J. H. Pratt — Capillary Electrometer. 149 



then passed through the electrometer and the deflection noted. 

 By reference to the E. M. F. curve for the electrometer the 

 potential (Y 7 ) for the given deflection was found. The electric- 

 ity being divided between the two instruments the proportion 

 will hold Y : V::C+C : C, where Y equals the potential of 

 the condenser, Y' equals the potential of the electrometer, and 

 C and C the capacities of each. By division, Y— Y f : Y':: C 



Y— V' 

 : C, or C' = v/ C. As Y ; is found to be small compared 



Y 

 with Y, the equation C——C will be approximately true. This 



method has some disadvantages which make it impracticable 

 for accurate results. The discharge from the condenser being 

 practically instantaneous, its full force is exerted at once on 

 the mercury, the inertia of which is apt to carry it beyond the 

 position to which an equal amount of electricity, in the ordinary 

 use of the instrument, would bring it. 



On the other hand, the tendency of the electrometer to dis- 

 charge itself acts in the opposite direction and prevents the ex- 

 act determination of the deflection. The results from this 

 method cannot be accepted as satisfactory. The only safe 

 conclusion is that the capacity of the electrometer is great com- 

 pared with that of the condenser. 



By the second method employed, the condenser and electrom- 

 eter were placed in the same circuit, and the current passed 

 through both in series. The ratio of the E. M. F. of the cur- 

 rent to the potential indicated by the deflection of the electrom- 

 eter gives the capacity of the electrometer compared with 

 that of the condenser. The self- discharge of the instrument 

 acted very disadvantageously. When the current, passing 

 through the electrometer, had charged the condenser, the latter 

 acted like an infinite resistance in the circuit, and the mercury 

 of the electrometer, instead of remaining at the point to which 

 it had been deflected, moved from it and gradually returned to 

 zero. As the readings were made with a micrometer, and 

 could not be taken immediately, this movement prevented 

 accuracy. The indication was, as in the other method, of a 

 capacity far greater than that of the condenser. 



To secure more satisfactory results a third method, without 

 condenser was resorted to. A resistance box of 250,000 ohms, 

 of which 240,000 ohms were used, was put in circuit with the 

 electrometer, and currents of known potentials were passed 

 through the two. As the passage of a definite amount of electri- 

 city through the electrometer is necessary to bring the meniscus 

 to a given position, the movement of the mercury was retarded 

 by the introduction of the large resistance. The slow motion 

 made it possible at any instant to determine the position of the 



