70 



Dr. Waghorn's Modification of the Ordinary 



the final result being that in ordinary cases the capacity of 

 the condenser may work out one, two, or even four per cent, 

 less than its true value. 



The first difficulty, the use of very high resistances, can be 

 avoided, as suggested by Prof. Ayrton, by reducing the E.M.F. 

 of the battery in a known ratio when obtaining the steady 

 deflexion ; but the second seems more troublesome to avoid, 

 unless we are supplied generally with Mr. Vernon Boys's 

 quartz fibres ; but the difficulty may be turned if, instead of 

 reading the permanent deflexion of the galvanometer, the 

 throw on completing the circuit is read. This throw would, of 

 course, if there were no damping, be double the permanent 

 deflexion, and the damping need not be allowed for, since it is 

 (at least very approximately) the same in the two observations 

 made ; that is, in the throw due to charging the condenser and 

 also in the first elongation of the needle due to the alteration 

 in the permanent field. 



The requisite apparatus becomes then very simple, and such 

 as is always likely to be at hand whenever" such observations 

 are required, and involves merely resistance-coils of, say, from 

 1 to 10,000 ohms with wandering leads, a sensitive galvano- 

 meter, and battery. A PohFs commutator or rocking key is 

 very convenient, but of course not indispensable, for com- 

 paring in rapid succession the two throws which form the 

 actual experiment. 



The ordinary form of this commutator slightly modified, as 

 shown, is very convenient in all the numerous electrical ob- 

 servations which require a comparison between any two 



