HISTORY AND PROGRESS. 75 



interrupts the short-circuit c L', but makes, at the same 

 time, contact between itself and the spring 6, and in so 

 doing, shunts the current from the half m of the electro- 

 magnet coils, by the circuit e, d, b, m. The short-circuit to 

 line c L' being broken, the current must pass from b through 

 IK, whose resistance is equal to the resistance m' ; thus the 

 total resistance of the circuit remains unaltered. The arma- 

 ture is now held by only the one pole m, and the spring /is 

 so adjusted that on the slightest decrease in the magnetism 

 of m, it exerts force enough to pull back the armature. 



The idea of attracting the armature a certain distance 

 by two poles, and this done, of holding it there by one, is 

 as novel as it is ingenious, and answers its purpose very 

 well in this instance. The plan is, however, complicated, 

 and the apparatus, of course, requires very nice adjust- 

 ment. 



55. Magneto-electric Pointer Telegraph of Siemens and 

 Hakke. More generally employed than their pointer 

 telegraph with voltaic currents, and than Kramer's, is the 

 convenient and trustworthy instrument which Siemens and 

 Halske some years since constructed for the Bavarian tele- 

 graph lines. The system is almost exclusively employed on 

 the lines of the Grande Socie'te des Chemins de Fer Busses, 

 by the London, the Danzig, and the Koenigsberg fire brigades, 

 and on various other lines in England and elsewhere. 



The apparatus consists of a battery of permanent magnets, 

 between the poles of which a coil of insulated wire on a 

 revolving armature of soft iron develops, on being turned 

 on its axis, alternately positive and negative currents. These 

 currents traverse the line one after the other, and passing 

 through the coils of an electro-magnet at the receiving 

 station, cause its armature to vibrate and turn an escapement- 

 wheel and pointer. 



Fig. 39 represents an external view of the complete 

 apparatus. A A is a cylindrical case, containing the trans- 

 mitter ; c, a handle which, in operating with the apparatus, 

 is turned round from letter to letter, marked on the hori- 

 zontal dial-plate, stopping always against the tooth opposite 



