heat generated in that conductor, is observed in one of three ways: by
the expansion of the conductor, by its change in resistance, or by the
production of an electromotive force in a thermo-electric couple
heated by the conductor. Any one of these three ways can be used to
get some idea of the amount of current which is received. None of them
gives an accurate knowledge of the forms of the waves which cause the
reproduction of speech in the telephone receiver.

[Illustration: Fig. 14. Oscillogram of Telephone Currents]

_Electromagnetic Method_. An electromagnetic device adapted to tell
something of the magnitude of the telephone current and also something
of its form, _i.e._, something of its various increases and decreases
and also of its changes in direction is the oscillograph. An
oscillograph is composed of a magnetic field formed by direct currents
or by a permanent magnet, a turn of wire under mechanical tension in
that field, and a mirror borne by the turn of wire, adapted to reflect
a beam of light to a photographic film or to a rotating mirror.

When a current is to be measured by the oscillograph, it is passed
through the turn of wire in the magnetic field. While no current is
passing, the wire does not move in the magnetic field and its mirror
reflects a stationary beam of light. A photographic film moved in a
direction normal to the axis of the turn of wire will have drawn upon
it a straight line by the beam of light. If the beam of light,
however, is moved by a current, from side to side at right angles to
this axis, it will draw a wavy line on the photographic film and this
wavy line will picture the alternations of that current and the
oscillations of the molecules of air which carried the originating
sound. Fig. 14 is a photograph of nine different vowel sounds which
have caused the oscillograph to take their pictures. They are copies
of records made by Mr. Bela Gati, assisted by Mr. Tolnai. The
measuring instrument consisted of an oscillograph of the type
described, the transmitter being of the carbon type actuated by a
2-volt battery. The primary current was transformed by an induction
coil of the ordinary type and the transformed current was sent through
a non-inductive resistance of 3,000 ohms. No condensers were placed in
the circuit. It will be seen that the integral values of the curves,
starting from zero, are variable. The positive and the negative
portions of the curves are not equal, so t