538 Mr. Sidney George Broivn [March 12, 



allow a large current to pass, condensers are placed across to short- 

 circuit the contact. 



These short-circuiting condensers are very important to the proper 

 working of the relay, as without their aid very little current indeed 

 could be obtained in the local circuit to do useful work. The cable 

 relay is a delicate instrument, and mechanical effects had to be pro- 

 duced by means of energy four millionths of that required to produce 

 one candle power of an ordinary carbon lamp. 



The operation of the relay throughout is quite automatic and 

 reliable, and no clerk is required to supervise. 



The drum relay has two properties that peculiarly fit it for cable 

 work. 



1, The relay contact is always made, because the contact arm 

 never leaves the surface of the drum. 



2. By the rotation of the drum the friction between the arm, to 

 side motion, and the surface of the drum is reduced in a most won- 

 derful way, so that the arm may be moved by the extremely feeble 

 forces received at the end of the cables. 



The relay has a fixed mechanical zero, the centre of the insulated 

 portion, to which the end of the arm must return after every signal 

 or group of signals, and the zero of tlie electrical signals has been 

 made by electrical adjustment to coincide with the mechanical zero. 

 Tf there were not this coincidence there would be mutilation of the 

 re-transmitted signals. 



The working of the relay is complicated by the requirements 

 of the service, which demand that a condenser should be included 

 in the suspended coil circuit. Tiie object of this condenser is to 

 exclude the possibility of interference from " earth " currents, which 

 sometimes flow along the cable. 



The presence of the " earth " current is due to outside electrical 

 influences, atmospheric or celestial. 



Now these " earth " currents, if allowed to flow through the 

 suspended coil, would produce deflections that would interfere with 

 the proper working of the relay. 



The magnetic shunt which is always placed across the coil does 

 shunt the " earth " current to a very great extent, but does not 

 always get rid of it, and so to make matters sure the "unshunted " 

 series or Varley condenser is included in the system. 



The condenser, unfortunately, polarises or charges up under a 

 series of signalling impulses of the same polarity or sign, and for 

 this reason" itself causes a wandering of the electrical zero of the 

 signals. We are therefore trying to stop one kind of variable zero 

 effect by a device that produces another one of its own. 



The effect of the wandering zero due to the series condenser can 

 be cured, because the wandering, unlike that of the " earth " currents, 

 follows a regular law, viz. the law of the signals themselves. 



The relay produces the signals and combination of signals in 



