Electromagnetic to the Electrostatic Unit of Electric Quantity. 283 



accurately by loss of charge. It would be possible, by dis- 

 charging through a delicate tangent-galvanometer, or some form 

 of absolute electrodynamometer, the charge of an air-condenser 

 many times per second, to determine the electrostatic value of 

 .the current so produced ; and as the electrostatic value of the 

 current is known from the number of discharges per second, 

 from the geometrical dimensions, and the difference of poten- 

 tials to which the plates are charged, we should have a fairly 

 accurate means, but one hitherto unemployed for determining v. 

 But it appeared, for the following reasons, to Mr. Perry 

 and myself that the method best suited for the accurate deter- 

 mination of v was one that had also not been previously em- 

 ployed, and which consisted in measuring the capacity of an 

 air-condenser (1) electromagnetically by the swing of the 

 needle of a ballistic galvanometer, and (2) electrostatically by 

 the measurement of the linear dimensions of the condenser. 



A. Because the equation connecting these capacities, 



Jc=v 2 K, 

 h being the absolute electrostatic capacity, 

 K „ „ electromagnetic capacity, 



leads to an equation involving only the square root of a resist- 

 ance ; so that if any unknown error existed in our coils only, 

 the square root of that error would be introduced into the 

 answer, whereas in all the methods previously employed the 

 error in v was directly proportional to that in the coils. 



B. Because only one accurate measuring instrument (a de- 

 licate ballistic galvanometer) needed to be employed ; whereas 

 in all other methods previously used two accurate instruments, 

 such as an absolute electrometer and galvanometer, &c, were 

 necessary. 



Two difficulties, of course, presented themselves in this in- 

 vestigation — difficulties that it took us many months to over- 

 come, labouring, as we were, under the disadvantage of expe- 

 rimenting in a country like Japan. They were : — 



1. To obtain a large air-condenser of which the plates had 

 sufficiently true surfaces that the electrostatic^capacity could 

 be accurately measured — at any rate when the plates were 

 not further than half a centimetre from one another. 



2. To obtain a galvanometric arrangement of sufficient sen- 

 sibility to measure the small capacity of such an air-condenser, 

 and sufficiently ballistic that the air-damping should be almost 

 inappreciable. 



3. The Condenser. 



ABC (fig. 1) represents a vertical section of the plane upper 

 brass plate 1324*96 square centimetres in area, and DEFGr 



