TELEGRAPHIC APPARATUS. 
255 
The current coming from the key is here divided into one 
portion that passes through the upper branch a, and thence by 
thelineL to the earth atthedistant station, andinto anotherpor- 
tion that traverses the lower branch to reach the earth through 
the artificial line A l. It is evident that perfect equality in the two 
branches^and J,is required to leave the needle motionless. The 
artificial line in this case, as in the former, must be identical 
with the real line in all its electrical conditions. The same 
phenomena attend the transmission of signals as we have 
already described in the former instance. 
The duplex system may therefore be defined by saying that 
it consists in sending two messages simultaneously between 
stations connected by a single wire. We must get rid of the 
notion of the messages passing each other in the wire, or that 
anything passes in opposite directions. It will be seen by 
what we have stated above that such is not the case. The 
problem has been practically and elegantly solved by so con¬ 
necting up ordinary apparatus that the currents at each 
extremity of the line are opposed to each other when required. 
The whole duplex system depends, then, on the particular 
action of certain apparatus that have long been known. 
We have seen that it is essential that the artificial line should 
be the equivalent of the real line in everything by which the 
current is affected. In short lines it is sufficient to balance 
the resistance of the line wire by a resistance coil of equal 
value. But it is a different matter with long lines (such, for 
example, as exceed 250 miles in length), in which the effects 
of induction come into operation. In this case there must be 
added to the circuit of the artificial line condensers, or other 
similar apparatus, that give rise to sudden charges of static 
electricity. When a great line is charged with electricity, the 
inductive action of the earth adds a static charge, which, at 
the first instant augments the dynamical charge resulting from 
the battery current. In like manner, a statical discharge takes 
place when the line is discharged of its dynamical electricity. 
These statical charges and discharges show themselves in the 
apparatus as momentary currents, and they cause vibrations 
or jerks of the indicator of the receiving apparatus that inter¬ 
fere with the slower movements constituting the signals. The 
remedy is to give the artificial line a static capacity equal to 
