SCIENTIFIC AND TECHNICAL LABOURS TO 1860. 229 



The arrangement just described of polarized 

 magnets - - i. e. those in which the oscillating bar or 

 magnet has two resting places, according as a positive 

 or negative current has last traversed the electro- 

 magnetic coils - - has obtained more considerable and 

 general importance through their being used for relays. 

 On the employment of polarized relays depends the 

 possibility of telegraphing the Morse alphabet with 

 short induced currents, the one direction of the current 

 initiating a dash on the paper strip, while the other 

 completes it. The length of the produced dash ac- 

 cordingly does not depend on the duration of the 

 current, but on the duration of the interval between 

 two short successive currents of alternate direction. 



On this principle depend several of our telegraphic 

 constructions, of which only the induction writing-tele- 

 graph need be mentioned here. In this the short 

 currents of alternate direction required for its working 

 were produced by a well-closed electro-magnet, round 

 which was wound a primary coil of short thick 

 wire and a secondary of long thin wire. In the 

 primary coil the currents required for telegraphing 

 the Morse alphabet were produced in the usual way. 

 In the secondary coils connected with line and earth 

 there then occurred, at the beginning and close of 

 the currents circulating in the primary conductor, 

 powerful induced currents of alternate direction, which 

 produced the required Morse signs in the telegraphic 

 apparatus at the receiving station. For the magnetic 

 inductors magnetically closed electro - magnets with 



