306 



THE ELECTRIC TELEGRAPH. 



or 1,000, according to the strength of the battery, and II is 

 varied until the needle of the galvanometer rests upon the 

 zero-line. When the balance is obtained with, say p = 100, 



we alter this side to 110 and get another reading for R^rRi. 



p 

 The relation of the electro-motive forces x is expressed by 



the equation 



E __ (E, R) + (110 - 100) 

 E' ~ (110 100) 



as will be explained afterwards. 



These are the principal applications of the beautifully 



arranged testing apparatus invented by Messrs. Siemens. 



With some trifling modifications it is equally applicable for 



measurements by other methods. 



51. British Association Bridge. An ingenious electrical 



balance has been arranged by the sub-committee appointed by 



the British Association in 1861. The purpose of this balance 



is for copying standard resistances with great exactness. 

 Instead of employing proportion resistances of some 



unalterable value, the ends of 

 the two branches A and c, 

 Fig. 145, enclose a wire, w x, 

 of sensible resistance, contact 

 being made with it by means 

 of a travelling point, u. Ac- 

 cording as u is moved to the one 

 side or the other, therefore, re- 

 sistance is added to one and 

 subtracted from the other branch; 

 and the resistance of w x being 

 small, a balance of great exact- 

 ness may be obtained between 

 the .proportion resistances A and 



c, which is otherwise liable to be temporarily deranged by 



inequality in the temperature of these coils. 



Fig. 146 shows a special plan of the board. The two 



branch coils A and c, of equal, or nearly equal, resistances, are 



