The document describes the construction and working of a resistance thermometer, which uses a Wheatstone bridge circuit to measure the changing resistance of a thermometer bulb in contact with an object to determine its temperature. Key components include a battery, galvanometer, thermometer bulb, fixed resistors A and B, and an adjustable slide wire resistor s. The resistance of the bulb r is determined through balancing the Wheatstone bridge and calculating the ratio of r to the portion of s in the circuit. Improved versions address issues like contact resistance affecting measurements.

1. VARADA SUJEETH
B130345CH

2. What is resistance thermometer?

3. CONSTRUCTION
 Consists 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.

4.  Adjustable slide wire, s.
 Slider, c.
 Resistors and slide wires are made of manganin, to
avoid any effect due to ambient temperature changes.

6. 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 s
Combining both, r =(A/B)s

7. WORKING
 Resistance 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.

8. DISADVANTAGES
 Contact 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.

10. CALLENDER – GRIFFITHS BRIDGE
 Improved version
 Slide 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

11.  Dividing
r=D*(A/S +f)/(B/S +1-f)
 Advantages: effect of contact resistance is avoided.

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