Slide 1
RESISTANCE THERMOMETER CIRCUITSVARADA SUJEETHB130345CH
What is resistance thermometer?
CONSTRUCTIONConsists of a Wheatstone bridge for the measurement
of resistance.A battery is connected across two junction points, to
provide a potential.Other 2 junctions are connected to a
galvanometer.Two fixed resistors A,B.Resistance thermometer bulb,
r.
Adjustable slide wire, s.Slider, c.Resistors and slide wires are
made of manganin, to avoid any effect due to ambient temperature
changes.
PRINCIPLE
Null method of measurement.At balance or null point ,potential
across A and B is equal to potential across r and s . i.e, ia A= ib
B ia r = ib sCombining both, r =(A/B)s
WORKINGResistance bulb is made in contact with the body ,whose
temperature is to be measured.Resistance of bulb changes
corresponding to the temperature.Wheatstone is balanced by the
movement of slider over the slide wire resistance.Movement of
slider can be measured using a calibrated scale.
DISADVANTAGESContact resistance vary with condition of slide
wire.Dirt, dust and wear can increase contact resistance.Resistance
of connecting lead wires changes with temperature.Current through
bulb causes Joule heating effect , which changes the
temperature.
CALLENDER GRIFFITHS BRIDGEImproved versionSlide wire, s is
placed at top of bridge so that it lies in both arms of
bridge.Balanced condition id D = ib(B+S-f*S) id r = ib(A+f*S)
f-fraction of slide-wire lying in arm A
Dividing r=D*(A/S +f)/(B/S +1-f) Advantages: effect of contact
resistance is avoided.
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