some other form of energy so that they could be passed over a wire, and
then change them back to their original form, reproducing each sound and
every peculiarity of the voice of the speaker in the ear of the hearer,
was the task that Professor Bell set for himself. Just as you would sit
down to add up a big column of figures, knowing that sooner or later you
would get the correct answer, so he set himself to work out this problem
in invention. The result of his study and determination is the
telephones we use to-day. Many improvements have been invented by other
men--Berliner, Edison, Blake, and others--but the idea and the working
out of the principle is due to Professor Bell.

[Illustration: "CENTRAL" TELEPHONE OPERATORS AT WORK
Since tiny lights have taken the place of bells to indicate the calls of
subscribers the central stations are quiet except for the low hum of
carefully modulated voices. The women standing behind the seated
operators are inspectors. They watch for mistakes and disturbances of
any kind.]

Every telephone receiver and transmitter has a mouth-and ear-piece to
receive or throw out the sound, a thin round sheet of lacquered
metal--called a diaphragm, and an electromagnet; together they reproduce
human speech. An electric current from a battery or from the central
station flows continuously through the wires wound round the
electromagnet in receiving and transmitting instruments, so when you
speak into the black mouthpiece of the wall or desk receiver the
vibrations strike against the thin sheet-iron diaphragm at the small end
of the mouthpiece; the sound waves of the voice make it vibrate to a
greater or less degree; the diaphragm is placed so that the core of the
electromagnet is close to it, and as it vibrates the iron in it
produces undulations (by induction) in the current which is flowing
through the wires wound round the soft iron centre of the magnet. The
wires of the coil are connected with the lines that go to the receiving
telephone, so that this undulating current, coiling round the core of
the magnet in the receiver, attracts and repels the iron of the
diaphragm in it, and it vibrates just as the transmitter diaphragm did
when spoken into; the undulating current is translated by it into words
and sentences that have all the peculiarities of the original. And so
when speaking into a telephone your voice is converted into undulations
or waves in an electric current conveyed with incredi