412 



Dr. J. E. Ives on the Law of 



later, and recorded in the same paper, make it untenable at 

 least for moderate speeds, since the speed of breaking, 70 cms. 

 per second or less, was found to have very little effect upon 

 the electromotive force induced in the secondary. 



An attempt has therefore been made to find by experiment 

 the relation between the optimum capacity and the other 

 constants of the primary circuit, viz., the current, the re- 

 sistance, and the inductance. 



For this purpose a primary coil was used without a secondary, 

 as it is well known that the action of the primary is unaffected 

 by the presence of an open secondary. The break consisted 

 of an amalgamated copper wire, one millimetre in diameter, 

 dipping into a cup of mercury. The surface of the mercury 

 was covered by two or three centimetres of distilled water. 

 The breaking was done by hand . In making an experiment 

 the capacity of the condenser, around the break, was gradually 

 increased, and the value noted at which the sparking suddenly 

 disappeared or became very small. The batteries used were 

 large lead chloride cells. 



A primary circuit may be regarded as made up of the three 

 branches shown in the accompanying diagram. 



The first branch, containing the primary coil and battery, 

 and having a resistance E^, an 

 inductance L, and an electro- 

 motive force E ; the second con- 

 taining the break, and having a 

 resistance Es, divided between 



the two sides ; and the third 



containing the condenser, and 



having a capacity K, and a 



resistance Ec. By suitable 



manipulation any one of these 



quantities can be varied while 



the others are kept constant. 

 This was done, and although 



I have not yet been able to 



derive a general law con- 

 necting the optimum capacity 



with these different variables, 



certain well-marked and some- 

 what remarkable results have 



been obtained. 



1. It was Pound that for the same current the optimum 



capacity is much greater when the breaking pole is negative 



than when it is positive. See Table I. and PL XYII. 



fig. I. The change of sign was obtained by reversing the 



cell connexions. 



vvvvvv 



f^3 



K 













