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.
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.
5.
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.
9.
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
