288 Sir W. Thomson on Mr. Varley's Reciprocal Electrophorus. 



metallic pieces that when a sufficient intensity of charge has been 

 reached, sparks pass across the air-intervals. Hence to give a 

 commencement of action to Mr. Varley's instrument, one of the 

 inductors must be charged from an independent source to a con- 

 siderable potential (that of several thousand cells for instance), 

 to make sure that sparks will pass between the carriers and the 

 metal piece (corresponding to one of my connecting springs) which 

 it passes under the influence of that inductor. In my " Reple- 

 nishes" however well discharged it may be to begin with, electri- 

 fication enough is reached after a few seconds (on the compound- 

 interest principle, with an infinitesimal capital to begin with) to 

 produce sparks and flashes in various parts of my instrument. In 

 Mr. Varley's instrument, what corresponds to my connector is de- 

 scribed as being connected with the ground ; and the effect is to 

 produce positive and negative electrification of the two inductors. 

 In this respect it agrees with the self-acting apparatus for multi- 

 ing and maintaining electric charges, described in a commu- 

 nication to the Royal Society last May"*. From this arrange- 

 ment I passed to the " Replenisher " by using a wheel with car- 

 riers as a substitute for the water-droppers, and arranging that 

 the connectors might be insulated and one of the inductors con- 

 nected with the earth, which, of course, may be done in Mr. 

 Varley's instrument, and which renders it identical with mine, 

 with the exception of the difference of spring-contacts instead 

 of sparks. This difference is essential for some of the applica- 

 tions of the " Replenisher," which I described, and have found 

 very useful, especially the small internal replenisher, for reple- 

 nishing, when needed, the charges of the Leyden jar of my hete- 

 rostatic electrometers. But the reciprocal-electrophorus prin- 

 ciple, which seemed to me a novelty in the communication to the 

 Royal Society and in the Philosophical Magazine article of last 

 January referred to, had, as I now find, been invented and pub- 

 lished by Mr. Varley long before, in his patent of 1860, when it 

 was, I believe, really new to science. 



Postscript. 



Glasgow College, March 20, 1868. — In looking further into 

 Mr. Varley's patent, I find that he describes an arrangement for 

 making spring-contacts instead of the narrow air-spaces for 

 sparks, — and that he uses the spring-contacts to enable him to 

 commence with a very small difference of potentials, and to 

 magnify on the compound-interest principle. He even states 

 that he can commence with such a difference of potentials as can 



* Proceedings of the Royal Societv, 1867 ; or Phil. Mag. November 

 1867. 



