magnetized soft iron, and the battery was omitted from the line. The
undulatory current in a system of two such telephones joined by a line
is _produced_ in the sending telephone by the vibration of the iron
diaphragm. The vibration of the diaphragm in the receiving telephone
is _produced_ by the undulatory current. Sound is _produced_ by the
vibration of the diaphragm of the receiving telephone.

Such a telephone is at once the simplest known form of electric
generator or motor for alternating currents. It is capable of
translating motion into current or current into motion through a wide
range of frequencies. It is not known that there is any frequency of
alternating current which it is not capable of producing and
translating. It can produce and translate currents of greater
complexity than any other existing electrical machine.

Though possessing these admirable qualities as an electrical machine,
the simple electromagnetic telephone had not the ability to transmit
speech loudly enough for all practical uses. Transmitters producing
stronger telephonic currents were developed soon after the fundamental
invention. Some forms of these were invented by Professor Bell
himself. Other inventors contributed devices embodying the use of
carbon as a resistance to be varied by the motions of the diaphragm.
This general form of transmitting telephone has prevailed and at
present is the standard type.

It is interesting to note that the earliest incandescent lamps, as
invented by Mr. Edison, had a resistance material composed of carbon,
and that such a lamp retained its position as the most efficient small
electric illuminant until the recent introduction of metal filament
lamps. It is possible that some form of metal may be introduced as the