THE TELEGRAPH. 
2$2 
in contact with the key, while the other pole is connected with 
the earth. The lever of the key is connected with the line 
through the receiving apparatus, but this receiving apparatus 
is arranged in a peculiar manner, for instead of its being sur¬ 
rounded by a single circuit, it has two surrounding the needle 
in opposite directions, so that if the same current passes 
through these two circuits simultaneously, no effect will be 
produced on the needle. One of these portions of the circuit 
is indicated in the diagram by a full line, and the other by a 
dotted line. Let us suppose that contact is made with the 
battery through the key; the current beginning at b has two 
circuits open for it; one (full line in diagram) carries it 
through the telegraph line, the other (dotted line) through a 
resistance (marked ligne artific.) takes it to the earth. It 
therefore divides into two parts, one of which will surround 
the needle of the receiving apparatus in one direction, and the 
other in the opposite direction. If these two currents are not 
equal, the needle will be deflected every time the key makes 
the battery contact: if, on the other hand, they are equal, the 
needle will remain at rest whatever number of battery contacts 
be made by the key. Let this be well understood, for it is 
the principle of the whole system. The operator at A must 
therefore so arrange the circuit of his receiving instrument 
that when he sends the line signals through his own receiving 
apparatus this shall remain entirely unaffected, though it is 
free to receive and mark signals made at B. This result can 
be obtained only by equalizing the circuits, that is to say, by 
introducing into the second circuit an “ artificial line ” equal 
or very nearly so to the real line, in all its effects on the 
currents. We have seen that at the point b, the current finds 
two paths, one by the circuit of the real line and the other 
through the artificial line to the earth. If the resistances and 
retardations of the circuits are equal, the whole current of the 
battery will divide itself at b into two equal parts ; and we 
have seen how the operator at either station might send signals 
to his correspondent through his own apparatus without 
making the latter move, while yet free to receive signals from 
the correspondent station. 
Let us see how it happens that these signals can be received 
from B at the same time that a is sending to that station. Let 
