Telephone. This instrument is designed to reproduce sounds at a distance by means of electricity. Sound is due to vibrations of the air, and these vibrations differ among themselves in, and are characterised by, three particulars—viz. frequency or number per second, amplitude or range of displacement, and co-existence of simultaneous vibrations affecting the same air. These physical data determine respectively the pitch, the loudness, and the character, quality, or timbre of a sound. The mechanical problem of telephony then is, given an air-vibration which presents these three characteristics in any determinate way, to reproduce a similar air-vibration at a distance; then this, being received by the ear, will be perceived as sound. Under the most complex vibration no particle of air can be in more than one place at once: hence the net result of superposition of the most complex possible series of vibrations, from single melody or smooth harmony to the most complicated noise or the most delicate inflexion of the human voice, is a movement of each air-particle which may be represented by a curve of an apparently irregular form, lying alternately above and below a straight line which threads the curve and represents the position of the air-particle when undisturbed. The reproduction of this irregular resultant motion is the most general form of the problem: the earliest attempts were confined to the reproduction of the frequency of a vibration—i.e. the pitch of a sound, the first of the particulars above mentioned. In 1860 Reis of Frankfurt first attacked the problem: he used a collodion membrane, upon which the sound-waves produced by the voice were made to impinge; this membrane, vibrating under their influence, alternately displaced and liberated a lever, which alternately made and broke the circuit of a galvanic battery; the intermittent current thus produced acted upon a distant electro-magnet which alternately attracted and failed to attract its armature; the armature thus oscillated, and a wing or plate attached to it acted upon the air, which was thus set in vibration, reproducing the pitch of the original tone or the inflection of the original voice. It appears from recent inquiry that Reis actually did more than this; but the next step is attributed to Mr Elisha Gray of Chicago, who sent successions of electrical current of varying strength as well as of varying frequency into the circuit, and thus enabled the relative loudness as well as the pitch of sounds to be transmitted; and who afterwards took the important step of using the variations of a steady current. These variations, positive and negative, are capable of representing all the back-and-fore variations of position of a particle of air, however irregular these may be: and he secured them by making the sound-waves set a diaphragm in vibration; this diaphragm carried a metallic point which dipped in dilute sulphuric acid; the deeper it dipped the less was the resistance to a current passing through the acid, and vice versa: so that every variation in the position of the diaphragm produced a corresponding variation in the intensity of the current: and the varying current acted upon a distant electro-magnet, which accordingly fluctuated in strength, and in its attraction for a piece of soft iron suspended on a flexible diaphragm: this piece of soft iron accordingly oscillated, pulling the flexible diaphragm with it; and the variations of pressure in the air acted upon by the diaphragm produced waves, reproducing the characteristics of the original sound-waves, and perceived by the ear as reproducing the original sound or voice. Mr Gray lodged a caveat for this contrivance in the United States Patent Office on 14th February 1876; but on the same day Professor Alexander Graham Bell filed a specification and drawings of the original Bell telephone. In this the flexible diaphragm upon which the sound-waves impinge bore a small bar-magnet, which was thus made to oscillate; it oscillated in the immediate neighbourhood of the pole of an electro-magnet round which passed the continuous current of a battery; the oscillation of the bar-magnet in the neighbourhood of the pole caused fluctuations in the strength of the electro-magnet; these fluctuations caused variations in the intensity of the current passing continuously round the electro-magnet, and away to the distant receiver; there the varying current found an electro-magnet to act upon, and this exerted varying tension upon a soft-iron diaphragm. Later on Bell disclaimed the use of a non-metallic flexible diaphragm in the transmitter, and used only a soft-iron diaphragm; and he also used a permanent magnet instead of an electro-magnet in that instrument, it being found that currents of sufficient intensity were produced thereby. He also used similar instruments at both ends of the line. For these two later modifications priority is claimed on behalf of Professor Dolbear. Mr Edison secured the requisite variations of the current in another way. He found that lamp-black and certain other substances could be prepared in masses which were slightly compressible, resilient, and semi-conductive, and which varied in conductivity as they became exposed to greater or smaller pressures; and that the variations in conductivity were very satisfactorily proportional to the variations of pressure. The vibrations of the diaphragm were accordingly made by him to concentrate themselves upon a small button of such a substance, and thus to induce variations in the conductivity of the circuit.
At the present time instruments of the Bell telephone type are mainly used as receivers only. In the Bell receiver itself both poles of the magnet are now brought into proximity to the magnet; and the instrument may be flattened down into the form of a watch. In the Gower receiver the form is also watch-shaped, and the sound is conveyed to the ear by tubes. In the Ader receiver the electro-magnetic field acting upon the ferrotypic iron diaphragm is improved, being rendered more intense and more uniform by a ring of soft iron placed outside the diaphragm. In other receivers devices are applied for economising wire, for decreasing the magnetic resistance of the magnetic circuit, for facilitating construction and regulation, &c. The transmitter is now almost always some modification of the Microphone (q.v.). Edison's instrument, above described, is in substance a microphone; it is, however, not operated by shaking the whole apparatus as in the original Hughes microphone, but by bringing variations of pressure to bear upon the carbon button. In the Blake transmitter the same principle is applied; the current of a battery passes through the button, and is there made to vary, and it also passes through the primary winding of an induction coil: the variations are taken cognisance of by the secondary winding which transmits them, adequately intensified, to the general circuit. The Crossley, the Gower, and the Ader transmitters are more obviously adaptations of the Hughes microphone, with eight, sixteen, and twenty loose contacts respectively instead of two, and attached to the under side of the vibrating diaphragm, which in the Gower and the Crossley is a thin sheet of wood. Other types are the Johnson, which is provided with a shunt so as to adjust the current transmitted, and thus prevent buzzing; the Hummings, in which powdered coke is used; the Swinton, which depends on swinging carbon rods resting by loose contacts against a carbon bar, there being thus no diaphragm in the popular sense, though the English courts have decided that the carbon rods are mechanically equivalent to a diaphragm; and Thompson's valve transmitter, in which a ball of carbon rested upon three carbon buttons and was impinged upon by air-waves from beneath.
The word 'telephone' is now usually applied to the complete instrument, comprising the transmitter, the receiver, and the various subsidiary appliances, such as switch bells worked by a battery or by a small magneto-electric machine, driven by a crank for signalling purposes. The arrangements which have to be devised for the purposes of a central telephone station increase rapidly in complexity with the number of subscribers. For these, and for the means adopted to minimise the effects of induction from neighbouring wires, &c., the reader may consult Allsop's Telephones (1891); Preece and Maier, The Telephone (1889); and Preece's Manual of Telephony (1893).
Of the five leading telephone patents for Great Britain, the last, the Crossley transmitter, was to expire 1st February 1893. In 1880 (21st December) Mr Justice Stephen decided in favour of the British Post-office that telephony was a form of telegraphy. Since that decision telephony has been carried on in Britain under the Postmaster-general's license, on payment of one-tenth of the gross receipts. In 1891 this tenth amounted to £40,000, in 1898 to £100,000. France, Germany, Austria, and the Continent generally are well supplied with trunk lines; and in England most of the chief towns are connected directly with London, and there is an increasing number of other lines. The first London-Paris line was opened in March 1891. In Berlin the system of laying wires underground was very thoroughly developed. On the Continent the telephone is largely used by the rural post-offices; and in France any person may telephone from a call-office to a post-office, and his verbal message will be transcribed and sent on as a telegram. In the United States the microphonic transmitter patent only began to run its course of seventeen years in 1891, on account of a dispute as to priority between Edison and Berliner. In the United States the telephone is very extensively employed, and the distances run are great, up to 1000 miles, with specially thick wire.
In 1892 the Telegraphic Act gave the Post-office entire control of the trunk wires throughout the kingdom. The intention was to provide a complete system of telephonic communication between the important towns, leaving the towns themselves to be dealt with by individual companies. The National Telephone Company, using mostly Blake transmitters and Bell receivers, gradually absorbed the other companies, obtaining a practical monopoly of the work; and it secured a license from the Postmaster-general, which expires in 1911. In 1898 it had 918 exchanges and 1424 call-offices in the United Kingdom. Dissatisfaction having been expressed as to the imperfect service of the National Company, the government in 1898 appointed a select committee, which reported that no system can be satisfactory which is worked under a practical monopoly by a private company for private profit, and that the work should be carried on like the Post-office, for the benefit of the whole country, and not only certain localities and limited areas. They recommended general competition, either by the Post-office or by municipalities acting under license, but favoured the former as promising a more general benefit. The proposed legislation on these lines (1899), to give facilities for extending municipal licenses, was opposed on the ground that the only proper remedy is for the Post-office to take over the whole work at once.