132 



SCIENTIFIC NE^VS. 



[Aug. 1st, 1887. 



pitch to those which it tends to produce if struck or other- 

 wise agitated. 



There is one shape which has practically no tendency to 

 vibrate to a particular sound or sounds ; it is equally ready 

 to take up all kinds of vibrations, and is the exact form 

 of that diaphragm or tympanic membrane of the human ear 

 previously mentioned, viz., a disc fixed round its circum- 

 ference or edge ; free everywhere else, but stretched and 

 flat. When such a disc is exposed to aerial vibrations, it 

 takes up the vibrations in the form of excursions to and fro, 

 which are greatest at its centre and smallest at its edge. 



It can be understood, then, that when such a disc is ex- 

 posed to a succession of varying sounds, such as a musical 

 air, or to spoken words, its vibrations change in conformity 

 with the changes in the sound waves, and correspond 

 to them. 



It would not be strictly accurate to say that a diaphragm 

 has no vibration period of its own, or that it responds with 

 equal readiness to all kinds and rates of vibration, but 

 practically its defects may be disregarded in considering the 

 main action. It may be further noted that a circular shape 

 is not essential to the practical working of a telephonic dia- 

 phragm, though it is probably the best form. 



Now, then, the problem of telephony, or the transmission 

 of sound to a distance, may be stated as the problem of 

 causing a diaphragm performing vibrations at one place, to 

 produce similar motions in a distant diaphragm. It has to 

 be remembered that the vibrations are extremely small in 

 amount and extremely rapid, and that they represent a very 

 small amount of energy ; also that failure to transmit very 

 slight changes in the rates and characters of the vibrations 

 means failure to reproduce exactly the niceties of articulation, 

 and consequently produces indistinct or unintelligible 

 speaking. 



This being the case, it follows that our medium of trans- 

 mission must be, so to speak, very free from friction and 

 inertia, because friction means dissipation of some of the 

 originally small amount of energy, and inertia means failure 

 to respond to the small changes in the vibrations. The first 

 medium that suggests itself is a mechanical one. If two 

 diaphragms are connected in the centre by a rod, wire, or 

 string, it seems natural that motions communicated to one 

 will be transmitted to the other. This is so, and most 

 people have seen and used the old " lovers' telephone," of 

 which perhaps the simplest form is two lobster or con- 

 densed milk tins, each with a hole punched in the centre of 

 the bottom, and a string knotted inside each and stretched 

 between them. Such mechanical telephones answer fairly 

 well for short distances, but the friction of the supports, and 

 the inertia and extensibility of the wire or string, render the 

 mechanical telephone useless over distances much exceeding 

 half a mile, even when straight runs can be obtained. 



Twenty-six years ago, Farrar, and three years later Bour- 

 seul, suggested that electricity might conceivably be used 

 for the transmission of sound and speech. Reis, in 1861, 

 set himself the task of experimentally proving this possi- 

 bility. He went about the work in the most scientific way, 

 studying the construction of the human ear, and endeavour- 

 ing to adapt in his instruments the lessons learnt from it. 

 Reis made instruments which actually transmitted sound, and 

 a receiver which was quite capable of reproducing speech. 

 The weak point was the transmitter. There can be little or 

 no doubt, that with sufficiently careful adjustment, his trans- 

 mitter could, and did, transmit speech. However that may 

 be, his labours had no immediate practical or commercial 

 result. Had Reis been able to inspire a capitalist with 

 some faith and imagination, the telephone industry might 

 to-day be one of twenty-five years' standing. But this is 

 touching on thorny ground, which we must avoid, as these 



remarks are not written with a controversial aim. The 

 telephone as a commercial and practical instrument is 

 associated with the name of Graham Bell, who, in 1876, 

 invented the instrument, which, in one form or another, 

 is the most widely used telephone receiver of the present 

 day. It has the remarkable property of being available 

 either as transmitter or receiver, and it is this feature which 

 constitutes its chief beauty, scientifically speaking, although 

 practically, it is very little used as a transmitter. 



Before this instrument is described, it is necessary to 

 give a brief account of the electrical laws upon which its 

 action is based, for we have to see how the transmitter 

 receiving sound waves, produces, or varies, an electric 

 current in the conducting line and the distant receiver, and 

 how that varying current reproduces in the receiving 

 diaphragm vibrations similar to those performed by the 

 sending diaphragm. The electrical laws involved are those 

 expressing certain relations between electric currents and 

 magnets, or magnetic fields (a magnetic field is the space 

 in which a magnet or an electrical current has a magnetic 

 influence). 



Briefly put, these laws may be thus stated : — 

 1st. An electric current can be set up in any electrical 

 circuit, by any change in the strength or direction of the 

 magnetic field in which any part of the circuit happens to 

 lie, provided that such change has the effect of causing a 

 larger or smaller number of " magnetic lines of force " to 

 be included in the circuit. The current will continue while 



Fig. 2.— Reis's Telephone. 



the change takes place, and will be stronger or weaker, in 

 exact proportion to the rate of the change, while its direction 

 will depend upon the nature of the change, i.e., currents in 

 reverse directions will be produced by increasing and 

 decreasing the number of lines of force included in the 

 circuit. 



2nd. An electrical conductor carrying a current produces 

 a magnetic field in its neighbourhood, and a piece of iron 

 or steel placed near the conductor will be magnetised in a 

 direction at right angles to the conductor. The magnetisa- 

 tion will be (approximately) proportional to the strength ot 

 the current, and its direction will depend upon the direction 

 of the current. 



The Bell telephone, shown in section in figure 3, contains 

 a steel magnet. A; a coil of insulated copper wire, B, 

 wound on a bobbin for convenience, and surrounding one 

 end of the magnet ; and a diaphragm, or disc of thin sheet 

 iron, C. All these parts are fitted into a case of wood, 

 or similar material, forming a handle and mouthpiece. 

 It will be observed that the iron disc, C, is clamped round 

 its edge by the wooden mouthpiece, that it is mounted at 

 right angles to the length of the magnet, and that its centre 

 is close to the end of the magnet which carries the coil of 

 wire. 



The coil of wire forms part of an electrically conducting 

 circuit, the remainder of which consists of two-line wires, 

 or one-Une wire and the earth, and the coil in the distant 

 instrument, which is exactly like the one described. 



