HISTORY AND PROGRESS. 155 



employment of a relay with coils wound with two separate 

 wires, in one of which the current of his line battery cir- 

 culate, and in the other that of an equating battery. These 

 coils, wound in opposite directions on the cores, have equal 

 and opposite magnetic effects on the relay when connected 

 up in their proper circuits ; so that, on pressing down the key, 

 although the whole of the current of the line battery passes 

 through the relay, the latter remains perfectly unaffected. 

 For convenience of closing the circuit of these two batteries 

 at the same moment, Gintl employs a double key, a b c, 

 and d b' (?, consisting of two separate levers insulated from 

 each other, being connected together by an insulating cross- 

 piece, and having in front a common knob. 



In the circuit of one series of the coils of the relay (usually 

 the outer and thicker) are inserted, by means of the leading 

 wires I and n, the equating battery, and the front and 

 middle contacts, a and b, of the right side of the key. The 

 front contact of the other side of the key is connected to the 

 positive pole by the line battery, the negative pole being to 

 earth; the middle contact, or lever, is connected with re- 

 maining coils of the relay, and thence, on the other side, to 

 the line wire ; and the back contact of the key to earth. 



On pressing down the knob of the key, the current of 

 the line battery, L B, goes from the + pole over the lever 

 a b', leading wire, terminal 1 of relay, the interior coils, 

 terminal 2, through the line to B, where it passes from 3 

 through the interior coils of the relay, 4, key b' d, 5, E, 

 earth, and, at station A, E to the pole of the battery. 



The current of the equating battery, at station A, goes, 

 at the same time, through its circuit : +, key, a, b, I, the 

 outer coils of the relay, 11, &c., neutralising the effect of the 

 line battery upon the relay. 



Suppose now that, while the key of station A is depressed, 

 that of station B is also pressed down, the line current 

 from station B will pass from + of the battery through 

 ', V, of the key, 4, coils of relay, 3, B to station A, where 

 it will enter the coils of the relay at 2, and go from 1 over 

 the key, b', a, through L B to earth, &c. Thus the equili- 



