ELECTROMOTIVE FORCE.] E L E T R I T T Y 95) n- of absolute measurements of the Peltier effect with Tait s isonof theory ; but, unfortunately, no data that we know of are d available for the purpose. It is absolutely necessary for usure- this purpose to have heat measurements and determina nts, tions of the lines of the metals in the same specimens. The data of Edlund 1 and Le Iloux are quite useless for such a purpose. One result of Le Roux s is, however, interesting. He finds for the amount of heat developed at the junction BiCu, the values 3 09 and 3 - 95 at 25 C. and 100 C. respectively. Since the neutral temperature of BiCu is very high, the Peltier effect ought, according to Tait s theory, to vary as the absolute temperature. The absolute temperatures corresponding to 25 C and 100 C, are 298 and 373, and we have 3 95 -=-3 09 = 1-278, while 373 -=-298= T252; the agreement between these numbers bears out the theory so far.- of General Considerations regarding the Seat of Electromo tive Force. Before proceeding to notice the remaining cases of the origin of electromotive force, in which the phenomena are more complicated, and the experimental conditions less understood, it may be well to call attention to a principle that appears to hold in most of the cases already examined. In most of these cases the seat of the electromotive force appears to be at the places where energy is either taken in or given out in the circuit. 3 It is very natural to ask ourselves what the consequences would be if we applied this principle to the voltaic circuit. It would probably be admitted by most that the energy in the voltaic circuit is taken in mainly at the surface of the electropositive metal. This admission, taken in conjunc tion with the general principle above stated, leads us to the conclusion that the electromotive force resides mainly at the surface of the electropositive metal. The absorption or evolution of energy at the junction of the dissimilar metals is quite insignificant, and we should, on the same view, deny that any considerable part of the electromotive force resides there. This view appears to be at variance with the theory of metallic contact, as now held by Sir William Thomson and others; and the burden of explaining the experiments made by him and others on the contact force of Volta is doubt less thrown on those who adopt this view. The position of such would very likely be that there is an uneliminated Bourcc of uncertainty in all these experiments 4 (see above, p. 85). On the other hand, those who adopt the con tact force of Volta at the junction of copper and zinc as the main part of the electromotive force of DanielPs element are under the necessity of distinguishing this from the electromotive force corresponding to the Peltier effect, which must be a distinct effect, since it is but a very small fraction of that of a Daniell s cell. We are, however, so very ignorant of the nature of the motion which is the essence of the electric current that the very form in which we have put the question may be misleading. If this motion be in the surrounding medium, as there is great reason to believe it to be, it would not be surprising to find that speculations as to the exact locality of the electromotive force in the circuit were utterly wide of the mark. The very language which we use implies a certain mode of analysing the problem which may be altogether wrong. The only thing of which we can as yet be sure is that the mathematical equations deduced 3 Wied. Galv., Bd. i. 694. 8 Since the above was written further experimental evidence in sup port of the theory has appeared. See Naccari and Bellati, Atli del R. Tst. Veneto di Sc. Litt. cd Arti, November 1877. 3 Maxwell, vol. i. 249. By "being taken in," in the case of heat for instance, is meant "disappearing as heat and appearing as electro- tinetic energy." In a thermoelectric circuit this transformation occurs wherever there is Peltier or Thomson effect.
- Maxwell, I.e.
from Ohm s law and other proximate principles are in exact accordance with experiment. Pyroclcctricity. Some account of this interesting subject has Pyn>- already been given in the Historical Sketch at the beginning of this elec- article. It will be well, however, to state here some of the conclu- trieity. sions of those who have recently investigated the matter. It seems now to be settled that it is not merely high or low temperature, but change </ temperature, which gives rise to the electrical phenomena of pyroclectric crystals. The properties exhibited by tourmaline may be described thus. One end A of the crystal is distinguishable from the other end B by the dissymmetry of the crystalline form. A is called the analogous pole of the crystal, and B the antilogous pole. When the temperature of the crystal is increasing uniformly throughout, the analagous pole is positively electrified and the antilogous pole negatively electrified. When the temperature is decreasing uniformly throughout, the analogous pole is negative and the antilogous pole positive. This law was originally dis covered by Canton, 5 but it seems to have been lost sight of again Canton, and rediscovered both by Bergman and by Wilcke in 1766. When the Wilcke. temperature is uniform, the positive and negative regions are sym- Berg- metrically distributed about the central zone of the crystal, which man. is neutral. If the ends be unequally heated, this symmetry no longer obtains. It must not be forgotten that complications may arise from the crystal becoming electrical as a whole by friction, usually positive, like most other vitreous bodies. Gaugain 6 made a series of interesting experiments on the elec- Gaugain. trical properties of tourmaline, and concluded that a tourmaline whose temperature is varying may be compared to a voltaic battery of great internal resistance, consisting of an infinite number of cells, each of infinitely small electromotive force ; so that the electro motive force is proportional to the length of the tourmaline, and its internal resistances is proportional to the section inversely and to the length directly. He also concluded that the amount of elec tricity furnished by a tourmaline, while its temperature varies either way between two given temperatures, is always the same. In order to explain the properties of the tourmaline, it has been Thorn. supposed 7 that the crystal is naturally in a state of electrical pola- son s rization, like that assumed by Maxwell in a medium" under the theory influence of electromotive force, or more nearly (since no sustaining force having an external origin is supposed) like that of a permanent magnet. The intensity of this polarization is supposed to be a function of the temperature. Supposing the tourmaline to remain for some time at the same temperature, a surface layer of electricity would be formed, which would completely mask the electrical polarization of the crystal, inasmuch as it would destroy all external electrical action. This neutralization would be instantly effected by running the crystal through the flame of a lamp. If, however, the temperature increase, then the polarization will, let us say, increase, so that the surface electrification no longer balances it. We shall thus get polar electrical properties of a certain kind. If the temperature decrease, the polarization will decrease, and we shall thus get polar properties of the opposite kind. In many pyroelectric crystals there are more than one electric axis, so that we have several analogous and corresponding antilo gous poles. An enumeration of the various crystals in which pyroelectric properties have been found, and a discussion of the peculiarities in their crystalline form, belongs more properly to the science of Mineralogy. Much has been done in this department by Kohler, 8 GustavRose and Kiess, !) and Hankel. 10 For some very in teresting researches by Friedel see Annalcs de Chimie ct de Physique, 1869. Frictional Electricity. In accordance with the general principle Contact laid down at the beginning of this section, we should expect to find of non- an electromotive force at the surface which separates two different condue- non-conductiug media, just as we have found it at the boundary of tors, two different conducting media. The effect of such a contact force would be very different however in the former of these cases, from what we have seen it to be in the latter. In the case of non-con ductors the electricity cannot leave the surface of separation, but will simply accumulate on the two sides of it, till the force arising from electrical separation is equal to the contact force. On separ ating the bodies, in certain cases, we may carry away with us these surface layers of electricity, and it is an obvious consequence of oiu- principles that the electrifications of parts of the two bodies that have been in contact must be equal and opposite. While the bodies are in contact the difference of potential between the layers of electricity corresponding to very considerable surface density may be very small, just as in Yolta s condensing electroscope (see above, p. 34) ; but when we separate the bodies work is done against the electrical attractions, and the potential increases enormously. 6 Phil. Trans., 1759. 6 Mascart, t. ii. 7 Thomson, Phil. Mag., 1878, p. 26; or Nichol s Cyclopaedia of the Physical Sciences, 1860. 8 Pogg. Ann., xvii., 1829. 9 Abh. dcr Bcrl. Abaci., 1836 and 1843.
10 Pogg. Ann., xlix., 1., Ivi., 1840-2; alsocxxxi., cxxxii., 1867, &e.