Method of Comparing Inductance with Capacity. 43 



placed the inductive coil L, and a small cloth rheostat, r7i, 

 the latter enabling- one to maintain a steady balance in case 

 of fluctuations in the temperature of L. The remaining arm 

 of the balance, R 1? consists of two resistance-boxes between 

 which is stretched a wire of known resistance per unit length. 

 The condenser — in this case a Muirhead divided microfarad 

 — is between the junction of the ratio arms and a sliding 

 contact on the stretched wire. 



A Cohen vibrating wire interrupter was used in con- 

 junction with a Campbell vibration galvanometer tuned to 

 resonance; the range of the galvanometer permitted the use 

 of only comparatively low frequencies. As the inductance 

 was known to be comparatively small, moderate values were 

 selected for the resistances in order to increase the sensitivity 

 of the arrangement. The resistance of the coil was first 

 roughly determined, and the arrangement was then set for a 

 "steady " balance, i\ and r 2 being made equal. Alternating 

 current was then turned on, and i\ and r 2 were altered 

 (keeping their total constant) until the deflexion became 

 very small ; as the point of balance was approached, a little 

 successive adjustment of the rheostat, rh t and the sliding 

 contact enabled one to obtain a very sharp zero. The in- 

 terrupter was first placed across AC and the galvanometer 

 across BD, and then these were interchanged. The method 

 worked satisfactorily in every way, and gave beautifully 

 sharp and well-defined balancing points. (A coil interrupter 

 and telephone were also tried ; they worked quite well, too, 

 but they were not so sensitive as the galvanometer.) The 

 results of a few typical measurements are here given. 



Test coil : short solenoid. 



Inductance (approximate calculation) 0*75 mh. 



„ (measured by Anderson's method) . 0*764 „ 



New method : — 



(i.) Interrupter BD; Galvanometer AC. 

 C = 0'2 microfarad ; R = 100 ohms. 

 r 1 = 21 , 3 ohms. r 2 = 78'7 ohms. 



.-. ^ = 100 2 -78'7 2 = 3-807xl0 3 . 

 ,\ L = 0-761 millihenry. 



