WILLIAM SIEMENS, F.R.S. 39 



gives us a direct comparison between insulators, although it does 

 not give us their resistance in units. Therefore, this method only 

 serves to determine the general quality of an insulating material, 

 but is inapplicable for measuring submarine cables. 



CHARGE IN CABLES. We have now gone through the different 

 methods of testing, and of determining resistances. We come to 

 another branch of the subject that of determining the charge or 

 lateral induction. If a current flows into an insulated wire, the 

 current does not pass through it without electrifying the insulating 

 substance surrounding it. Part of the current leaks through the 

 insulating material ; but a larger amount of it is found there 

 remaining, as it is in a Leyden jar, and forms what is called static 

 electricity. It is evident that before a current can pass through 

 the cable from one end to another, it must charge the whole length 

 of its insulating covering ; in fact, the gutta-percha surrounding 

 this wire acts the same as the glass of a Leyden jar. We may 

 consider a long submarine cable like an infinite succession of Ley- 

 den jars, which have to be charged one after the other before the 

 electricity can flow out and produce its effect at the extreme end. 

 It is evident that this retardation or this absorption of current 

 should be made as small as possible, in order that each current 

 may as soon as possible arrive at the other end ; but we find that 

 different substances absorb different amounts of electricity in 

 charge. Thus india-rubber absorbs only three-fourths of what 

 gutta-percha would absorb, and is in this respect the preferable 

 material to use. But, independently of this consideration, we 

 sometimes want to know the amount of charge which a cable will 

 take, in order to determine by it, its length. If, for instance, we 

 know our insulating material, the size of our conductor, and of the 

 insulated wire, we can tell beforehand what will be the amount of 

 charge in a given length of cable. We measure the charge by the 

 deflection of a galvanometer, and in this way by reading the angle 

 of a single deflection, in putting the battery to the cable, we find 

 the charge K to be equal to the sine of half the angle of deflection 

 divided by the electro-motive force, or the number of cells em- 

 ployed, or, in mathematical language, we have K = 5- which 



formula represents the capacity of a cable for the unit of electro- 

 motive force. 



