TRANSACTIONS OF SECTION A. 285 



the labour of comparing resistance coils, condensers, absolute electrometers, &c, 

 against the standards to be lodged by the British Association for the purpose. 



Value of the condenser from comparison with four con- "I .noo _.* 

 densers lent by Messrs. Clark and Forde 1869 .J 



Value of condenser determined by Dr. Muirhead, January ~1 „„, » 

 1870, from throw of galvanometer needle . . . ./' 



Values obtained by fall of potential through small resistances — 10,000 — ■ 

 vary according to time ; for short time, - 001 sec, the condenser comes 

 out -3305 ; for much longer time, -1 sec, its capacity comes out -335. 



9. Note on the Capacity of a certain Condenser, and on the value of V. 



By C. Hockin, M.A. 



The observations given in this paper were first begun with the object of re- 

 determining the value of the capacity of certain condensers employed in the practical 

 testing of cables, and in terms of which the capacity of many cables now submerged 

 have been recorded and published. 



Dr. Muirhead and Professor Ayrton stated to me that they proposed to draw 

 the attention of the British Association to the desirability of recognising some one 

 condenser or condensers as a provisional standard. 



Fully concurring in their views on this point, I have determined in various ways 

 the capacity of a condenser made by Dr. Muirhead several years ago, and the 

 particulars of which he has given in his paper. 



The agreement of the value of the condenser now with the value it had when 

 first made is satisfactory, as showing the permanence of a well made condenser. 



The order of the experiments made was as follows : — 



1. A condenser was built up of silvered glass plates insulated from each other 

 by three small fragments of shellac. 



2. The capacity of the glass condenser was determined by the ' ballistic ' method, 

 the deflection of a Thompson's galvanometer needle being observed. 



3. The glass or air condenser was compared with three other condensers by the 

 null method, that is to say, the glass condenser was charged to a definite positive, 

 say, potential, and one of the other condensers to some lower negative potential 

 such that when the two condensers were connected the potential of each fell to zero 

 immediately after the connection. 



4. The capacities of the three condensers last mentioned were determined by 

 the throw of the needle of a Thompson's galvanometer moving freely, and having a 

 period of oscillation which was varied from 2'9 seconds to 25 seconds. 



5. The rate at which the condensers lost their charge when the opposite plates 

 were connected by a known large resistance was determined. 



6. A correction in one case has been applied for absorption determined by 

 observing the rate at which the potential of the condenser varied when after 

 discharging through a known resistance for given times, the circuit of the high 

 resistance was suddenly broken. 



The glass condenser was thus made. 



A hundred circles of the best flat plate glass were obtained and silvered on both 

 sides by the chemical process. They were supplied by Messrs. Farmiloe & Sons. 



These were carefully examined, and any spots observed not covered by the 

 silver were covered with gold leaf secured by a little very weak gum or by a trace 

 of lard. 



Connection was made with the surfaces of the glass plates by soldering a 

 thin copper wire to them with an alloy of cadmium and bismuth melting at a 

 very low temperature. 



Fifty plates had a diameter of 127 mm. and 50 were of considerably greater 

 diameter. 



The plates were built up thus. 



A plate of shellac was made with flat sides by pouring melted shellac on a 



