WITH ELECTEOMAGNETIC FORCE. 127 



comparison with the capacity of a sphere of known radius, and electromag- 

 netically by passing the discharge from the condenser through a galvanometer. 



The Electrical Committee of the British Association have turned their 

 attention to the means of obtaining an accurate measurement of this velocity, 

 and for this purpose have devised new forms of condensers and contact-breakers ; 

 and Sir William Thomson has obtained numerical values of continually increasing 

 accuracy by the constant improvement of his own methods. 



A velocity which is so great compared with our ordinary units of space 

 and time is probably most easily measured by steps, and by the use of several 

 different instruments ; but as it seemed probable that the tune occupied in the 

 construction and improvement of these instruments would be considerable, I 

 determined to employ a more direct method of comparing electrostatic with 

 electromagnetic effects. 



I should not, however, have been able to do this, had not Mr Gassiot, 

 with his usual liberality, placed at my disposal his magnificent battery of 2600 

 cells charged with corrosive sublimate, with the use of his laboratory to work in. 



To Mr Willoughby Smith I am indebted for the use of his resistance- 

 coils, giving a resistance of more than a million B. A. units, and to Messrs 

 Forde and Fleeming Jenkin for the use of a galvanometer and resistance-coils, 

 a bridge and a key for double contacts. 



Mr C. Hockin, who has greatly assisted me with suggestions since I first 

 devised the experiment, undertook the whole work of the comparison of the 

 currents by means of the galvanometer and shunts. He has also tested the 

 resistances, and in fact done everything except the actual observation of equi- 

 librium, which I undertook myself. 



The electrical balance itself was made for me by Mr Becker. 



The electrostatic force observed was that between two parallel disks, of 

 which one, six inches diameter, was insulated and maintained at a high 

 potential, while the other, four inches diameter, was at the same potential as 

 the case of the instrument. 



In order to insure a known quantity of electricity on the surface of this 

 disk, it was surrounded by the "guard-ring" introduced by Sir W. Thomson, 

 so that the surface of the disk when in position and that of the guard-ring 

 were in one plane, at the same potential, and separated by a very narrow 

 space. In this way the electrical action on the small disk was equal to that 

 due to a uniform distribution over its front surface, while no electrical action 



