TELEGRAPHIC APPARATUS. 
235 
same time contact is made at E with the battery P, and the 
current flows through the line to the distant station. 
It is often necessary, in very long overhead lines, and with 
submarine cables, to have recourse to compensation currents, 
which are made to destroy the induction effects of the.cable or 
line. The key shown in fig. 169 is used by the English Post 
Fig. 168. 
Office, and is very useful on certain overhead and submarine 
lines. The line wire is connected with 3, the earth, and one 
end of the relay coil with 7, and the other end with 4. The 
positive pole of the battery is at c, and the negative at z ; l, l' 
are two levers insulated from each other and moving on the 
same axis when the knob is pressed down ; s , s'" are two 
insulated springs connected with z ; s' s" two similar springs 
connected with c. When the key is pressed down, a positive 
current is transmitted by s', along the lever 1 ! to the stud 0 in 
