Apparatus for enabling the same line to be used for telegraph signals
and telephoning.

One type consists in coils with iron cores or simply electromagnets.
These act to retard the current in reaching its full power and also
prolong it. This gives a graduated effect to the signals, so that the
telephone diaphragm is not audibly affected by the impulses.

The telephoning current is so slight and so rapid in its characteristic
changes that it is without effect upon the ordinary telegraph.