564 



RATIO OF UNITS. 



have a difference of potential E ; and let 3 be the deflection, cor- 

 rected for the graduation, which would be produced in the same 

 galvanometer by the current due to an electromotive force E in a 

 resistance R ; the capacity C is expressed by 



(i) 



If c is the value of the same capacity in electrostatic units, it follows 

 that 



2 c v 8 



= C = ' R Ta' 



The advantage of the method is that the ratio a only depends on 

 the square root of a resistance R, so that the relative error in this 

 latter quantity only affects the value of a by less than one-half. 



1135. In the experiments of Professors Ayrton and Perry,* the 

 condenser was square, and the area of the plate A comprised within 



the guard-ring was 1325-14 square centimetres. The distance of the 

 two plates was 0-7728 cm. 



The pile P, composed of 382 Daniell's elements, was closed by a 

 resistance AB (Fig. 238) of about 10,000 ohms. The difference of 

 potential of the ends of this resistance was used to charge the con- 

 denser. With this view, the key K was pressed down, and the 

 insulated fork PQ, movable about the point O, was turned so as to 

 connect the branch P with the rod F, which supports the disc M, 



* AYRTON and PERRY. Journal of Soc. Tel. Engineers, Vol. vin., p. 126. 1879. 



