530 STANDARD ELECTRICAL DICTIONARY.

Thermaesthesiometer.

An electro-therapeutic instrument for testing the sensitiveness of the surface of the body to changes of temperature. Vessels of mercury are provided with thermometers to indicate their temperature. One vessel is surrounded by an electric conductor wound in a number of turns. The temperature is raised by pa.s.sing a current through this. By successive applications of the vessels to the same spot upon the skin the power of differentiating temperatures is determined.

Thermo Call.

(a) An electric alarm or call bell operated by thermo-electric currents.

It may serve as a fire alarm or heat indicator, always bearing in mind the fact that differential heat is the requisite in a thermo-electric couple.

(b) See Thermo-electric Call.

Thermo-chemical Battery.

A voltaic battery in which the electro-motive force is generated by chemical action induced by heat.

The chemical used generally is sodium nitrate or pota.s.sium nitrate. The positive plate is carbon. On heating the battery the nitrate attacks the carbon, burning it and produces potential difference. For negative plate some metal unattacked by the nitrate may be employed.

Fig. 330. POUILLET'S THERMO-ELECTRIC BATTERY.

Thermo-electric Battery or Pile.

A number of thermo-electric couples q. v., connected generally in series.

In n.o.bili's pile the metals are bis.m.u.th and antimony; paper bands covered with varnish are used to insulate where required. In Becquerel's pile copper sulphide (artificial) and German silver, (90 copper, 10 nickel) are the two elements. The artificial copper sulphide is made into slabs 4 inches long, 3/4 inch wide, and 1/2 inch thick (about).

Water is used to keep one set of junctions cool, and gas flames to heat the other set. In Fig. 331, c, d represent the binding screws. The couples are mounted on a vertical standard, with adjusting socket and screw B, so that its lower end can be immersed in cold water, or raised therefrom as desired.

531 STANDARD ELECTRICAL DICTIONARY.

FIG. 331. BECQUEREL'S THERMO-ELECTRIC BATTERIES.

Fig. 332 shows one couple of the battery. S is artificial antimony sulphide; M is German silver; m is a protecting plate of German silver to save the sulphide from wasting in the flame.

Fig. 332. ELEMENTS OF BECQUEREL'S THERMOELECTRIC BATTERIES.

Clamond's pile has been used in practical work. The negative element is an alloy of antimony, 2 parts, zinc, 1 part. The positive element is tin plate. Mica in some parts, and a paste of soluble gla.s.s and asbestus in other parts are used as insulators. They are built up so as to form a cylinder within which the fire is maintained. The air is relied on to keep the outer junctions cool. The temperature does not exceed 200? C.

(392? F.)

Sixty such elements have an electro-motive force of 300 volts and an internal resistance of 1.5 ohms. Such a battery requires the consumption of three cubic feet of gas per hour. (See Currents, Thermo-electric. )

532 STANDARD ELECTRICAL DICTIONARY.

Thermo-electric Call.

A thermostat arranged to ring a bell or to give some indication when the temperature rises or falls beyond certain points. It may be a compound bar of bra.s.s and steel fixed at one end and free for the rest of its length. Its end comes between two adjustable contacts. As the temperature rises it bends one way (away from the bra.s.s side) and, if hot enough, touching a contact gives one signal. If the temperature falls it curves the other way, and if cold enough touches the other contact, giving another signal. (See Thermostat, Electric.)

Thermo-electric Couple.

If two dissimilar conductors form adjacent parts of a closed circuit, and their junction is at a different temperature than that of the rest of the circuit, a current will result. Such pair of conductors are called a thermo-electric couple. They may be joined in series so as to produce considerable electro-motive force. (See Thermo-electricity and other t.i.tles in thermo-electricity.)

The efficiency of a thermo-electric couple according to the second law of thermo-dynamics is necessarily low--not over 10 per cent.

Thermo-electric Diagram.

A diagram indicating the change in potential difference for a fixed difference of temperature between different metals at different temperatures. It is laid out with rectangular co-ordinates. On one axis temperatures are laid off, generally on the axis of abscissas. On the other axis potential differences are marked. Different lines are then drawn, one for each metal, which show the potential difference, say for one degree centigrade difference of temperature between their junctions, produced at the different temperatures marked on the axis of abscissas.

Fig. 333? THERMO-ELECTRIC DIAGRAM, GIVING POTENTIAL DIFFERENCE IN C. G. S. UNITS.

Thus taking copper and iron we find at the temperature 0? C. (32? F.) a difference of one degree C. (1.8? F.) in their junctions will produce a potential difference of 15.98 micro volts, while at 274.5? C. (526.1?

F.) the lines cross, and zero difference of potential is indicated.

Taking the lead line on the same diagram it crosses the iron line a little above 350? C. (662? F.), indicating that if one junction is heated slightly above and the other is heated slightly below this temperature no potential difference will be produced. Lead and copper lines, on the other hand, diverge more and more as the temperature rises.

533 STANDARD ELECTRICAL DICTIONARY.

Thermo-electric Inversion.

The thermo-electric relations of two conductors vary at different temperatures. Sometimes at a definite point they have no electro-motive force and after pa.s.sing this point the positive plate becomes a negative one and vice versa. This is inversion, or reversal. (See Thermo-electric Diagram.)

Synonym-- Thermo-electric Reversal.

Thermo-electricity.

Electric energy, electro-motive force or electrification produced from heat energy by direct conversion. It is generally produced in a circuit composed of two electric conductors of unlike material, which circuit must possess at least two junctions of the unlike substances. By heating one of these to a higher temperature than that of the other, or by maintaining one junction at a different temperature from that of the other a potential difference is created accompanied by an electric current.

In many cases differential application of heat to an identical material will develop potential difference. This effect, the converse of the Thomson effect, is not used to produce currents, as in a closed circuit the potential differences due to differential heating would neutralize each other.

Thermo-electric Junction.

A junction between two dissimilar conductors, which when heated or cooled so as to establish a differential temperature, as referred to the temperature of the other junction, produces potential difference and an electric current.

Thermo-electric Pile, Differential.

A thermo-electric pile arranged to have opposite faces subjected to different sources of heat to determine the ident.i.ty or difference of temperature of the two sources of heat. It corresponds in use to a differential air thermometer.

Thermo-electric Power.

The coefficient which, multiplying the difference of temperature of the ends of a thermo-electric couple, gives the potential difference, expressed in micro-volts. It has always to be a.s.signed to a mean or average temperature of the junctions, because the potential difference due to a fixed difference of temperature between two metals varies with the average temperature of the two junctions. (See Thermo-electric Diagram.)