56 THE SCIENTIFIC PAPERS OF 



angle of readjustment of the instrument, we have (if the number 

 of discharges per second remains the same) 



K : K l = sin A : sin A 



KI sin A 



sin A l 



By permission of the British Government, we have been enabled 

 to test the Government experimental cables by this method. 



The results of these experiments show satisfactorily the accuracy 

 of the methods employed. They also prove that the formula em- 

 ployed in calculating the specific inductive capacities which 

 Professor W. Thomson and Mr. Werner Siemens obtained in 

 entirely different ways, can be relied upon in practice. 



The specific induction of all gutta-percha covered wires is shown 

 to be nearly the same and to be entirely independent of its specific 

 conductivity, while india-rubber and its compounds are far inferior 

 in specific induction to gutta-percha. The specific induction of 

 gutta-percha being taken as a unit, that of india-rubber is equal 

 to 0'7 only, and that of Wray's mixture = 0*8. 



We have still to make mention of those methods which have 

 been frequently resorted to of late of ascertaining by means of 

 sensible electrometers the decrease of tension in a heavily charged 

 cable when left to itself. 



If E represents the tension of a galvanic battery in communica- 

 tion with the cable, as observed by a sine electrometer, y the 

 remaining tension in the cable after an interval of time t, K the 

 capacity, and w the resistance of the insulator, there will be, ac- 

 cording to the law of Ohm, after the interval t a current of discharge 



= % by which the tension is decreased during the time dt by 

 w 



d y. Hence we obtain the equations : 



Kdy = lt dt 



w 



dy _ dt 

 y Kw 



/*f 1 v 



