Before describing the construction of various forms of telephone, it is necessary to explain the principles underlying its mode of action. The sensation felt in the organ of hearing, and known as a "sound," is due to waves or vibrations in the air acting upon the tympanum of the ear. In this transmission of sound, the particles of air or other conductor are not transported, but the vibration of one particle is communicated to the next, and so on, the intensity becoming less as the distance increases. This constitutes the main fault of the so - called "string telephone," the earliest and simplest form of apparatus for communicating speech - its range is limited. The telephone proper differs from other instruments of a like class, in that it reproduces instead of merely conveying vibrations, and has thus a practically unlimited range.

Forms

Various forms of telephonic apparatus may now be discussed.

(1) String Telephone

This is formed of 2 metallic or cardboard tubes, in the shape of a cylindrical cone; one end is closed by a tightly - stretched parchment membrane, in whose centre the string intended to connect the 2 cylinders is fastened by a knot. When 2 such tubes are thus united, and the cord is tightly stretched, words may be conveyed along it by the speaker placing the opening of one tube to his month, and the listener putting his in the same way to his ear. The distance which may thus be traversed does not exceed 170 yd. The best results are said to be got from silken cord and the worst from hempen; cords of plaited cotton are generally used for economy. Some preference is given to nickel silver as the material for the mouth - pieces. Several modifications hare been proposed. Millar ascertained that by means of a telegraphic wire, stretched and connected by 2 copper wires with 2 vibrating disks, musical sounds might be conveyed to a distance exceeding 160 yd., and that by stretching these wires through a house, and connecting them with mouth - and - ear holes in different rooms,xommunicarions between them became perfectly easy. For the vibrating disks he employed wood, metal, or gutta-percha, in the form of a drum, with wives fixed in the centre.

The sound seems to become more intense in proportion to the thickness of the wire. Heaviside and Nixon ascertained that the most effective wire was No. 4 B.W.G. They employed wooden disks 1/2in. thick,and these may be placed in any part of the length of the wire. When the wire was well stretched and motionless, it was possible to hear what was said at a distance of 230 yd., and it seems that Huntley, by using very thin iron diaphragms, and by insulating the line wire on glass sup - ports, was able to transmit speech for 2450 ft., in spite of the zigzags made by the line on its supports.

(2) Kennedy's Telephone (Figs. 106, 107) is of the "Bell" type, but differs in its principle of action and in construction. No coils of wire are used on the magnets; but 2 coils are used one on each side of the ferrotype plate, the wooden case being turned out so as to form 2 spaces for the wire,also to grip the plate, leaving 2 in. of the centre of the plate for vibrating; the plate is 4 in. to 6 in. diameter; 2 to 4 oz. of No. 30 wire may be used in each space. The action of the telephone is this:- On current passing through the coils, the plate is magnetised, with its centre a S. pole, say; the plate is therefore drawn inwards. On a reverse current passing, the centre of the plate is magnetized as a N. pole; the centre of the plate is therefore repelled outwards - that is the action when it is used as a receiver of alternately reversed currents, such as produced in the Bell telephone or in the carbon telephone used with an induction coil as a transmitter. It may be used as receiver or transmitter just the same as Bell's telephone.

A pair of them tried on the same circuit with a pair of Bell's, spoke much louder, and, what is of more importance, the articulation was much more distinct.

(3) Thompson's

Prof. S. P. Thompson's improvements relate to telephonic transmitters based upon the principle discovered by Reis, of employing current - regulators actuated, directly or indirect - ly, by the sound - waves produced by the voice. By "current - regulator "is meant a device similar to that employed by Reis, wherein a loose contact between 2 parts of a circuit (in which are included a battery and a telephonic receiver) offers greater or less resistance to the flow of the electric current, the degree of intimacy of contact between the conducting pieces being altered by the vibrations of the voice. In Reis's transmitter} in Edison's, and in other well - known forms of this instrument, the action is indirect, a tympanum of stretched membrane or other equivalent organ, such as a diaphragm of mica, being used to collect, magnify, or concentrate the vibrations of the voice, and to convey them to the points of loose contact that regulate the current. In other forms of transmitter (for example, some of Hughes's microphones), the mode of action is direct, the air - waves beating directly upon the conductors or electrodes which are in loose contact, without the intermediation of a tympanum or diaphragm.

Of these 2 classes of telephone transmitters - viz. those in which the current - regulator is combined with a tympanum or diaphragm, and those in which the current - regulator is acted upon directly by the air - waves of the voice - the improvements relate to the latter only, as Prof. Thompson dispenses with the membrane tympanum used by Reis, the tympanic diaphragm of mica used by Edison, and does not even employ any diaphragm in the sense of a partition between the current - regulator and the air - waves of the voice, as in Theiler's transmitter. In Thompson's transmitters the air - waves act directly upon the current - regulator itself. As a result, the articulation is clearer for some of the consonantal sounds, which are only imperfectly or difficultly transmitted by telephones in which the current - regulator is affected indirectly through a tympanum, diaphragm, or partition. Transmitters of the class to which the improvements relate are ordinarily liable to 2 defects. Firstly, they do not articulate so loudly as transmitters in which there is a tympanum or diaphragm to collect or magnify the vibrations.