Metliod of Determining Electromagnetic Capacity. 69 



formed by the method given in the Philosophical Magazine 

 for 1885 (vol. xx.), which fall on the coloured disk and the 

 rotating sectors, which form a grey in white light. There is 

 less difficulty in this case than in ascertaining the luminosity, 

 as the colour falling on the mixture of black and white is the 

 same as that falling on the pigment. In this case, too, it 

 may be necessary to add a fixed amount of black to the coloured 

 disk in order to get a reading. 



YIII. Note on a Modification of the Ordinary Method of 

 Determining Electromagnetic Capacity. By J. W. W. 

 Waghokn, D.Sc* 



THE object of the proposed modification of the ordinary 

 well-known method is to enable the capacity of a con- 

 denser or other conductor to be determined with sufficient 

 accuracy without the costly apparatus generally employed. 



The usual process consists, as is well known, in reading the 

 " throw,-'^ ^j, of a galvanometer-needle under the impulse due 

 to the flow of a quantity of electricity which charges the con- 

 denser to the potential of a certain battery, and in afterwards 

 reading the steady deflexion, c/g (not very different from fZj), 

 when the galvanometer is connected in series thi'ough a total 

 resistance R. 



Then the required capacity, F (in farads) = ^ — ry^ ; 



where t is the time of one complete \abration of the needle. 



But as '' damping " reduces the throw of the needle, the 

 logarithmic decrement has to be determined and allowance 

 made accordingly. 



In attempting this determination of capacity with a sensitive 

 galvanometer not specially designed for ballistic observations, 

 the difficulties occur that in the first place the resistance, R, 

 must be very great if di is not to be very different from d^ ; 

 if the capacity of the condenser is \ microfarad, for example, 

 and the time of vibration two seconds, the resistance required 

 is more than 600,000 ohms, and these large resistances are not 

 always available. Secondly, by reason of the shortness of the 

 fibre suspension and its imperfect elasticity, the needle does 

 not immediately take up its true position under the forces 

 acting upon it, an imperfection which causes the deflexion 

 due to the throw of the needle to be unfairly lessened in com- 

 parison with the steady deflexion against which it is compared; 



* Communicated by tlie Physical Society : read December 8, 1888. 



