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HISTORY AND PROGRESS. 109 



R! and R 2 are two polarised relays of the construction in- 

 vented by Mr. Yarley, each consisting of a soft iron bar 

 turning horizontally on points within the two coils of a 

 cylindrical electro-magnet. Each end of the bar plays 

 between the poles of a permanent horse-shoe magnet. The 

 contacts 1 and 2 limit its motion ; 1 being a platinum-point 

 forming, with the soft iron bar, part of the local circuit ; and 

 2, an insulated rest. 



One of the relays, R 2 , is used for closing the circuit of the 

 Morse-instrument, and that of a key or spacing-apparatus ; 

 and the other, %, for closing the circuit of the spacing- 

 apparatus alone. 



The spacing- apparatus, s, performs the functions of a key ; 

 it consists of a metal beam, with an armature of soft iron at 

 one end, held in close proximity to the poles of an electro- 

 magnet. The beam is supplied with two contact points, one 

 at each end, which strike upon two upright anvils, and is 

 held by a spiral spring upon the reposing contact 2, until 

 the armature is attracted by the electro-magnet, when it is 

 deflected and makes contact with the other anvil, 1. 



An important adjunct to this apparatus is a series of in- 

 duction plates, i, connected across the terminals of the coils 

 of s. It is composed of several similar plates of lead or 

 carbon, made up in the form of a battery, and charged with 

 dilute sulphuric acid. By itself a system of this kind gives 

 no current, but it becomes polarised by the current of a 

 battery being sent through it ; and on removing the battery 

 and replacing it by a conductor, a current passes through 

 the latter in the reverse direction to the battery current, 

 decreasing in strength until the polarisation is neutralised. 

 Its purpose, in this instance, is to prevent for a moment the 

 armature of the spacing- apparatus being released from the 

 poles of its electro-magnet, when the circuit of the local 

 battery is interrupted by the relay on the cessation of the 

 positive current in the line. The polarisation current mag- 

 netises the cores before the armature is released, and retains 

 the beam on the contact 1 until the opposite current in the 

 line deflects the bar of the relay R, which closes the local 



