THE MEASUREMENT OF HIGH TEMPERATURES 235
meter, however, throws the bridge out of balance. The resulting deflection of the galvanometer pointer causes one of the wires to make contact with the moving wheel, and this in turn energizes one of the electro-magnets and so releases the clock. The-pen arm is then drawn along the bridge wire until the balanced condition is restored, when the contact of the galvanometer pointer is broken. In this way a continuous, record of the variations in temperature of the thermometer is obtained. A 4-volt
accumulator is necessary to operate the recorder. The drum of the recorder rotates once in twenty-five liours, giving a time scale of J-iiich per hour, but a simple change-speed gear is fitted, by means of which the speed may be increased to one revolution in about two hours.
The resistance pyrometer is of a very accurate order, being extremely sensitive and responsive to the slightest changes of temperature. For work up to 1,200°, in which. accuracy is of the first importance, the platinum resistance instrument is to be preferred to all others. As compared with. the thermo-electric pyrometer, it has the advantage of a more pronounced effect for a given temperature, and, con-sequently, greater sensitiveness. On the whole it may be said, however, that whereas the resistance instrument is of special valne for laboratory and research work in which. accuracy is essential, the thermo-electric pyrometer is preferable for ordinary industrial work. For continuous
Fig. 135.—Callendar Recobding Pykometer.
use the resistance pyrometer should not be taken much above 900°.
THERMO-ELECTRIC PYROMETERS
If a closed circu.it is formed of two different metals, and one of the ju actions is-heated, a current will flow round. the circuit. For a given circuit, the current producecL will depend upon the difference of temperature between the hot and cold junctions. This gives a means of measuring very high or very low temperatures.
Let ABC and ADC be two pieces of wire of different materials, fused or soIderecL together at A and C ; then if the junction at A is maintained at a higher temperature than that at C, a current will be produced which may flow round th.e circuit in either direction, according to the temperature of A. The two wires are said to constitute a thermo-couple. For measurements of very high, or very low temperatures a thermo-