622 Lord Kelvin [May 21, 



let it down very rapidly a hundred times, so as to produce one hun- 

 dred cycles of operation — break contact between discs, make and 

 break contact between upper disc and knob, make contact between 

 discs. Lastly, I lift the upper plate of the condenser ; you see now a 

 great divergence of the gold leaves, many of you can see it direct on 

 the leaves, while all of you can see it by their shadows on the screen. 

 Now, keeping the upper plate of the condenser still unmoved, I bring 

 a stick of rubbed sealing-wax into the neighbourhood of the electro- 

 scope ; you see the divergence of the leaves is increased, I remove 

 the sealing-wax and the divergence diminishes to what it was before. 

 This proves that the gold leaves diverge in virtue of resinous elec- 

 tricity upon them, and therefore that the insulated plate of the 

 condenser received resinous electricity from the copper disc. If now 

 I interchange the two discs so that the upper is zinc and the lower 

 copper, and repeat the experiment, you see that the rubbed sealing- 

 wax diminishes the divergence as it is brought from a distance into 

 the neighbourhood, and that a glass rod rubbed with silk increases 

 the divergence. Hence we conclude that in the separation of two 

 discs of copper and zinc the copper carries away resinous electricity 

 and the zinc vitreous electricity. 



§ 2. Experiment 2. — The same apparatus as in Experiment 1, 

 except that the polished zinc and copper discs have their opposed 

 faces varnished with shellac, and are provided with wires soldered to 

 them for making metallic connection between them when the upper 

 rests on the lower, as shown in Fig. 2. All operations are the same 

 as in Experiment 1, but now with this addition — when the upper 

 disc rests on the lower, make and break metallic contact by hand as 

 shown in the diagram. The results are the same as those of Experi- 

 ment 1, except that the quantity of electrification given to the gold 

 leaves by a single cycle of operations is generally greater than in 

 Experiment 1, for this reason : In Experiment 1 at the instant of 

 breaking contact between the zinc and copper there is generally some 

 degree of inclination between the two discs, while at the corresponding 

 instant of Experiment 2 they are parallel and only separated by the 

 insulating coats of varnish. If great care is taken to keep the 

 discs as nearly as possible parallel at the instant of separation, the 

 effect of a single separation may be made greater in Experiment 1 

 than in Experiment 2 (see § 3 below). 



§ 3. An instructive variation of Experiment 1 may be made by 

 giving a large inclination, 5°, or 10°, or 20°, of the upper plate to 

 the lower, while still in contact and at the instant of separation. By 

 operating thus the experiment may be made to fail so nearly com- 

 pletely that no divergence of the leaves will be observed even after 

 one hundred cycles. 



§ 4. These two experiments, with the variation described in § 3, 

 put it beyond all doubt that Volta's electromotive force of contact 

 between two dissimilar metals is a true discovery. It seems to have 

 been made by him about the year 1801 ; at all events he exhibited 



