362 



THE ELECTRIC TELEGRAPH. 



A similar arrangement, with the exception of the form of 

 commutator u, which we have altered, is used by Messrs. 

 Siemens on board ship while paying out submarine cables, 



and also at the land 

 stations. The key acts 

 between the lever 3 and 

 back contact 2, as a short 

 circuit across the galvano- 



-*^= c&a| meter coil a , the longer of 



+ * J 3 -. *l the two, and between the 



lever and front contact 1, 

 as contact key in the cir- 

 cuit of the galvanometer 

 coil ft. This application of 

 the key is useful in insu- 

 lation tests, when the cur- 

 rent is allowed to run, for 

 a certain time, into the 

 cable before the observa- 

 tion is made, and the 

 battery circuit on the other 

 side closed, at the same 

 instant that the insulation 

 current is allowed to go through the galvanometer. The 

 other connections and pieces are obvious. 



The constant of the instrument is measured by allowing 

 the current of the smaller battery b to go, at the same time, 

 parallel through both the coils a and /}. This is done 

 by means of contact plugs in the holes i in of u x , in the holes 

 1 3, 2 4, and 45 of u, and in all the holes of the pro- 

 portion resistance coils r and p ; all other stoppers are left 

 out. The current of b goes through the circuit b, z, I, IIT of 

 u,, K (1, 3) (the key being pressed down) divides into 



' r'q ^ i\ -o ' 'fK "n ! of battery 6. The resistance 

 ( a, r, p, TJ (3, 1), B, u (5, 4), c j 



of the coil ft and its circuit remains unaltered ; R is adjusted 

 until the deflecting forces of the currents in and ft are 

 balanced, in which case 



