308 



THE ELECTRIC TELEGRAPH. 



being in the battery- circuit, 3 at a bottom contact-point, 4 

 in the circuit of the galvanometer. T is an ebonite button, 

 on which the finger is placed to depress it, and Q a piece of 

 ebonite intended to prevent 2 and 3 making contact with 

 each other, and to push 3 down upon 4. 



The resistance- wire R being approximately adjusted is 

 placed opposite to the standard in the bridge, and the point 



Fig. 147. 



u moved along the wire w x until, on depressing the key, the 

 galvanometer indicates no current. The commutator D is then 

 reversed, by which R and s exchange places, and the contact- 

 point u moved again until the balance is obtained as before. 

 If the balance is obtained without moving u, it is evident 

 that R and s are equal to each other, and that the resistances 

 A -|- x u and c -|- w u are also equal, indicating, at the same 

 time, a small inequality between the values of A and c. If 

 the balance is only obtained by moving u, the 'direction in 

 which this movement takes place shows whether the wire R 

 is too long or too short, and if R is a known length of the 

 same wire as w x, the distance through which the contact 

 is moved gives a measure of the excess or deficiency of the 

 length of R. When the length of R has been adjusted till 

 the balance is attained,' the proportion coils, A and c, are 

 removed and replaced by others, A' and C', whose resistances 

 are ten times as great. A second adjustment is made with 

 these coils, which are afterwards substituted by two others 

 of still greater resistance ; and in this way any required 

 degree of accuracy may be attained. 



The connections (shown by dotted lines) between the 

 mercury-cups, &c., are made with stout copper wires. The 



