ELECTRICAL THERMOMETRY The convenience of the mercurial thermometer lies in the fact that it is complete in itself, and can be read without sub sidiary appliances beyond a magnifying glass. Its weakness lies in the very limited range of each single instrument, and in the troublesome and often uncertain corrections which must be applied to its readings in all work of precision. Electrical thermometers have the disadvantage of requiring auxiliary apparatus, such as galvanometers and resistances, the use of which involves some electrical training. But they far surpass the mercurial thermometer in point of range, delicacy and adapt ability, and can be applied to many investigations in which ordinary thermometers are quite useless.
There are two kinds of electrical thermometers, which depend on different effects of heat on the electrical properties of metals: (I) The Thermocouple, or Thermopile, which depends on the production of a thermoelectric force when the junctions of different metals in an electric circuit are at different tempera tures; and (2) the Electrical Resistance Thermometer, the action of which depends on the fact that the resistance of a pure metal to the passage of an electric current increases very con siderably when the temperature is raised. The theory of the thermocouple is fully discussed in the article ELECTRICITY, as it possesses many points of interest, and has been studied by many skilful experimentalists. The electrical resistance ther mometer is of more recent origin.; but although the theory has been less fully developed, the practice of the method bids fair to surpass all others in the variety and accuracy of its applica tions. In order to secure the widest possible range and the greatest constancy, in either variety of electrical thermometer, advantage is taken of the great stability and infusibility charac teristic of the metals of the platinum group. Other metals are occasionally used in work at low temperatures with thermo couples for the sake of obtaining a larger electromotive force, but the substitution is attended with loss of constancy and uncertainty of reduction, unless the range is greatly restricted.
depth of the plane of junction can be measured without difficulty to the thousandth of an inch. The insertion of the wire makes the least possible disturbance of the continuity of the metal. There is no lag, as the thermometer itself is part of the metal. The instantaneous value of the temperature at any particular point of the stroke can be measured separately by setting a periodic contact to close the circuit of the galvanometer at the desired point. A further advantage is gained by measuring only the difference of temperature between two junctions of a thermo couple at different depths, instead of the whole interval from some fixed point. None of these advantages could be secured by the use of any ordinary thermometer; some depend on the fact that the method is electrical, but some are peculiar to the thermo couple, and could not be otherwise attained.
On the other hand, the thermocouple is not well suited for thermometry of precision on account of the smallness of the electromotive force, which is of the order of ten microvolts only per degree for the most constant couples. By the use of very delicate galvanometers it is possible to read the hundredth or even in special cases to the thousandth of a degree on this small difference, but unfortunately it is not possible to eliminate acci dental thermal effects in other parts of the circuit due to small differences of temperature and material. These accidental effects seldom amount to less than one or two microvolts even in the best work, and limit the accuracy attainable in temperature measurement to about the tenth of a degree with a single platinum thermocouple. This limit can be surpassed by using couples of greater thermoelectric power and less permanence, or by using a pile or series of couples, but in either case it is doubtful whether the advantage gained in power is not balanced by loss of sim plicity and constancy. A method of avoiding these effects, which the writer has found to be of great use in delicate thermoelectric researches, is to make the whole circuit, including all the terminals and even the slide-wire itself, of pure copper. Platinoid, german silver, constantan and other alloys most commonly used for resistance and slide-wires, are particularly to be avoided, on account of their great thermoelectric power when connected to copper. Manganin and platinum-silver are the least objectionable, but the improvement effected by substituting copper is very marked. It is clear that this objection to the use of the couple' does not apply so strongly to high temperatures, because the electromotive force of the couple itself is greater, and the accuracy attainable is limited by other considerations.