Both line currents may coincide in direction. Then when the two keys are
depressed, a line current of double strength goes through both receiving
instruments and both work by preponderance of the double line current
over the compensation current. In other respects the operation is the
same as before described.
Fig. 315. DUPLEX TELEGRAPH, DIFFERENTIAL SYSTEM.
Fig. 316. DIFFERENTIAL DUPLEX TELEGRAPH CONNECTIONS.
The cut shows a diagram of the operation of one end of the line. R and R
are resistances, E and E are earth contacts, and the two circles show
the magnet of the receiving instrument wound with two coils in
opposition. The battery and key are also shown. It also illustrates what
happens if the key of the receiver is in the intermediate position
breaking contact at both 1 and 2. The sender's line current then goes
through both coils of the receiving instrument magnet, but this time in
series, and in coincident direction. This actuates the instrument as
before. Owing to the resistance only half the normal current passes, but
this half goes through twice as many coils or turns as if the receiver's
key was in either of the other two positions.
In actual practice there are many refinements. To compensate for the
varying resistance of the line a rheostat or resistance with sliding
connection arm is connected in the compensation circuit so that the
resistance can be instantly changed. As the electro-static capacity of
the line varies sectional condensers are also connected in the
compensation circuits.