THERMO-ELECTRICITY. 215 THERMOMETER. is zero is called the 'neutral" teniiicrature, and is a constant for any two boilies. These thernio-electrit- currents are due to molec- ular actions at the junctions and also through- out the conductins; wires, owing to their non- uniformity of temperature. Where the two dif- ferent metals join there are electric forces called the 'Peltier electromotive forces;' while the electric forces through the conductors themselves are called 'Thomson electro-motive forces.' The existence of these electro-motive forces is proved by forcing a current around a circuit by means of a battery or cell: at the junctions there is rise or fall of temperature, depending upon the direction of the current, showing a force oppos- ing or helping on the current; similarly the conductor itself has its temperature raised or lowered, depending upon the direction of the cur- rent — quite apart from the usual heating effect of a current. The properties of thermo-electric currents are best studied by diagrams. Some one metal is 1 chosen as a standard; lead is generally selected because it has no 'Thomson effect.' A circuit is made part of lead and part of another metal; one junction is kept at 0° C, and while the tem- perature of the other is varied continuously, both above and below zero the total electro- motive force around the circviit is measured. These values of the E.M.F. for different values of the temperature of the variable junction are plotted in a curve, having E.M.F. for ordinates and temperatures for abscissse. These curves when drawn for different bodies — all with refer- ence to lead — are found to be approximately parabolas. Another form of diagram is made as follows: The junctions of the two metals, one of which is lead, are kept at temjicratures T and T + AT, where AT is a small quantity; the resulting E.M.F. around the circuit is measured, AE call it AE. The ratio ^ is called the 'thermo-electric power:' it is evidently a func- tion of the temperature T, and when plotted on a diagram having temperatures as abscissie, the curves are practically straight lines. A thermo-couple is one of the most sensitive instruments known for detecting differences in temperature. It can be improved by joining in a zigzag series several couples : first, a piece of bismuth, then one of antimony, then one of bis- muth, etc., the first end of the first piece of bismuth and the last end of the last piece of antimony being joined by a wire passing around a galvanometer. Thus every other junction of bismuth and antimony is turned to form one face of the zigzag; and. if one face is at a higher tem- perature than the other, each couple helps the others, and thus a considerable current may be produced through the galvanometer. Such a combination of couples forms a 'thermopile.' It was invented by Xobili in 1834. THERMOMETER (from Gk. eipti-n, therme, heat -|- liirpov, nirtron, measure). An instru- ment for measuring temperature, but in early times erroneously supposed to measure heat. A brief account of the underlying theory and the history of thermometry are given under the title Thermometry. This article will deal only with those forms of thermometers that are at present in ordinary use. The Air or Oas Thermometeb. This form of the instrument is accepted as the standard and the indications of mercurial, alcohol, or other thermometers are all supposed to be corrected so as to agree with the standard adopted by the International Bureau of Weights and Measure.? at Paris. This standard consists of a hollow bulb of platinum filled with pure, dry nitrogen. When kept at a uniform pressure the volume of the gas expands in direct proportion to its teni- peralure. By calling the volume at the melting ])oint of ice V, and the volume at the boiling ])oint of water V -f 100°, we establish a scale of centigrade degrees, which may lie extended above or below these limits, so that, for instance, the temperature of the absolute cold or entire absence of heat would be — 27.'? C. and the temperature of the boiling point of mercury would be about -|-.357° C. Numerous precautions are needed in the use of the air ther- mometer, all of which are explained in the Travaux et memoires of the In- ternational Bureau of Weights and Measures, and in Guillaume, Ther- momeirie (Paris, 1890). According to the theories of thermodynamics the peculiarities of platinum, nitrogen, or of any other materials that may be used, introduce small ir- regularities, so that the scale of equal changes of volume is not exactly proportional to the quan- tity of heat or the change in temperature. There- fore some recommend that a slight correction be ap- plied to the nitrogen gas thermometer in order to obtain the ideal thermo- dynamic equivalent. In Fig. 1 we see the simplest or portable form of air thermometer, as arranged by Joly, for measuring temperatures below the boiling point of water. The large glass bulb on the left is filled with dry air, or dry nitrogen, and ends in a short vertical glass tube attached to the flexible rubber tube whose other end opens into another short glass tube, shown on the right-hand side, whose upper end is hermetically closed and constitutes the vacuum chamber of ii barometric column of mercury. The glass bulb is immer.sed in a liquid or gas whose tem- perature is to be measured. If the volume of the inclosed gas expands with the higher tem- perature that is to be measured, then this ex- pansion is counterbalanced by raising the vacuum chamber and the flexible tube on the right until the increased pressure of the mer- cury column compresses the expanding gas in the large bulb so as to keep it at a constant volume, which is indicated by the fact that the mercury in the left-hand tube just touches the tip of a fine glass point that is permanently fastened inside the tube. The pressure exerted by the Fio. 1. JOLY AIR THERMOMETEB-
