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 come into practical use.

Variation of Resistance. Variation of resistance proportional to the vibrations of the diaphragm is the method which has produced the present prevailing form of transmission.  Professor Bell’s Centennial exhibit contained a water-resistance transmitter.  Dr. Elisha Gray also devised one.  In both, the diaphragm acted to increase and diminish the distance between two conductors immersed in water, lowering and raising the resistance of the line.  It later was discovered by Edison that carbon possesses a peculiarly great property of varying its resistance under pressure.  Professor David E. Hughes discovered that two conducting bodies, preferably of rather poor conductivity, when laid together so as to form a loose contact between them, possessed, in remarkable degree, the ability to vary the resistance of the path through them when subject to such vibrations as would alter the intimacy of contact.  He thus discovered and formulated the principles of loose contact upon which the operation of all modern transmitters rests.  Hughes’ device was named by him a “microphone,” indicating a magnification of sound or an ability to respond to and make audible minute sounds.  It is shown in Fig. 8.  Firmly attached to a board are two carbon blocks, shown in section in the figure.  A rod of carbon with cone-shaped ends is supported loosely between the two blocks, conical depressions in the blocks receiving the ends of the rod.  A battery and magneto receiver are connected in series with the device.  Under certain conditions of contact, the arrangement is extraordinarily sensitive to small sounds and approaches an ability indicated by its name.  Its practical usefulness has been not as a serviceable speech transmitter, but as a stimulus to the devising of transmitters using carbon in other ways.  Variation of the resistance of metal conductors and of contact between metals has served to transmit voice currents, but no material approaches carbon in this property.