364 THE ELECTRIC TELEGRAPH. 



One of the fundamental laws of magnetism is that the 

 magnetic force decreases as the square of the distance between 

 the points acting upon each other increases. The galva- 

 nometer coils, in which currents are circulating, being mag- 

 nets, are subject to the same law. It, therefore, becomes con- 

 venient to so arrange the coil (3 that its distance from the 

 needle may be varied, by which the constant relation K may 

 be obtained of any value most convenient in the calculation 

 of x, as, for example, some power of 10, or the resistance x 

 may be calculated by the distance ; which is, in that case, 

 varied by means of a michrometer screw, and observed with 

 a nonius. The latter method, where time and trouble are 

 not of importance, is less to be recommended, however, as any 

 error occurring in the observation of the distance, obviously 



comes into x in the square. Nothing is easier than to make 

 TT ~R 



-= = 10 6 , and then the resistance of the cable is 

 



x = (E -f r) millions. 



The apparent complication of the board is caused by the 

 arrangements of the pieces for measuring by the other 

 methods as well as by this one. 



79. Beyond the limits within which the differential 

 _ 2_ method just explained may be em- 

 ployed, we use the method of deflection. 

 For this purpose Professor Thomson's 

 reflecting galvanometer, and the sine- 

 multipliers, are the best measuring in- 



istruments. One end of the cable, a b, 

 Fig. 167, is connected with the galvano- 

 meter coil, the other end of the coil 

 being in connection with a contact key, 

 K, and, through this, with one pole of 

 Fig. 167. a battery, B, whose other pole is to 



earth. When in this position the key is pressed down, the 

 charge current passes through the galvanometer. This is 

 avoided by a short circuit or shunt of no appreciable resist- 



