The telephone was invented in 1875 by Alexander Graham Bell, a resident of the United States, a native of Scotland, and by profession a teacher of deaf mutes in the art of vocal speech.  In that year, Professor Bell was engaged in the experimental development of a system of multiplex telegraphy, based on the use of rapidly varying currents.  During those experiments, he observed an iron reed to vibrate before an electromagnet as a result of another iron reed vibrating before a distant electromagnet connected to the nearer one by wires.

The telephone resulted from this observation with great promptness.  In the instrument first made, sound vibrated a membrane diaphragm supporting a bit of iron near an electromagnet; a line joined this simple device of three elements to another like it; a battery in the line magnetized both electromagnet cores; the vibration of the iron in the sending device caused the current in the line to undulate and to vary the magnetism of the receiving device.  The diaphragm of the latter was vibrated in consequence of the varying pull upon its bit of iron, and these vibrations reproduced the sound that vibrated the sending diaphragm.

The first public use of the electric telephone was at the Centennial Exposition in Philadelphia in 1876.  It was there tested by many interested observers, among them Sir William Thomson, later Lord Kelvin, the eminent Scotch authority on matters of electrical communication.  It was he who contributed so largely to the success of the early telegraph cable system between England and America.  Two of his comments which are characteristic are as follows: 

To-day I have seen that which yesterday I should have deemed
impossible.  Soon lovers will whisper their secrets over an
electric wire.

* * * * *

Who can but admire the hardihood of invention which devised such slight means to realize the mathematical conception that if electricity is to convey all the delicacies of sound which distinguish articulate speech, the strength of its current must vary continuously as nearly as may be in simple proportion to the velocity of a particle of the air engaged in constituting the sound.

Contrary to usual methods of improving a new art, the earliest improvement of the telephone simplified it.  The diaphragms became thin iron disks, instead of membranes carrying iron; the electromagnet cores were made of permanently magnetized steel instead of temporarily 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.