ELKCTROMOTIVE TOKCE.] ELECTRICITY 83 the effect of this by supposing the representative magnet NS carried onwards a little with the disc, or, which amounts to the same thing, we may suppose the pole M to lag a little behind at M (lying, say, on BOT perpendicular to ZOY.) The action of N will now"preponderate, and the resultant force on M will be in the direction M F. This force, when resolved parallel to OY, OZ, OX, gives a tangential component as before, a repulsive normal com ponent, and a radial component, which will be directed to or from the centre of the disc, according as the representative magnet lies farther from or nearer to the centre of the disc than the foot of the perpendicular from M . The original explanation of rotation magnetism ( Faraday, Exp. Res., 81, &c.) should be consulted by the reader who wishes to pursue the subject. An account of the researches of Nobili, Matteucci, and others will be found in Wiedemann, IM. ii. 860, tfcc. The mathematical theory has been treated by Jochmann, who neglected the inductive action of the currents on each other (Crelle s Journ., 1864 ; Pogg. Ann., 1864; also Wiedemann, I.e.). A complete theory of the induction of currents in a plane conducting sheet has been arrived at by Maxwell by means of an extremely elegant application of the method of images (Proc. R.S., 1872 ; also Electricity and Magnetism, vol. ii. 668, 669). On the Origin of Electromotive Force. It remains for us now to view the transformations of energy which take place in the voltaic circuit from the other side, and to inquire whence comes the energy that is evolved in so many different forms by the electric current. Two distinct questions are here involved. First What form of energy is being absorbed, and at what part of the circuit does the absorption take place? Secondly Where is the electromotive force which drives the current situated 1 lser . To the first of these questions experiment has given, on ion of the whole, a very satisfactory answer. The electric circuit T oT is, indeed, one of the best instances of the great law of conservation, which states that the appearance of energy anywhere is always accompanied by the disappearance somewhere of energy to an equal amount. No general dis cussion of this first question is necessary; it will be suffi cient to indicate the application of the general principle when we deal with particular instances. Unfortunately the answers, both experimental and theoretical, that have been at different times given to our second question, are not so concordant as could be desired. The reader is, therefore, cautioned against accepting with out due examination 1 anything that may be here advanced. tact Perhaps the most general principle concerning the e - origin of electromotive force recognized by physicists of the present day is the following: When two different substances are in contact, there exists in general an electromotive force at the surface 1 of separation, tending to displace electricity across that surface. This electromotive force is commonly called the " electro motive force of contact," or simply the "contact force." In the particular case of two conductors in contact, the effect of this force would simply be to maintain a certain differ ence of potential between them. Although the earliest known case of electrification viz. amber rubbed with woollen cloth is an instance in point, and although many experiments on electrification by the friction of different substances were made, yet this prin ciple was not recognized fully till the experiments of Galvani and Volta directed the attention of men of science to the matter. ? irt Volta was the first to demonstrate clearly the existence of the contact force in the case of metals. A simplified [1 ? form of his fundamental experiment is the following. The
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B ,;, ^ This applies particularly to any indications of the views of Hying physicists. upper and lower plates of a condensing electroscope (see above, p. 34) are made of different metals, say copper and zinc. Let the upper plate be laid upon the lower, and the metallic contact ensured by connecting them for an instant by means of a wire. If the upper plate be now lifted verti cally upwards, the gold leaves of the electroscope diverge, indicating that the zinc plate is now positively electrified to a considerable potential. This is explained as being due to the contact force at the junction of the copper wire with the zinc plate, by virtue of which the zinc is at a higher potential than the copper. Suppose the upper plate to be connected with the earth, then if E be the contact force, the potential of the zinc plate is E. Now E is very small, but as soon as the upper plate is raised the potential of the lower plate is increased in the same ratio as its capacity is diminished; hence the divergence of the leaves. Yolta Law of found that he could arrange the metals in series, thus Volta. 7n
Fe ! i Pb 5 Cu ....! 2 Sn 1 Ag... ....! 3 such that, when any metal is placed in contact with one below it in the series, it takes a higher potential ; and he found that the electromotive force between any two metals in the series is the sum of the electromotive forces between every adjacent intervening pair. Thus, if Zn|Pb denote the electromotive force from lead to zinc, we get from the above table, Zn;Pb = 5,PbjSn = l, ZnjSu = ZnlPb + PbjSn = 6 , Pb|Cu = Pb;Sn + Sn; Fe + Ft>;Cu = 6 , and so on. It follows from Volta s law that, if a number of metals be connected up in series, the difference of potentials be tween the extreme metals is independent of the intermediate metals, and, in particular, is zero if the extreme metals be the same. We cannot, therefore, have a resultant electro motive force in a closed circuit consisting of metals merely. This is entirely in accordance with experiment, provided the temperature be the same everywhere. While one party of physicists neglected or attempted to explain away Volta s contact force, another took up the investigation, and endeavoured to obtain precise measure ments of it in different cases. Careful experiments of this Experi kind were made by Kohlrausch 2 and Gerland, 3 by a method ,n ents due to the former. Kohl- A condenser is used whose plates are made of the metals rausc to be tested, say zinc and platinum. The plates are first placed parallel to each other at a very small distance apart, and touched simultaneously with a wire (say of platinum). A difference of potentials is thereby established, so that if the potential of the Pt be zero that of the Zn is Zn|Pt. (Here we neglect the contact force between air and zinc and between air and platinum. No experimental proof that we know of has been given in support of this, see below, p. 85). In consequence of this difference of potentials the Zn plate becomes positively charged. The wire is now removed, the plates of the condenser separated to a considerable distance, and the Zn plate connected with one electrode of a Dellmaim a electrometer, the other elec trode of which is connected to earth. The reading is pro portional to the potential difference Zn!Pt increased in the ratio in which the capacity of the Zn plate has been decreased by the separation. Hence, if A be the reading, Zn;Pt-A (1). The condenser plates are now brought into their former position, and connected through a Daniell s cell, consisting
- Pogg. Ann., 1853.
