TELEGRAPHIC APPARATUS . 
253 
us suppose that the operator at B has arranged the con¬ 
nections of his apparatus of battery, artificial line and key, 
under conditions absolutely identical with those at A, so that 
when he sends to A, his own apparatus is not affected. If 
now, b is sending a message at the same time as A also sends 
his current, b makes a battery contact at the same time as A, 
and the result is that no current traverses the line. A stops, 
so to say, b’s current, and similarly b stops A’s current, for 
they use equal batteries and the two currents balance each 
other. But it is precisely by having the power of annulling each 
other’s currents, that A and b have the control of the apparatus 
opposed to them. For so long as the currents from a pass 
freely through the apparatus there, the one into the artificial 
and the other into the real line, this apparatus remains 
A —L ' B 
stationary ; but when the line current is opposed, the balance 
of the two circuits is destroyed, and the current from a which 
passes through the dotted circuit to the earth moves the needle 
and produces a signal, b, therefore, has the power of stopping 
the current which A sends into the line ; and consequently can 
produce signals on a’s apparatus without disturbing the 
balance of his own. The like is true of a’s power over the 
apparatus at b. 
We shall endeavour to make this still clearer by an illustra¬ 
tion taken from the flow of a liquid. Let us suppose that 
fig. 179 represents a peculiar arrangement of water-pipes, in 
which A and B are turbines. If water under pressure (which 
may be taken as corresponding with the electro-motive force) 
passes from b into A, a part will flow on one side of the turbine 
into l and another part on the other side into the pipe A l. If 
the section and resistances of the two pipes are equal, half the 
water will flow on one side and the other half on the other, so 
that the turbine, acted on by equal and opposite forces, will 
