For bis.m.u.th and antimony at 19.5? C. (67.1? F.) it is 103 microvolts per degree Centigrade (1.8? F.). This means that if one junction is heated to 19? C. and the other to 20? C. (66.2? F. and 68.0? F.) a potential difference of 103 micro-volts will be produced.
The potential difference is approximately proportional to the difference of temperature of the two junctions if such difference is small. Hence for large differences of potential the thermo-electric power coefficient does not apply.
As a differential function it is thus deduced by Sir William Thomson, for expressing the E. M. F. in a thermo-electric circuit: If a circuit is formed of two metals with the junctions at indefinitely near temperatures, t and t + dt, and dE is the E. M. F. of the circuit, then the differential coefficient dE/dt is called the thermo-electric power of the two metals for the temperature t.
534 STANDARD ELECTRICAL DICTIONARY.
Thermo-electric Series.
The arrangement of possible thermoelectric elements, q. v., in a table in the order of their relative polarity. Bis.m.u.th and antimony form a couple in which when their junction is heated the bis.m.u.th acts as the positive or negatively charged element and antimony as the negative or positively charged. Between these two extremes according to Seebeck the series runs as follows:
Antimony, Silver, Copper, a.r.s.enic, Gold, Platinum, Iron, Molybdenum, Palladium, Steel, Tin, Cobalt, Cadmium, Lead, Nickel, Tungsten, Mercury, Bis.m.u.th.
Zinc, Manganese,
A differential temperature of 1? C. (1.8? F.) in a bis.m.u.th-antimony couple maintains a potential difference of 103 micro-volts.
Matthiessen gives a different series; it is arranged in two columns; the first column has positive coefficients annexed the second has negative.
On subtracting the greater one from the lesser, which, if the two elements are in different columns, of course amounts to adding after changing the negative sign, the relative potential difference due to the combination is obtained.
+ - Bis.m.u.th 25 Gas c.o.ke 0.1 Cobalt 9 Zinc 0.2 Pota.s.sium 5.5 Cadmium 0.3 Nickel 5 Strontium 2.0 Sodium 3. a.r.s.enic 3.8 Lead 1.03 Iron 5.2 Tin 1 Red Phosphorous 9.6 Copper 1 Antimony 9.8 Silver 1 Tellurium 179.9 Platinum 0.7 Selenium 290
Thus the relative E. M. F. of a bis.m.u.th-nickel couple, as both are in the + column, would be 25 - 5 = 20; that of a cobalt-iron couple, one being in the + column the other in the - column, would be 9 + 5.2 = 14.2. Alloys are not always intermediate to their const.i.tuents, and small amounts of impurities affect the results largely. This may account for the discrepancies of different observers. Other compounds could be introduced into the series.
Artificial silver sulphide has been used by Becquerel in a thermo-electric battery.
535 STANDARD ELECTRICAL DICTIONARY.
Thermo-electric Thermometer.
A species of differential thermometer. It consists of two thermo-electric junctions connected in opposition with a galvanometer in the circuit. Any inequality of temperature in the two ends or junctions produces a current shown by the galvanometer. It may be used to determine the temperature of a distant place, one of the junctions being located there and the other being under control of the operator. If the latter junction is heated until no current is produced its temperature is evidently equal to that of the distant couple or junction. The heating may be done with hot water or mercury, or other melted metal.
The temperature of the water, or other substance, gives the temperature of the distant place.
Thermolysis.
Decomposition by heat; dissociation. All compound bodies are decomposable by heat if it is intense enough. Hence at very elevated temperatures there can be no combustion.
Synonym--Dissociation.
Thermometer.
An instrument for indicating the intensity of heat. Three scales of degrees of heat are used in practise, the Fahrenheit, R?amur, and Centigrade, each of which is described under its own t.i.tle. (See Zero, Thermometric-Zero, Absolute.) The ordinary thermometer depends on the expansion of mercury; in some cases alcohol is used. Besides these the compound bar principle as used in the thermostat (see Thermostat, Electric) is employed.
Thermometer, Electric.
(a) A thermometer whose indications are due to the change of resistance in conductors with change of temperature. Two exactly similar resistance coils maybe electrically balanced against each other. On exposing one to a source of heat, its resistance will change and it will disturb the balance. The balance is restored by heating the other coil in a vessel of water when the temperature of the water gives the temperature of both coils. The coils are enclosed in water-tight metallic cases.
Synonym--Electric Resistance Thermometer.
(b) A differential thermometer may be made by connecting with a pair of conductors, two thermo-electric couples in opposition to each other, and including a galvanometer in series. On heating the junction of one couple more than that of the other a current at once goes through the galvanometer.
(c) (See Thermometer, Kinnersley"s.)
Synonym--Thermo-electrometer.
536 STANDARD ELECTRICAL DICTIONARY.
Fig. 334. KINNERSLEY"S THERMOMETER.
Thermometer, Kinnersley"s.
A thermo-electrometer. A large gla.s.s tube is mounted on a standard and communicates with a small tube parallel to it. Water is poured in so as to rise in the small tube. Two wires terminating in bulbs enter the large tube by its top and bottom. The upper wire can be adjusted by moving up and down through a stuffing box. On discharging a Leyden jar through the s.p.a.ce between the k.n.o.bs on the two wires the water for a moment rises in the small tube. There is little or no accuracy in the instrument. It is allied to the electric mortar (see Mortar, Electric) as a demonstrative apparatus.
Synonyms--Electric Thermometer--Thermo-electrometer.
Thermo-multiplier.
A thermo-electric battery including a number of couples. The term is generally applied to a small battery with its similar junctions facing in one direction and used for repeating Melloni"s experiments on radiant energy, or so-called radiant heat.
537 STANDARD ELECTRICAL DICTIONARY.
Thermophone.
An apparatus for reproducing sounds telephonically by the agency of heat; a receiving telephone actuated by heat. Thus a wire may be attached to the centre of a diaphragm and kept in tension therefrom, and the transmitting telephone current may be caused to pa.s.s through it. The wire changes in temperature and consequently in length with the pulses of current going through it and vibrates the diaphragm, reproducing the sound. It is to be distinguished from the thermo-electric telephone which involves the action of potential difference produced by thermo-electric action.
Thermostat, Electric.
A thermostat or apparatus, similar to a thermometer in some cases, for closing an electric circuit when heated. It is used in connection with automatic fire alarms to give warning of fire. For this use a temperature of 52? C. (125? F.) is an approved one for setting one at, to complete the circuit. It is also applied to regulation of temperature, as in incubators.
(a) One kind of thermostat consists of a compound bar wound into a spiral and fastened at one end, to which a terminal of a circuit is connected. The bar may be made of two strips of bra.s.s and iron riveted together, and wound into a spiral. When such a bar is submitted to changes of temperature it bends in different directions, because bra.s.s expands and contracts more under changes of temperature than does iron.
A contact point, to which the other terminal is connected, is arranged to make contact with the spiral at any desired degree of temperature, thus closing an electric circuit and ringing a bell, opening or closing a damper, or doing anything else to notify an attendant or to directly change the temperature.
If the bra.s.s forms the outside of the spiral, increase of temperature makes the bending of the spiral bring the coils still closer. If the bra.s.s forms the inside, increase of temperature makes the spiral tend to become less close. As shown in the cut, the bra.s.s should lie along the inside of the spiral.
Sometimes a straight compound bar is used, one of whose ends is fastened and the other is free. As the temperature changes such a bar curves more or less, its free end moving to and fro. Two contact screws are provided, one on each side of its free end. If the temperature falls it makes contact with one of these; if the temperature rises, it makes contact with the other. Thus it may close one of two circuits, one for a fall and the other for a rise in temperature.
It is well to introduce a third bar between the bra.s.s and iron ones, made of some material of intermediate coefficient of expansion.
(b) Another kind of thermostat comprises a vessel of air or other gas, which, expanding by heat, actuates a piston or other device and closes an electric circuit. Synonym--Electro-pneumatic Thermostat.
(c) Another form utilizes the expansion of mercury. The mercury is made part of an open electric circuit. As it expands it comes in contact with the other terminal of the circuit, thus completing it, when the current gives an alarm or does as is provided for in the apparatus employed.
Thermostats may be worked on either open or closed circuits; normally the circuit may be open as described and may close on rise of temperature, or it may be normally closed and open as the temperature rises.
Fig. 335. ELECTRIC THERMOSTAT.