The human speaking voice is full of changes; unlike the notes from a musical instrument, there is no uniformity in it; the rise and fall of inflection, the varying sound of the vowels and consonants, the combinations of words and syllables—­each produces a different vibration and different tone.  To devise an instrument that would receive all these varying tones and inflections and change them into 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 incredible swiftness to the receiving instrument, and these are translated back into the vibrations that produce speech.  This is really what takes place when you talk over a toy telephone made by a string stretched between the two tin mouth-pieces held at opposite sides of the room, with the difference that in the telephone the vibrations are carried electrically, while the toy carries them mechanically and not nearly so perfectly.