54 THE SCIENTIFIC PAPERS OF 



Recent experiments hereafter given prove that the specific 

 inductive capacity of insulating materials is more to be relied upon 

 for permanency than their specific conductivity ; the inductive 

 capacity is, moreover, independent of local defects in the insula- 

 ting covering, being dependent chiefly upon the general geometrical 

 form of the insulator. In ascertaining, therefore, the inductive 

 capacity of a length of cable, as compared with a standard Leyden 

 jar, and in comparing this result with the total capacity due to the 

 material employed, a means is obtained of ascertaining with great 

 certainty whether the material is disposed throughout its length 

 in equal thickness round the conductor, or whether the wire lies 

 partly eccentric. A knowledge of the inductive capacity of a 

 cable is, moreover, absolutely necessary, in order to determine the 

 position of a break in the conductor when the broken end remains 

 insulated. 



According to Faraday's conception, the inductive action is 

 communicated, say from the interior electrified covering of a 

 Leyden jar to the exterior, from atom to atom, through the 

 dialectric. In our case the jar is represented by the cable, the 

 inner covering of which is formed by the surface of the copper 

 wire, the exterior by the water. 



The laws which apply to the motion of heat and electricity in 

 conductors are accordingly directly applicable to electro-induction, 

 which may be expressed by the conductivity multiplied by a 

 constant varying with the nature of the insulating material. 



Starting from this point of view the inductive capacity of any 

 insulated wire will be represented by the formula 



in which the inductive capacity / takes the place of the specific 

 conductivity X of the previous formula. The unit measure of in- 

 ductive capacity is assumed to be the capacity of a Leyden jar of 

 two square plates of the unit of measure in area, and placed at 

 unit distance apart. 



Professor W. Thomson has obtained the same formula in a 

 direct and most elegant manner, which differed from that of 



