore induced to construct an engine adapted for experiment. . .” (JSP 47)

This refers to the motor described in Joule’s 6th letter (30 August 1839). Joule was therefore hot on the trail of further quantitative relations. He had already known, before constructing his motor, why the dream of an infinite source of power was to remain unfulfilled but he pursued the investigation of the “duty” and “laws” of the new source of power.

The results of his investigation appeared in the 7th letter, which has naturally drawn the attention of readers of Joule’s works. But we must remember that this is only the first of three letters under the common title On Electromagnetic Forces. I would suggest that these could mark the early stages of a plan to treat the various effects of current electricity by quantitative measurement. Having seen how his first introduction of quantitative measurement had led to the law of attraction between electromagnets, what - was more natural than to develop the method and to seek out other quantitative relations which much exist? Perhaps Joule did not see it as clearly as it is stated here, but there are several pointers in this direction. Joule already had the confidence of an experienced experimenter and had been studying the works of other researchers. He could thus see how undeveloped the subject was, and how lacking in precision. More important still might be the fact that he had been studying Faraday (JSP 28). Who would not be inspired by Faraday?

In support of the suggestion we begin by recalling Joule’s intentions in investigating the behaviour of electric motors. But more directly we can quote Joule himself. At the beginning of his 8th letter we read that he intends to employ an absolute unit and to put the study of current electricity on a firm quantitative foundation: “The great difficulty, if


not the impossibility, of understanding experiments and comparing them with one another, arises in general from incomplete descriptions of apparatus and from the arbitrary and vague numbers which are used to characterize electric currents. Such a practice might be tolerated in the infancy of the science; but in its present state of advancement greater precision and propriety are imperatively demanded. I have therefore determined for my own part to abandon my old quantity numbers, and to express my results on the basis of a unit which shall be at once scientific and convenient.” (JSP 27)

The experiments on electrical heating appeared in the Royal Society’s Proceedings in December 1840 and must have been started soon after the 7th letter was written, probably in August 1840 (L 3). If we proceed on the assumption that Joule was systematically investigating current electricity and its effects, the heating effect was an obvious candidate for study. The recognized properties of current electricity at the time were (following Faraday): production of heat, production of magnetism, chemical decomposition, physiological phenomena, and production of sparks. Joule had already investigated the very new motive power of electromagnetism and was well advanced in his investigation of electromagnets. The chemical decomposition had already been treated by Faraday so he was left with physiology, sparks, or the heating effect.

We can now mention one of the few things we know of the content of Joule’s notebook from this period. Lowery records four pages of “observations made by Joule between 3 1st May and 1 l th June, 1841, on the effects produced by the application of electric currents to the body.” (L 5) Thus Joule had covered most of the phenomena mentioned above, and in particular one which had no bearing on energy conversion or engines. The physiological effects seem, however, not to have lent themselves so readily to Joule’s method as the heating effect had done.

His treatment of the latter was very successful. Not only was he able to announce the 12R law, but also to connect the number of atoms concerned in producing the current, the battery intensity, and the quantity of heat evolved. This preoccupation with the amount of material consumed in the battery has a parallel in the 7th letter where the duty of the electric motor is under consideration. I t is also highly reminiscent of Far- aday’s results on electrolytic decomposition. I do not feel any necessity to assume that Joule had already recognized any form of equivalence before this stage.

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A theoretical view from 1840/41

Whether we admit or deny that Joule made a systematic attempt to quantify the study of current electricity, it is at this point that we must deviate from the view of Joule as a pure experimenter. In his paper on electrical heating there appears an explicit statement of a theoretical view-what Joule was to call “the beautiful electrical theory of chemical heat.” (JSP121) “Berzelius thinks that the light and heat produced by combustion are occasioned by the discharge of electricity between the combustible and the oxygen with which it is in the act of combination; and I am of the opinion that the heat arising from this, and some other chemical processes, is the consequence of resistance to electric conduction.” (JSP 78)

It was this doctrine which prompted Joule’s next investigation, to prove that the heat of combustion is the consequence of resistance to electrical conduction (JSP 81). That he was driving towards this theoretical goal is evident in that, when faced with a discrepancy between experimental and theoretical results, it was the experimental results which Joule questioned. (JSP 101)

There is no hint, however, in this theory of any wider philosophical implications. The equivalence postulated here is by no means the same as the “equivalence between the various forms of force”, nor is there yet any necessity to consider the nature of heat.

In the papers which follow Joule meets with difficulties in explaining the heat evolved during electrolysis, but having sorted these out, he is ready on 24 January 1843 to give a summary of his findings. This appears (JSP 119) as the seven observations, which, as we have seen (page 202), contain the declaration regarding the proportionality and con- vertibility of mechanical and thermal energy. Perhaps, after all, there has been some hidden philosophy behind Joule’s progress.

Joule, the cautious philosopher?

We are forced to admit Joule’s idea of equivalence in the restricted sense of equivalence between electrical and chemical heating. We are also forced to admit an assumption common to all scientists, the existence of laws of nature. But our main concern is to ascertain whether Joule had a view of equivalence between various forms of energy, whether he ad-


hered to a dynamical theory of heat, and to what extent these views directed his experimenting.

Various statements seem to indicate Joule’s belief in the equivalence between physical agencies and in general conservation. We have, in No- vember 1841 , his recognition of the role of the “lost” electrical energy in electrolysis in raising elements to their gaseous state: “But, then, the intensities of affinity were obtained by comparing currents which had been produced under peculiar circumstances with regard to the condi- tion of the elements of the galvanic arrangements: in one case the hydrogen was evolved in a gaseous state; whilst in the other, the hydrogen, by combining with free and condensed oxygen, did not escape. Now we shall see from the following experiments that electric intensity is expended in the act of converting a body into the gaseous state.” (JSP 92)

This was, however, referred to the “recondite operations of resistance to the electric current”, but no further.

We have, too, the statement from January 1843 postulating the impossibility of annihilation of part of the power of the circuit without corresponding effect: “If the resistance to electrolysis which is over and above that due to chemical change were not accounted for elsewhere, it would prove the annihilation of part of the power of the circuit, without any corresponding effect. We shall see that this is not the case, but that in the evolution of heat,where the excess of resistance takes place, an exact equivalent is restored.” (JSP 1 15)

We might also include the curious attempt to correct for loss of light in the experiments on heats of combustion (JSP 106, June 1842). I t is possible for a reader today to read a great deal into this, but Joule gives no stress to the matter and we can give it no more weight than to say that, like the other examples given here, it indicates some feeling for equivalence in Joule’s mind.

We can safely admit the feeling for equivalence without accepting any suggestion that this feeling had a decided influence on the course of Joule’s investigations. We can, in fact, admit it and show at the same time that it had no prominent place in the planning or the interpretation of the experiments. The first general declaration of a belief in equivalence in the present sense came in August 1843 (JSP 158) when Joule states that “the grand agents of nature . . . are indestructible”. This declaration was only added to the paper announcing the mechanical equivalent of heat in a postscript. This is, I feel, proof enough that other

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thoughts were foremost in Joule’s mind as he wrote the paper and plan- ned the experiments. Reynolds mentions (Re 70) another point worth remembering in this connection, Joule’s failure to correct for the “considerable friction” in the bearings of his “magneto-electrical” apparatus. This is probably the major reason for the high values Joule obtained, but there is no mention of it whatsoever. It is only in the postscript that Joule recalls Rumford’s experiments; therefore the idea of equivalence of mechanical and thermal energies had only occurred to him after July 1843.

There is, of course, the possibility that Joule was being ultra-cautious; that he had carefully concealed his innermost convictions and even in announcing the conversion of heat into work still hesitated to bring forward his theoretical views. But Joule had not been afraid to mention his philosophy in other cases. There is no reticence when he states a prefer- ence for one of the two theories of magnetism he discussed in 1841 (JSP 52). Nor is there when his enthusiasm for the electrical theory of chemical heat is stressed and repeated many times. However, the outstanding evidence that the role of ultra-cautious philosopher does not fit Joule is in the 5th observation. The prediction it contains is com- pletely unsupported by experimental evidence. This means either that there was some theoretical view behind the 5th observation-in which case Joule was not cautious, or there was none-in which case Joule was no philosopher. In saying that Joule was not cautious, however, we need not imply that he was rash. It is quite probable that Joule was not aware of the significance of his words until later-hence the footnote.

We conclude that Joule certainly had the feeling for equivalence but that his idea of it was unconscious, vague, and perhaps little more than the intuition of a gifted experimenter who finds himself in an environment possessing a “book-keeping mentality”. It had no influence on the course of Joule’s investigations.

As far as the view that heat is a form of motion is concerned, there can be no doubt that Joule was a firm supporter. However, the effect which this had on the direction of his experimenting is far from obvious. Apart from a mention in 1841 there is no word of it before July 1843, at the start of the paper which confirmed the dynamical view. Here I feel it is reasonable to assume, with Rosenfeld, that Joule was aware of the philosophical implications regarding the theories of heat. One cannot escape the fact that Joule mentions them in his introduction, “. . .


when we consider heat not as a substance, but as a state of vibration, there appears to be no reason why it should not be induced by an action of a simply mechanical character, such, for instance, as is presented in the revolution of a coil of wire before the poles of a permanent magnet” (JSP 123), and then develops the conclusion, “We have therefore in magneto-electricity an agent capable by simple mechanical means of destroy- ing or generating heat.” (JSP 146)

But when did Joule’s awareness develop? From the text it might appear that the footnote of 18 February 1843 marks the earliest date of Joule’s realization that in his 4th and 5th observations he had unsuspect- ingly proposed a method of deciding between the two theories of heat -hence the haste to get the experiments under way and mentioned in print. Yet Rosenfeld claims that the idea of the dynamical nature of heat inspired Joule “from the start” (Ro 173). The statement depends, of course, on what one understands by the “start”, but I can see no evidence in the text to suggest that Joule was guided by a belief in the dynamical nature of heat before, at the very earliest, February 1843.

Of course, Joule declares a strong attachment to “the theory which regards heat as motion among the particles of matter” (JSP 121, Feb- ruary 1844). Perhaps this is why he did not find the 5th observation so startling at first, but only saw later that here was a way to give strong support to a belief which was previously little more than intuitive. That he did not include the note in the original paper indicates that his attention had been drawn to this shortly after the reading of the paper, and the appendix of 20 February 1844 tells us that Joule had given much thought in the intervening period to the question, “What is heat?”

How then are we to regard the seven observations? Let us return to the assumption that Joule had embarked on a programme of experiments to quantify the study of current electricity. The programme corn- menced shortly before August 1839 (i. e. before Joule’s 6th letter). Not until 24 January 1843 was he ready to summarise the basic relations discovered since 1839.

Interpretation of Joule’s seven observations of January 1843.

The 1st and 2nd observations give the conclusions from the paper in which they appear-those on heating during electrolysis. The 3rd observation gives the general principle behind these. We cannot be sure that

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this principle of conservation of “caloric” was in Joule’s mind before the experiments but it is reasonable to suppose that it could be part of his “book-keeping’’ philosophy. However, the extension of Black‘s conservation of heat in calorimetric experiments to an application in this case is not great, and there is no hint of extension to a case involving more than one physical agency. The apparent transport of heat by electricity was, though, a new factor. Joule returned to this in his 6th observation.

The 4th observation is a very succinct statement of the results from the experiments on motors and on electrical heating. By referring to Far- aday’s discoveries that the quantity and “intensity” of electricity from a battery are rigidly determined by, respectively, the number of equivalents consumed and the “sum of chemical affinities”, Joule is able to combine his own results in a very tidy relation because he has shown that, for a given quantity of electricity, both the mechanical and thermal effects of a battery are proportional to the “intensity”. Thus he is enabled to state very concisely the essence of the results of his early experiments and to connect them with those of Faraday. For a young man seeking out the basic relations of electrical effects, this must have been very satisfying. There is no reason to suspect any undercurrent of philosophy.

Looking at the 5th observation in the light of what precedes it, we can see it as merely an illustration of the previous statement. Joule had already devised the means of demonstrating the conversion of work into heat and of confirming the proportionality quantitatively. If he had realized that his statement was tantamount to a denial of the substantial theory of heat, he would have been aware that he was on un- proved ground and surely would have shown more care in the wording, at least mentioning in the text that experiments were under way. That he considered experimental evidence unnecessary indicates clearly that he had not thought deeply over his proposals, except as an illustration of his previous results.

Not only is the 5th observation unsupported, it is also unemphasised. To a modern reader the 4th and 5th observations stand out as being of major importance. To Joule they were apparently of major interest (otherwise they would not have been included), but standing as they do in the middle of a fairly long conclusion, they show that Joule did not yet value them as highly as the “electrical theory of chemical heat” which he supported in the last two observations.


The 6th observation introduces Joule’s general comments on the results of his research programme. He begins with a flourish: “Electricity may be regarded as a grand agent for carrying, arranging, and converting chemical heat.”

There is no mention whatever of the nature of this chemical heat. In fact, there is nothing in these observations which indicates one way or the other how Joule regards heat. Joule did deny that he had ever held the view of heat as a substance-though not before a full year had passed, during which time he formulated a theory of heat. This very eventful year had somewhat altered his view of electricity for we read now (JSP 122) that “electricity is a grand agent for converting heat and other forms of mechanical power into one another.”

All this seems to indicate that in January 1843 Joule was pleased to round off a programme of research with the seven observations, but that in the year which followed he became aware that his words had contained much more than he had realized. The 7th observation shrank in stature; Joule had been so taken up with this “grand finale” that he had missed important pointers in his own words. The success he had in put- ting his theory “beyond all question” had blinded him, but only tempo- rarily.

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Joule's Early Researches