Callender and Griffith’s Bridge

Object: To find the temperature coefficient of resistances for platinum, using a platinum resistance thermometer and a callender and Griffith’s bridge.

Apparatus Used: Callender and Griffith’s bridge, platinum resistance thermometer, galvanometer and connection wires.

Formula Used: The temperature coefficient of resistance,α , is given by the formula: α= R2–R1 / R1 t2–R2 t1 Where R1= resistance of the platinum wire at t1o CR2= Resistance of the platinum wire at t2o C Using a Callendar and Grafith bridge the resistance R, is given by the formula:2 x ρ, R = r Where ρ= resistance per unit length of the potentiometer wire.r = resistance introduced in resistance box.x = distance of null point from middle point (electrical zero.) The proper sign is used according as the balance point lines to the right or left of the electrical zero.Procedure:(a) Determination of electrical zero:Complete the connections as shown in fig. Short circuit the gaps PP and CC i.e. connect the terminals by a thick copper wire. Make the resistance R zero by moving the key of variable resistance. Now make the cell circuit on by inserting the plug key. Determine the position of null point by moving the jockey on the bridge wire. The null point gives the position of electrical zero.(b)Determination of resistance per unit length (ρ) of the bridge wire:To determine the value of ρ, the gap (PP) and (CC) are short circuited. The value from R is adjusted to a small value r to obtain the balance. Let the balance be on the left side of O at a distance x’, then,0 = r -2ρx’ρ = r / 2x’ Determination of R2 and R1 at steam temperature and room temperature respectively:(i) Connect the compensating leads and platinum leads of platinum resistance thermometer to the callender and Graffith’s bridge at CC and PP respectively. Other connections are made according to the circuit diagram shown in fig.(ii)Place the bulb of thermometer in a tumbler of ordinary water bath at room temperature and wait for some time to ensure that it has acquired the temperature of water. Note the temperature of water with the help of mercury thermometer. By introducing a suitable resistance, in the circuit of compensating leads with the help of moving coil resistance, balance is obtained. This length of the bridge wire is noted. Using the following formula the resistance of platinum wire R1 at room temperature is calculated: R1= R 2ρx Where R =resistance introduced in the moving coil resistance.x = length adjusted on bridge wire to get the null point.(iii) Now the platinum resistance thermometer is placed in hypsometer or in a hot water bath and wait for some time such that it acquires the constant temperature of the bath. Note this temperature also and determine again the resistance of the platinum wire as described above in the point (ii) let the resistance at this temperature be denoted by R2.(iv) Calculate the value of α using the relation,α= R2 –R1/R1 t2–R2 t1

Result:

The temperature coefficient of resistance for platinum = …….per oC