84 ELECT RICI T Y [KLECTROMOTIVE FORCE of a strip of zinc immersed in a porous vessel filled with zinc sulphate, which is itself immersed in a vessel containing copper sulphate, into which dips a strip of copper. In the first instance, the copper strip is connected with the zinc plate, and the zinc strip with the copper plate of the con denser. The difference between the potentials of the con denser plates is easily found by an application of Volta s law 1 to be D + ZnjPt, where D denotes the difference between the potentials of the two pieces of copper forming the terminals of a Daniell s cell ; hence if B be the electro meter reading, after removing the Daniell and separating the plates as before, we have D + Zn|Pt---AB (2). If we connect up the Daniell the opposite way with the condenser, then we get a reading C, such that D-ZniPt-AC (3). From (2) and (3) we get Zn|Pt-J:^D (4), which gives the contact force Zn|Pt in terms of the electromotive force of a Daniell. From (1), (2), (3) we get B-C=2A, an identical relation which the observations ought to satisfy, and which, therefore, affords the means ot testing their accuracy. In this way Kohlrausch found for ZnjCu the value -48D , or in other words, that the contact force from copper to zinc is about equal to half the electromotive force of a Daniell s cell. As an instance of the general nature of the results, we give two series of numbers from the observations of Kohlrausch. The contact force is between zinc and the metal mentioned in first column in each case, and Zn|Cu is taken = 100. Cu 100 100 An 112 115 Ag 105 109 Pt 106 123 Fe 75 In the second set of experiments the metals were care fully cleaned, whereas in the first set they may have been a little oxidized. This may very well account for the differences, for Kohlrausch found oxidized zinc strongly negative 2 to freshly cleaned zinc. In fact, he found 7,n !ZnO = about -4Zn!Cu. In order to test Volta s law, a further series of observations was made, giving the contact force between iron and several metals. The following table gives the results observed directly and calculated from the table last given : Observed. Calculated. Cu . ... 31-9 25-3 Pt 32-3 32-3 An 397 38-0 As . 29-8 30-9 i It will be seen that, with the exception of the values for FelCu, the agreement is very fair. Oiiiind It is not necessary to give here the results of Gerland "<! and Hankel. 3 The latter made a great number of very HnukeL care t a i experiments. He showed that the results depend 1 The truth of wliiuh, therefore, is assumed. The assumption of course is justified a posteriori.
- A rnetal is said to be nejrntive to another when it assumes the
lower potential in contact, ami positive when it assumes the higher potential. a MJ>. der Ktoirf. fftchs. Grsfllichafl, 1861. 1865. on the nature of the surface of the bodies, being different when the surface is filed and when it is polished with rouge or other powder. His tables also show the gradual change effected in the contact force when the plates are exposed to the action of the air. According to Volta, the contact forces between metals ^ l" and liquids are either very small, and do not follow the same ail( | law as the contact forces between metals, or else are abso lutely non-existent. Subsequent observers, however, de monstrated the existence of contact forces in this case also, but showed that they do not obey Volta s law like the con tact forces in the case of metals. Becquerel 4 placed the fluid to be examined in a capsule Bec<j of the metal, say copper. The capsule was placed on the rel upper plate of a condenser consisting of two copper plates in connection with a gold-leaf electroscope. The fluid and the lower plate of the condenser were touched each with a finger for a short time, and then the upper [date was removed. The divergence of the gold leaves was taken to indicate the contact force. In this way Becquerel found that zin_, copper, and platinum were mostly negative to alkaline solutions ; but the metals were in general positive to concentrated sulphuric acid. It is obvious, however, that the result of the experiment is complicated by the contact of the hand with the liquid and with the metal of the condenser. Similar objections apply to the results of Pfaff 5 and Peclet. 6 Buff 7 made the lower plate of his condenser of the metal to be examined, of zinc for example ; upon this was laid a thin glass plate on which was spread a thin layer of the liquid to be examined, or a piece of filter paper soaked with it. A zinc wire was used to bring the liquid and the lower plate of the condenser into communication ; this wire was then removed and the glass plate with the liquid lifted. The divergence of the leaves was taken to indicate the contact force between zinc and the liquid. Although this method is an improvement on the methods of Becquerel and Peclet, it is still unsatisfactory, owing to the presence of the glass. The most extensive and at the same time most careful Har experiments at present on record are those of Hankel. 8 met The fluid (L) to be examined was placed in a wide- mouthed funnel. The condenser was formed by the surface of the liquid and a copper plate, which could be placed parallel to it at a very short distance apart, and raised as usual. The stem of the funnel was bent at a right angle twice, and ended in a wider portion, into which dipped a strip of the metal (M) to be examined, M was connected for a moment by a platinum wire with the copper plate and also with the earth. The wire was then removed, the plate was raised, and its potential determined by means of Hankel s dry pile electroscope. The reading (A) is pro portional to Cu|Pfc + PtjM + M L, or, by A olta s law, to CuM + ML. Hence In the next place, the funnel is emptied and a plate of the metal M placed on its mouth. The copper plate is lowered so that it is at the same distance as before, contact estab lished by means of the platinum wire, and so on. The reading being B, we have Cu|M=xB. The plate of M is replaced by a plate of zinc, and the experiment repeated, and we have, C being tie third read ing, 4 Ann. de, Chim. et de Phys., 1824. l Pogg. Ann.. 1840. 6 Ann. de Chim. et de Phys., 1841. 7 Ami. der Chcm. u. Pkarm., 1842.
8 Abk. der. Ktmigl S&chs. GeseUschafl, 1805