You probably all know that the telephone is an electrical instrument by
which one person may talk to another who is at a distance. Not only can
we talk to a person who is in a different part of the city, but such
great improvements have been made in these instruments that we can talk
through the telephone to a person in another city, even though it be
hundreds of miles away.
The main principle of the telephone is electromagnetism, as in the
telegraph, but there are other important points in addition to those we
mentioned in describing the latter.