nger-nail. Soon after-wards came a "buzzer," and
then the magneto-electric bell. In 1887 Joseph O'Connell, of Chicago,
conceived of the use of tiny electric lights as signals, a brilliant
idea, as an electric light makes no noise and can be seen either by
night or by day. In 1901, J. J. Carty invented the "bridging bell," a
way to put four houses on a single wire, with a different signal for
each house. This idea made the "party line" practicable, and at once
created a boom in the use of the telephone by enterprising farmers.

In 1896 there came a most revolutionary change in switchboards. All
things were made new. Instead of individual batteries, one at each
telephone, a large common battery was installed in the exchange itself.
This meant better signalling and better talking. It reduced the cost of
batteries and put them in charge of experts. It established uniformity.
It introduced the federal idea into the mechanism of a telephone system.
Best of all, it saved FOUR SECONDS ON EVERY CALL. The first of these
centralizing switchboards was put in place at Philadelphia; and other
cities followed suit as fast as they could afford the expense of
rebuilding. Since then, there have come some switchboards that are
wholly automatic. Few of these have been put into use, for the reason
that a switchboard, like a human body, must be semi-automatic only. To
give the most efficient service, there will always need to be an expert
to stand between it and the public.

As the final result of all these varying changes in switchboards and
signals and batteries, there grew up the modern Telephone Exchange. This
is the solar plexus of the telephone body. It is the vital spot. It
is the home of the switchboard. It is not any one's invention, as the
telephone was. It is a growing mechanism that is not yet finished, and
may never be; but it has already evolved far enough to be one of the
wonders of the electrical world. There is probably no other part of
an American city's equipment that is as sensitive and efficient as a
telephone exchange.

The idea of the exchange is somewhat older than the idea of the
telephone itself. There were communication exchanges before the
invention of the telephone. Thomas B. Doolittle had one in Bridgeport,
using telegraph instruments Thomas B. A. David had one in Pittsburg,
using printing-telegraph machines, which required little skill to
operate. And William A. Childs had a third, for lawyers only, in New
Y