resistance medium for telephone transmitters, and that such a change
as has taken place in incandescent lamps may increase the efficiency
of telephone transmitting devices.

At the time of the invention of the telephone, there were in existence
two distinct types of telegraph, working in regular commercial
service. In the more general type, many telegraph stations were
connected to a line and whatever was telegraphed between two stations
could be read by all the stations of that line. In the other and less
general type, many lines, each having a single telegraph station, were
centered in an office or "exchange," and at the desire of a user his
line could be connected to another and later disconnected from it.

Both of these types of telegraph service were imitated at once in
telephone practice. Lines carrying many telephones each, were
established with great rapidity. Telephones actually displaced
telegraphic apparatus in the exchange method of working in America.
The fundamental principle on which telegraph or telephone exchanges
operate, being that of placing any line in communication with any
other in the system, gave to each line an ultimate scope so great as
to make this form of communication more popular than any arrangement
of telephones on a single line. Beginning in 1877, telephone exchanges
were developed with great rapidity in all of the larger communities of
the United States. Telegraph switching devices were utilized at the
outset or were modified in such minor particulars as were necessary to
fit them to the new task.

In its simplest form, a telephone system is, of course, a single line
permanently joining two telephones. In its next simplest form, it is a
line permanently joining more than two telephones. In its most useful