Suppose that B is sending a dash, and it begins while A's key is raised.
The line and compensation currents in B's receiving instrument
neutralize each other and no effect is produced, while A's receiving
instrument begins to register or indicate a dash. Now suppose A starts
to send a dash while B's is half over. He depresses his key. This sends
the two opposite currents through his magnet. His line current
neutralizes B's working current so that the compensation currents in
both receiving instruments hold the armatures attracted for the two
dashes. Meanwhile A's dash ends and he releases his key. At once his
line current ceases to neutralize B's line current, his receiving
instrument is actuated now by B's line current, while B's receiving
instrument ceases to be actuated by the compensation current.
Two assumptions are made in the above description. The line currents are
assumed to be equal in strength and opposite in direction at each
station. Neither of these is necessary. The line current received at a
station is always weaker than the outgoing line current, and it is the
preponderance of the compensation current over the partly neutralized
line current that does the work. As this preponderance is very nearly
equal to the line current received from the distant station, the signals
are actuated by almost the same current, whether it is compensation or
line current.