way could be accomplished the simultaneous transmission
of a number of telegraphic messages along a single wire, the number
being limited only by the delicacy of the listener's ear. The idea of
increasing the carrying power of a telegraph wire in this way took
complete possession of my mind, and it was this practical end that I had
in view when I commenced my researches in electric telephony.

[Illustration: Fig. 1]

In the progress of science it is universally found that complexity leads
to simplicity, and in narrating the history of scientific research it is
often advisable to begin at the end.

In glancing back over my own researches, I find it necessary to
designate, by distinct names, a variety of electrical currents by means
of which sounds can be produced, and I shall direct your attention to
several distinct species of what may be termed telephonic currents of
electricity. In order that the peculiarities of these currents may be
clearly understood, I shall project upon the screen a graphical
illustration of the different varieties.

The graphical method of representing electrical currents shown in Fig. 1
is the best means I have been able to devise of studying, in an accurate
manner, the effects produced by various forms of telephonic apparatus,
and it has led me to the conception of that peculiar species of
telephonic current, here designated as _undulatory_, which has rendered
feasible the artificial production of articulate speech by electrical
means.

A horizontal line (_g g'_) is taken as the zero of current, and impulses
of positive electricity are represented above the zero line, and
negative impulses below it, or _vice versa_.

The vertical thickness of any electrical impulse (_b_ or _d_), measured
from the zero line, indicates the intensity of the electrical current at
the point observed; and the horizontal extension of the electric line
(_b_ or _d_) indicates the duration of the impulse.

Nine varieties of telephonic currents may be distinguished, but it will
only be necessary to show you six of these. The three primary varieties
designated as intermittent, pulsatory and undulatory, are represented in
lines 1, 2 and 3.

Sub-varieties of these can be distinguished as direct or reversed
currents, according as the electrical impulses are all of one kind or
are alternately positive and negative. Direct currents may still
further be distinguished as positive or negative, according as the
impulses