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
readily comprehended if it be remembered that a telephone is sensitive
to the changes in the strength of the current if those changes occur
with a frequency of some hundreds or in some cases thousands of times
_per second_. On the other hand, currents vibrating with such rapidity
as this are utterly incompetent to affect the moving parts of
telegraphic instruments, which cannot at the most be worked so as to
give more than 200 to 800 separate signals _per minute_.

[Illustration: Fig. 1]

[Illustration: Fig. 2]

The simplest arrangement for carrying out this method is shown in Fig.
1, which illustrates the arrangements at one end of a line. M is the
Morse key for sending messages, and is shown as in its position of
rest for receiving. The currents arriving from the line pass first
through a "graduating" electromagnet, E2, of about 500 ohms
resistance, then through the key, thence through the electromagnet, R,
of the receiving Morse instrument, and so to the earth. A condenser,
C, of 2 microfarads capacity is also introduced between the key and
earth. There is a second "graduating" electromagnet, E1, of 500 ohms
resistance introduced between the sending battery, B, and the key.
When the key, M, is depressed in order to send a signal, the current
from the battery must charge the condenser, C, and must magnetize the
cores of the two electromagnets, E1 and E2, an