TELEPHONE, an instrument for transmitting sounds or speech to distances where such would be inaudible through aerial sound waves. This definition ex cludes speaking tubes, which act simply by preserving and concentrating sound waves. Telephonic action depends on the fact that sound waves in air are capable of communicating vibrations to a stretched membrane, and if by any means such vibrations can be transmitted with true resemblance to another mem brane at any distance, such receiving membrane will reproduce the sound. This capacity of a single vibrating membrane to reproduce the most complicated sounds, as of speech, is in reality the greatest mystery connected with the mat ter; all else relates to the mechanism of transmission only. The essential na ture of the operation is well shown in the common toy telephone sold in the streets, in which the floors of two small tin cups consist of stretched membranes, or even of paper. The two membranes are con nected by a long piece of twine. If now one cup be held to the mouth and spoken into, the voice communicates vibrations to the membrane. The stretched twine communicates similar vibrations to the membrane of the other cup, and if its cavity be held to the ear the sounds will be heard. This is the true mechanical telephone. The term is more commonly applied to the electrical telephonic ap paratus so much used in modern life, but the principle is precisely similar. Such apparatus generally belongs to one of two main classes.
The true inventor of the first telephone was undoubtedly Philip Reis, who showed, in 1861, that variations in an electric cur rent caused by a vibrating membrane could reproduce the necessary vibrations. Reis in this way transmitted musical sounds and even words; but his ap paratus was imperfect, and it was re served for Alexander Graham Bell to per fect that which is still commonly used and known as the Bell telephone, though it is the nearly unanimous opinion of electricians that Bell's patent has been held by courts of law to cover far more ground than is really due to him, much to the public detriment and to the hin drance of progress. In Bell's telephone there is a cylindrical steel magnet, sur rounded at one end by a coil of wire,whose ends are connected by wire with the cir cuit or line wire. It will now be under stood that any change in the power of the magnet will cause currents in this wire. Near, but not touching, the .magnet's end is stretched a very thin sheet of iron, as a membrane, which is spoken to through the mouthpiece. Thus made to vibrate, the iron membrane approaches to and recedes from the magnet; and as it acts toward this as an armature, tending to close the magnetic circuit, the effect is to produce fluctuating degrees of free magnetism, which again produce fluctu ating or undulating currents in the line wire. But if these fluctuating currents
are received in a precisely similar instru ment, they in its coil produce variable magnetic force in the magnet, and this reproduces vibrations in the second iron membrane, which reproduce the sound. The • second class of instruments are based on the microphone. If part of a galvanic current is composed of two or three pieces of matter (preferably charcoal) in loose contact, variations in the current produce variations in the con tact pressure of the loose pieces, and the converse. Hence, instead of a vibrating membrane causing undulating currents by means of a magnet as in the Bell method, it may abut against such a series of mere contacts, and thus cause an un dulating or variable current, which again is capable of converse action. A micro phone is thus capable, with more or less modification, of being used as a tele phone, and the employment of either method is a question of practical con ditions. The Bell telephone is indepen dent of any battery, being self-acting; but its feeble currents are incapable of transmitting speech to a distance; hence most of the modifications in magnetic telephones have had the design of in creasing the power, as by using both poles of the magnet, and in other ways. The microphone, on the other hand, uses the power of a battery in its circuit, but in some respects appears less delicately sensitive than the free membrane. In practice it is very general to employ some form of microphone as the transmitting or speaking instrument, and the Bell tele phone, or one of its modifications, as the receiving or hearing instrument.
There are now many forms of tele phone in use, the principal varieties being the bi-telephone, in which there are two receivers, one for each ear ; the capillary telephone, in which electro-capillarity is used to produce telephonic effects; the chemical telephone, in which chemical or electrolytic action is utilized; the elec trostatic telephone, which utilizes elec trostatic disturbances in the reproduc tion of sound; the reaction telephone, in which two mutually reacting coils are used; and the thermoelectric telephone, in which a thermoelectric battery is used. The last-named telephone has never been used in practice. In 1892 a long-distance telephone was erected be tween Chicago and the larger E. cities and such service has been extended everywhere. In 1902 a patent was ob tained in the United States for a wire less telephone service. De Forrest and others have since filed many patents. In 1915 the American Telegraph and Tele phone Co. sent a message from Washing ton to Paris without wires and subse quently from Washington to Hawaii. Portable telephones were largely used in the World War.
See WIRELESS TELEGRAPHY AND TELE PHONY.