Though possessing these admirable qualities as an electrical machine, the simple electromagnetic telephone had not the ability to transmit speech loudly enough for all practical uses.  Transmitters producing stronger telephonic currents were developed soon after the fundamental invention.  Some forms of these were invented by Professor Bell himself.  Other inventors contributed devices embodying the use of carbon as a resistance to be varied by the motions of the diaphragm.  This general form of transmitting telephone has prevailed and at present is the standard type.

It is interesting to note that the earliest incandescent lamps, as invented by Mr. Edison, had a resistance material composed of carbon, and that such a lamp retained its position as the most efficient small electric illuminant until the recent introduction of metal filament lamps.  It is possible that some form of metal may be introduced as the 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 form, it is a line joining a telephone to some means of connecting it at will to another.