232 
THE TELEGRAPH , 
The best-known form of relay is Siemens’ polarized relay 
(fig. 166). S is the south pole of a tempered steel magnet, the 
north pole of which, sr, terminates in branches to the two cores 
of the electro-magnet m. Between the upper parts of these 
cores, the slip a oscillates about its pivot d. The coils are 
wound in opposite directions round the two branches of the 
electro-magnet, so that the current in one direction tends to 
make n a north pole, and n' a south pole, while a current in 
the opposite direction makes n south, and n' north. AThough 
Fig. 165, 
the piece A D is made of soft iron, it forms a prolongation of 
the pole s s of the magnet, and is therefore magnetized in the 
same way. 
Eelays can also be made to transmit positive and negative 
currents, according as they receive the one or the other current; 
and the Morse inking apparatus can also be made into a relay 
for throwing local currents into circuit, instead of marking the 
paper. The two operations may even be combined. It is in 
this way that the whole distance from London to Teheran is 
worked at one manipulation, with five transferences all working 
automatically. 
Yet, notwithstanding the delicacy of adjustment in the 
Siemens relays, it often happens, especially with sub-marine 
cables, that the armature does not respond to a small change 
of the intensity of the current. It then becomes necessary 
