Contact Electricity of Metals. 85 



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 completely 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 dis- 

 covery. It seems to have been made by him about the year 

 1801 ; at all events he exhibited his experiments proving it 

 in that year to a Commission of the French Institute 

 (Academy of Sciences). It is quite marvellous that the 

 fundamental experiment (§ 1 above), simple, easy, and sure 

 as it is*, is not generally shown in courses of lectures on elec- 

 tricity to students, and has not been even mentioned or 

 referred to in any English text-book later than 1845, or at 

 all events not in any one of a large number in which I have 

 looked for it, except in the 'Elementary Treatise on Elec- 

 tricity and Magnetism/ founded on Joubert's i Traite Ele- 

 mentaire d J JSlectricite,' by Foster and Atkinson, 1896 

 (p. 136). The only other places in which I have seen it 

 described in the English language are Rogers article in the 

 1 Encyclopaedia Metropolitana ' referred to above ; Tait's 

 1 Recent Advances in Physical Science,' 1876 ; and Professor 

 Oliver Lodge's most valuable, interesting, and useful account 

 of all that had been done for knowledge of contact electricity 

 from its discovery by Yolta till 1884, in his Report to the 

 British Association ol that year, ' On the Seat of the Electro- 

 motive Forces in the Voltaic Cell/ 



§ 5. The reason for this unmerited neglect of a great 

 discovery regarding properties of matter is that it was over- 

 shadowed by an earlier and greater discovery of its author, by 

 which he was led to the invention of the voltaic pile and crown 

 of cups, or voltaic battery, or, as it is sometimes called, the 



* Fully and clearly described in Roget's article on 

 the ' Eucydopadia Metropolitana/ vol. iv. edition 1845, p. 210. 



