them speaks the sound vibrations impinge
upon the diaphragm and cause it to act as a vibrating armature. By
reason of its vibrations, this diaphragm induces very weak electric
impulses in the magnetic coil. These impulses, according to Bell's
theory, correspond in form to the sound-waves, and, passing over the
line, energize the magnet coil at the receiving end, thus giving rise to
corresponding variations in magnetism by reason of which the receiving
diaphragm is similarly vibrated so as to reproduce the sounds. A single
apparatus at each end is therefore sufficient, performing the double
function of transmitter and receiver. It will be noticed that in this
arrangement no battery is used The strength of the impulses transmitted
is therefore limited to that of the necessarily weak induction currents
generated by the original sounds minus any loss arising by reason of
resistance in the line.

Edison's carbon transmitter overcame this vital or limiting weakness
by providing for independent power on the transmission circuit, and by
introducing the principle of varying the resistance of that circuit with
changes in the pressure. With Edison's telephone there is used a closed
circuit on which a battery current constantly flows, and in that
circuit is a pair of electrodes, one or both of which is carbon. These
electrodes are always in contact with a certain initial pressure,
so that current will be always flowing over the circuit. One of the
electrodes is connected with the diaphragm on which the sound-waves
impinge, and the vibrations of this diaphragm cause corresponding
variations in pressure between the electrodes, and thereby effect
similar variations in the current which is passing over the line to the
receiving end. This current, flowing around the receiving magnet, causes
corresponding impulses therein, which, acting upon its diaphragm, effect
a reproduction of the original vibrations and hence of the original
sounds.

In other words, the essential difference is that with Bell's telephone
the sound-waves themselves generate the electric impulses, which are
therefore extremely faint. With Edison's telephone the sound-waves
simply actuate an electric valve, so to speak, and permit variations in
a current of any desired strength.

A second distinction between the two telephones is this: With the Bell
apparatus the very weak electric impulses generated by the vibration of
the transmitting diaphragm pass over the entir