72 On an Electric Machine founded on Induction and Convection,. 



duced by disconnecting the two inductors from one another and 

 connecting them with the two plates of a single Daniell's cell, 

 when either the zinc or the copper lining is left in one of them. 

 These differences are very approximately in simple proportion 

 to the differences of potentials between the pairs of the opposite 

 quadrants of the electrometer in the different cases. The dif- 

 ference between the effects of zinc and of copper in this arrange- 

 ment is of course in the direction corresponding to the posi- 

 tive electrification of the quadrants connected with the spring 

 whose point of contact is exposed to the zinc-lined inducing 

 surface. It must be remembered, however, as is to be expected 

 from HankePs observations, that the difference measured will be 

 much affected by a slight degree of tarnishing by oxidation, or 

 otherwise, of the inner surface of either inductor. When the 

 copper surface is brought to a slate-colour by oxidation under 

 the influence of heat, the contact difference between it and 

 polished zinc amounts sometimes, as I found in experiments 

 made seven years ago, to 125, that of a single cell of Daniell's 

 being called 100. 



A useful application of the little instrument represented in 

 fig. 4 is for testing insulation of insulated conductors of small 

 capacity, as, for instance, short lengths (2 or 3 feet) of submarine 

 cable, when the electrometer used is such that its direct appli- 

 cation to the conductor to be tested would produce a sen- 

 sible disturbance in its charge, whether through the capacity of 

 the electrometer being too great, or from inductive effects due to 

 motion of the moveable part, or parts, especially if the electro- 

 meter is " heterostatic "*. In this application one of the inductors 

 is kept in connexion with a metal plate in the water surrounding 

 the specimen of cable to be tested ; and the other is connected 

 with the specimen, or is successively connected with the different 

 specimens under examination. The springs are connected with the 

 two electrodes of the electrometer as usual. The small constant ca- 

 pacity of the insulated inductor, and the practically perfect insula- 

 tion which may with ease be secured for the single glass or vulca- 

 nite stem bearing it, are such that the application of the testing 

 apparatus to the body to be tested produces either no sensible 

 change, or a small change which can be easily allowed for. It 

 will be seen that the small metal pegs carried away by the turn- 

 ing-wheel from the point of the insulated spring, in the arrange- 

 ment last described, correspond precisely to the drops of water 

 breaking away from the nozzle in the water-dropping collector 

 for atmospheric electricity. 



A form bearing the same relation to that represented in the 



* Lecture on Atmospheric Electricity (Proceedings of the Royal Insti- 

 tution, 1860). 



