SECTION LXIX.

THERMO-ELECTRIC CIRCUITS.

Apparatus required: Water baths, thermo-circuits, fourway key and mirror galvanometer.

If a circuit consists of wires of different materials, and if one of the junctions of two dissimilar wires is heated, an electric current flows through the circuit, and continues to flow so long as the difference of temperature between the heated junction and the rest of the circuit is maintained.

This electric current is due to an electromotive force in the circuit produced by the inequality of temperature of the two junctions, and it is found, for small differences of temperature between the two junctions, to be nearly proportional to the difference. For greater differences, if t, is the temperature of the hot junction, to that of the rest of the circuit, the electromotive force e in a circuit, of two metals is given by the equation

where A and T are constants depending on the two materials of the circuit, T being a temperature known as their "neutral temperature.

To verify the above statements the apparatus shewn in Fig. 124 is provided.

It consists of two vessels containing water, in which are placed two test tubes containing the junction of the wires to be experimented on, and thermometers for indicating their temperatures. The rest of each tube is filled with clean sand

or with petroleum, to improve the thermal connection of the junctions and thermometers with the water.

The circuits to be tested consist of lengths of No. 25 iron, nickel, and lead wire, to one end of each of which a length of copper wire is soldered, and brought to a binding screw placed on the board through which the test tubes pass. The other ends are soldered to copper wire brought to a fourth screw on the board.

The binding screws except that connected with the lead wire should be joined by copper connecting wires to a four-way key, so that each wire may in turn be connected together with the lead wire to a galvanometer of about 50 ohms resistance. With a galvanometer of this resistance, the effect of the different resistances of the circuits may be neglected, and the deflections taken as proportional to the electromotive forces acting in the various circuits.

Fill the two vessels with water at the temperature of the room, and connect the thermo-circuits in turn through the four-way key to the galvanometer. Verify that there is no current in any of the circuits.

Now raise the temperature of the vessel which has no binding screws over it, to about 70°C. and keep it constant for 10 minutes. Then connect the galvanometer to each circuit in turn, and deterinine the deflections, noting the temperature

before and after each observation. If the sensitiveness of the galvanometer can be altered adjust it so that the greatest deflection observed is nearly to the end of the scale. Decrease the temperature to about 60° C. by adding cold water and repeat.

Continue till the hotter vessel reaches about 20° C., then raise its temperature to about 70° C. in steps of about 10° C., taking observations in the same way during the process.

Note the direction in which the current flows through the hot junction in each case, and enter the deflection as positive when the current flows from the lead to the other metal through the hot junction.

Represent the observations for each circuit by a curve taking to as abscissae and the deflections as ordinates. Indicate the observations taken while the temperature was decreasing by a cross, those while it was increasing by a circle. The curves will be found to be almost straight lines, and from this it is evident that if we put the equation (p. 334) into the form

e=E。 (t-to) (1-bt, + to), where E, AT and b =

b must be small.

=

A

2E.'

The "thermoelectric power or height" of a metal at a given temperature with respect to lead, which is taken as the standard, is defined as the ratio of the small increase of E.M.F. produced

when a junction of lead and the metal is heated, to the small increase of temperature of the junction, and is counted positive if the E.M.F. tends to produce a current from the lead to the metal through the heated junction.

In the above case, if the deflection obtained say for leadnickel at 59° is subtracted from that obtained at 70° and the difference divided by 70° - 59°, the quotient is proportional to the thermoelectric power of nickel at (70+59)/2 = 64.5° C. on the scale used. To reduce the results to an absolute scale, in micro-volts per degree, make use of the fact that the thermoelectric power of nickel with respect to lead at about 60° is 25 micro-volts per degree.

Determine from the observations the thermoelectric powers of the metals used, and draw a "thermoelectric diagram," taking temperatures as abscissae and thermoelectric powers as ordinates.

If more accurate results are required, a low resistance galvanometer must be used, and the resistance of each circuit be made equal, or the total resistance of the galvanometer and each circuit be taken into account in comparing the electromotive forces of the circuits.

S. P.

22

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SECTION LXX.

MEASUREMENT OF THE MECHANICAL EQUIVALENT OF HEAT BY THE ELECTRICAL METHOD.

Apparatus required: Covered calorimeter, thermometers, heating coil, standardised ammeter and voltmeter, storage cells, watch.

When the whole of the work done on a body is converted into heat, the amount of work done bears a fixed ratio to the amount of heat produced, in whatever way the work is performed, and the work which has to be done to generate one gram-degree of heat is, we have seen (p. 142), known as the "mechanical equivalent" of heat. To determine this quantity, any convenient method of generating heat by performing work on the body may be adopted, and it is proposed in this section to do work by sending a current of electricity through an insulated wire immersed in water.

=

= EAt

If A is the current passing through the wire, and E the electromotive force at the ends of the wire, the rate at which work is done on the wire per second is EA watts, and if the current flows for t seconds, the total work done joules EAt. 107 ergs. If the water rises in temperature 0 degrees, and the water equivalent of the calorimeter thermometer and coil w, the heat generated, supposing no heat is lost by radiation etc. we. If J joules are necessary to ωθ. generate one gram-degree of heat, we have

=

=

EAt = w0J,

from which J can be found.

Weigh the calorimeter and stirrer provided. Nearly fill the