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, and is thereby retarded in
rising to its full strength. Consequently no sound is heard in a
telephone, T, inserted in the line-circuit. Neither the currents which
start from one end nor those which start from the other will affect
the telephones inserted in the line. And, if these currents do not
affect telephones in the actual line, it is clear that they will not