104 



Potts — Rowland's New Method. 



ing upon the period of the current flowing through the con- 

 denser. The method chosen for the measurement of capacity 

 was one described in this Journal, December, 189T. 



Method. — This consisted in the use of a divided circuit. 

 One branch, a, contained a resistance R,„ either the fixed or 

 hanging coils of the electro-dynamometer, and a condenser 

 whose capacity c was to be studied. The other arm, b, con- 

 tained a resistance R & , a coil with which the capacity c is com- 



pared, and either the hanging or fixed coils of the electro- 

 dynamometer. Let L be the coefiicient of self-induction of 

 the coil plus that of the coils of the electro-dynamometer in 

 that arm, and let I be the self-induction of the coils of the 

 electro-dynamometer in the arm b. Let an electromotive force 



E = E/ m ) 



be applied to the terminals de. Now representing the maxi- 

 mum values of the current by C a and C & and the phases by 4> a 

 and cf) b we have for the branch a 



and for branch b 



qJ{U+^) = E e (-R a + ibl- -j- 

 C i e^ w+ * 6 ) = E.(R, + <&L) 



If we adjust the resistances until there is no deflection of the 

 electro-dynamometer, we shall have the difference of phase 

 ((j) b — (f) a ) = 90° or cos ((ji b — (j) a ) = 0. Further, 



9l e *(**-*«) = 



R 6 + ^L 



K a + iM- 



(12) 



be 



Hence since cos (<f> b — </>«) = we must have the real part of this 

 equal to zero, or 



( 1 - Pel) (b*cL) = &VR a R& 

 or 



= R.Rv 



