THE CABLE SHIP ON REPAIRS. 187 



and the front cheek of the coil is splayed out to prevent inter- 

 ception of the reflected ray. The signals are much improved 

 by introducing a soft-iron core within the tube. The core 

 becomes magnetised by every current received through the 

 coil, and therefore produces greater amplitude in the signals, 

 while, owing to the fact that it does not give up or reverse its 

 magnetism as rapidly as the signalling currents pass through 

 the coil, the signals are steadied and rounded off, a considerable 

 advantage on short lines. This improvement was introduced 

 by Mr. Walter Judd, Electrician-in-Chief of the Eastern and 

 Associated Telegraph Companies, and has been found to lessen 

 the interference of earth currents on signals. The core has a 

 shoulder at one end to prevent it touching the mirror by 

 entering too far in the tube, while by adjusting the length of 



Fig. 96. 



core within the tube the signals can be brought to any required 

 size. This addition to the mirror is very useful in signalling 

 through faulty lines, as a smaller battery power can be used. 

 The soft iron also acts as a damper and checks the oscillations 

 of the needle. 



The suspension silk need not be a single fibre, but the thick- 

 ness can be suited to the sensitiveness required. The silk is 

 best fastened on by laying the mirror face downwards on a 

 table or flat board as at M (Fig. 96), one end of the silk having 

 been fastened down to the board by a little beeswax at A, while 

 the other end is held in the lef d hand. The silk can then be 

 held taut and slightly raised, while the mirror is shifted round 

 so as to bring the magnet in a position at right angles to the 

 silk. By means of a light copper rod (Fig. 96) heated at the 

 end in a spirit flame, a little melted beeswax or shellac is 

 applied to the back of the mirror to fasten the silk down. 



