A condenser is a device composed of two or more conductors insulated from each other by a medium called the dielectric.  A pair of metal plates separated by glass, a pair of wires separated by air, or a pair of sheets of foil separated by paper or mica may constitute a condenser.  The use of condensers as pieces of apparatus and the problems presented by electrostatic capacity in lines are discussed in other chapters.

Measurements of Telephone Currents.  It has been recognized in all branches of engineering that a definite advance is possible only when quantitative data exists.  The lack of reliable means of measuring telephone currents has been a principal cause of the difficulty in solving many of its problems.  It is only in very recent times that accurate and reliable means have been worked out for measuring the small currents which flow in telephone lines.  These ways are of two general kinds:  by thermal and by electromagnetic means.

Thermal Method.  The thermal methods simply measure, in some way, the amount of heat which is produced by a received telephone current.  When this current is allowed to pass through a conductor the effect of the 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.