ovement has been made since its first
invention. It is inadequate as a transmitter, and as early as 1876,
Professor Bell exhibited other means than electromagnetic action for
producing the varying currents as a consequence of diaphragm motion.
Much other inventive effort was addressed to this problem, the aim of
all being to send out more robust voice currents.

[Illustration: Fig. 6. Telegraph Equipment Converted into Telephone
Equipment]

Other Methods of Producing Voice Currents. Some of these means are
the variation of resistance in the path of direct current, variation
in the pressure of the source of that current, and variation in the
electrostatic capacity of some part of the circuit.

_Electrostatic Telephone._ The latter method is principally that of
Dolbear and Edison. Dolbear's thought is illustrated in Fig. 7. Two
conducting plates are brought close together. One is free to vibrate
as a diaphragm, while the other is fixed. The element _1_ in Fig. 7 is
merely a stud to hold rigid the plate it bears against. Each of two
instruments connected by a line contains such a pair of plates, and a
battery in the line keeps them charged to its potential. The two
diaphragms of each instrument are kept drawn towards each other
because their unlike charges attract each other. The vibration of one
of the diaphragms changes the potential of the other pair; the degree
of attraction thus is varied, so that vibration of the diaphragm and
sound waves result.

Examples of this method of telephone transmission are more familiar to
later practice in the form of condenser receivers. A condenser, in
usual present practice, being a pair of closely adjacent conductors of
considerable surface insulated from each other, a rapidly varying
current actually may move one or both of the conductors. Ordinarily
these are of thin sheet metal (foil) interleaved with an insulating
material, such as paper or mica. Voice currents can vibrate the metal
sheets in a degree to cause the condenser to speak. These condenser
methods of telephony have not become commercial.

[Illustration: Fig. 7. Electrostatic Telephone]

_Variation of Electrical Pressure._ Variation of the pressure of the
source is a conceivable way of transmitting speech. To utilize it,
would require that the vibrations of the diaphragm cause the
electromotive force of a battery or machine to vary in harmony with
the sound waves. So far as we are informed this method never has com