44 THE ADDRESSES, LECTURES, ETC., OF 



vary in its condition by changes of temperature or by moisture. 

 Considering the difficulty of obtaining a permanent condenser, I 

 prefer to employ another method, which is represented in Fig. 4, 

 Plate 2. 



Between the two permanent branches of a Wheatstone diagram a 

 delicate galvanometer is placed, and the cable is inserted between 

 the open end of these resistances or branches. The cable, being in 

 a water-tank, is connected on its outside with the battery, the 

 second pole of which is joined to the resistance-coils. Now, if the 

 cable is mechanically perfect, the insulation-resistance must be 

 everywhere the same al6ng its whole length, and this being the 

 case, there must be as much electricity flowing from the battery 

 through the two resistances, and through every portion of the 

 covering of the cable to earth ; the leakage from the cable would, 

 in fact, produce the same effect as one collective leakage at the 

 exact middle of that cable. If there should not be that perfect 

 balance, but if there was a faulty joint at a point near one end of 

 the cable, then the discharge of the battery current to earth would 

 gravitate to this point, and that branch of the balanced resistances 

 near at hand would pass more current than the other branch. 

 The result would be that the needle, in the connecting link or 

 bridge, would be deflected. Now, if an artificial resistance were 

 inserted at the end of the balanced resistance, nearest the fault, 

 this could be so regulated as to re-establish the balance of currents 

 on the galvanometer, and we should get an indication of the 

 importance of that fault. This method is exceedingly sensitive, 

 and a simple means of furnishing a continued verification of the 

 normal condition of a cable in course of manufacture. The 

 slightest inequality between the sides will tip the needle over to 

 one side or the other, as may be illustrated by the diagram before 

 us, in which hydrostatic pressure is again substituted for electrical 

 force. If we take a flat bar of wood, accurately balanced upon 

 pivots in the middle, and bring it under a general rain, then the 

 rain falling down equally upon it, will keep it perfectly in 

 balance, because it will fall everywhere alike. But if at a point 

 near one end there should be an extra amount of rain-fall, this 

 would immediately cause the bar to tip, for the same reason that 

 the electrical balance tips under the influence of the extra leakage 

 of a faulty joint. 



