DISCUSSION ON"A THYRATRON STROBOSCOPE "•

Mr. A. Glynne (communicated): The stroboscopeis to be used to observe the difference in frequencybetween a supply of standard frequency and the speedof rotation of a machine. An ultra-short flash has noadvantage for this purpose. If a small neon tube besupplied with 120 volts (d.c.) together with about 40volts of the standard frequency (a.c.), the tube will giveflashes of 3 or 4 milliseconds duration. If the image ofthe tube be now examined in a flat mirror attached tothe end of shaft of the machine at a small angle to theplane of rotation, the slip speed may conveniently beobserved in daylight. The load on the a.c. supply isof the order of 0-005 volt-ampere, i.e. less than 1 percent of that mentioned in the paper.

In the author's stroboscope the rate of change of theflux in the transformer core is reduced by the secondaryampere-turns and by the eddy currents in the iron.The circuit shown in the Figure avoids this effect, requires

Diode

1 Thyratron

only an a.c. supply, and is not subject to the troublesmentioned in the paper. In operation the condensercharges up to the peak value of the a.c. potential duringthe first half of the positive half-cycle. Nearly 90°later the negative potential on the grid of the thyratronbecomes sufficiently reduced to allow the condenser todischarge through the neon tube. As there is noinductance in this circuit and very little resistance, thedischarge is very rapid. I estimated the duration of theflashes, using the method described below, to be about10 microseconds.

I should like to know how much current is taken fromthe d.c. supply in the author's apparatus. Supposethat it is 30 milliamperes and that the discharge throughthe thyratron takes place in 50 microseconds; this wouldgive a peak value of current in the thyratron in excess of12 amperes. It is well known that the cathodes ofthese tubes can easily be damaged in this way. Thedifficulty can be avoided by using a mercury valve with

• Paper by Mr. R. S. J. SPILSBURY (see vol. 79, p. 585).

a mercury-pool cathode. I have tried, with success,using, in place of the thyratron, one anode, one grid, andthe cathode of a mercury-arc rectifier. In one experi-ment the a.c. voltage was raised to 620, the condenserwas reduced to 0-03 [xF, and the mean current was4 milliamperes. The neon tube used was of the " beehive " type without resistance, and its light illuminatedthe edge of a disc moving at 2 500 cm per sec. Theapparent thickening of radial lines 0-25 mm wide wasestimated to be 50 per cent, which gives the duration ofthe flashes as 5 microseconds. If a phase-shiftingtransformer be used to supply the alternating current tothe apparatus, the operation of any machinery runningat synchronous speed can be studied.

Mr. R. S. J. Spilsbury (in reply): With regard toMr. Glynne's first suggestion, it appears that the totalinput to the circuit would be of the order of 0*02 volt-ampere, or about l/500th of that taken by the devicedescribed in the paper, so that the light given is pre-sumably very feeble: this feature greatly reduces theutility of a stroboscope. As to the power demand, itshould perhaps be emphasized that the load imposed bymy device on the controlled-frequency supply is duepractically entirely to the resistor R3: if the supply istaken from an output transformer, R3, C3, and C4 canbe omitted, when the consumption is merely that of thethyratron grid circuit and is extremely small.

Mr. Glynne's second circuit, shown in the figure, issimilar in principle to mine, but it employs a rectifieda.c. supply instead of d.c. mains and omits the inductioncoil. It would be of no use for my purpose, since ittakes the whole of the power for the neon lamp from thecontrolled-frequency source: it might, however, haveadvantages for applications for which the mains fre-quency was sufficiently accurate.

The d.c. consumption of the circuit described in thepaper is approximately 45 milliamperes, and the dura-tion of the discharge about 80 microseconds. Thethyratron employed has a peak rating of 12-5 amperes,and two samples have each operated the circuit forabout 1 300 hours without failure. In these circum-stances it hardly seems worth while to adopt the cold-cathode tube, with its attendant ignition difficulties.

As a matter of interest the values of the componentsof my circuit for a standard frequency of 50 cycles persec. are as follows:—

Rt= 4 000 ohms.R2= 10 000 ohms.#„ = 100 000 ohms.

G1 =

= CA = 2

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