1909] 



on Modern Suhnarine Telegraphy. 



537 



received to operate a relay : they were too ill-defined, and the zero 

 line wandered too greatly to insure that a relay with a fixed me- 

 chanical zero would work satisfactorily. 



These two difficulties were overcome by the invention of the 

 " drum " cable relay and the magnetic shunt. 



The drum cable relay (Fig. 8) is very similar to the siphon re- 

 corder. It is the same so far as the suspended coil and connecting 

 fibres are concerned, but, in place of the siphon tube, a relay contact 

 arm is provided. 



Fig. 8. — Drum Cable Eelay. 



The end of this arm is arranged to press upon the surface of a 

 revolving drum. The outer drum surface of gold or silver is divided 

 into three parts : a central insulated portion, upon which the end 

 of the contact arm normally rests when no signals are received, and 

 portions one on each side of the central one. These outer divisions are 

 included in the circuit of a local battery, and two post-office pattern 

 relays. 



When the relay arm is deflected to one side or the other, upon 

 the receipt of the signal, it slides or skates into contact with one or 

 other of the outer portions of the drum, and thus closes circuit of the 

 battery through one or other of the post-office relays : this second re- 

 lay is thus operated and in turn works a "sounder" key to re-transmit 

 the signal into a second cable. 



To reduce the electrical resistance that is found to exist in the 

 contact between the relay pointer and the revolving drum, and to 



2 N 2 



