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