The method previously adopted by Van Rysselberghe, to prevent
induction from taking place between the telegraph wires and those
running parallel to them used for telephone work, was briefly as
follows: The system of sending the dots and dashes of the
code--usually done by depressing and raising a key which suddenly
turns on the current and then suddenly turns it off--was modified so
that the current should rise gradually and fall gradually in its
strength by the introduction of suitable resistances. These were
introduced into the circuit at the moment of closing or opening by a
simple automatic arrangement worked exactly as before by a key. The
result, of the gradual opening and gradual closing of the circuit was
that the current attained its full strength gradually instead of
suddenly, and died away also gradually. And as induction from one wire
to another depends not on the strength of the current, but on the rate
at which the strength changes, this very simple modification had the
effect of suppressing induction. Later Van Rysselberghe changed these
arrangements for the still simpler device of introducing permanently
into the circuit either condensers or else electro-magnets having a
high coefficient of self-induction. These, as is well known to all
telegraphic engineers, retard the rise or fall of an electric current;
they fulfill the conditions required for the working of Van
Rysselberghe's method better than any other device.

Having got thus far in his devices for destroying induction from one
line to another, Van Rysselberghe saw that, as an immediate
consequence, it might be concluded that, if the telegraph currents
were thus modified and graduated so that they produced no induction in
a neighboring telephone line, they would produce no sound in the
telephone if that instrument were itself joined up in the telegraph
line. And such was found to be case. Why this is so will be more