yune(i, 1878] 



NATURE 



161 



break key of so delicate a constniction that the sound-waves in 

 the air are able to work it. 



The air vibrations set in motion a stretched membrane like a 

 drumhead, with a piece of platinum fastened to it. This piece 

 of platimrm, when vibrating, strikes against another piece of 

 platinum, and so completes the circuit every time contact is 

 made. 



At every point of the circuit there is thus a series of currents 

 corresponding in number to the vibrations of the drumhead, and 

 by causing these to pass through the coil of an electromagnet, 

 the armature of the electromagnet is attracted every time the 

 current passes, and if the annature is attached to a resonator of 

 any kind, the succession of tugs will set it in vibration, and 

 cause it to emit a sound, the pitch of which is the same as that 

 of the note sung into the transmitter at the other end of the 

 line, 



[Mr. Gower here played the " March of the Men of Harlech " 

 on the telephone harp placed in the Geological Museum. The 

 instrument consists of a set of steel reeds worked by percussion, 

 which make and break contact on the battery circuit, of which 

 the primary wire of an induction coil forms part. The receivers 

 are worked by the secondary current There were four receivers, 

 one of them Prof. Bell's original one, placed in different parts of 

 the Senate-house.] 



If the pitch of a sound were the only quality which we are able 

 to distinguish, the problem of telephony would have received its 

 complete solution in the instrument of Reis. But the human 

 ear is so constructed, and we ourselves are so trained by con- 

 tinual practice, that we recognise distinctions in sound of a far 

 more subtle character than that of pitch ; and these finer dis- 

 tinctions have become so much more important for the purposes 

 of human intercourse than the musical distinction of pitch, that 

 many persons can detect the slightest variation in the pronuncia- 

 tion of a word who are comparatively indifferent to the variations 

 of a melody. 



Now, the telephone of I'rof. Graham Bell is an articulating 

 telephone, which can transmit not only melodies sung to it, but 

 ordinary speech, and that so faithfully that we can often recog- 

 nise the speaker by his voice as heard through the telephone. 

 How is this effected ? It is manifest that if by any means we can 

 cause the tinned plate of the receiving instrument to vibrate in 

 precisely the same manner as that of the transmitter, the impres- 

 sion on the ear will be exactly the same as if it had been placed 

 at the back of the plate of the transmitter, and the words will be 

 heard as if spoken at the other side of a tinned plate. 



But this implies an exact correspondence, not only in the 

 number of vibrations, but in the type of each vibration. 



Now, if the electrical part of the process consisted merely of 

 alternations between current and no current, the receiving instru- 

 ment could never elicit from it the semblance of articulate speech. 

 If the alternations were sufficiently regular, they would produce 

 a sound of a recognisable pitch, which would be very rough music 

 if the pitch were low, but might be less unendurable if the pitch 

 were high ; still, at the best, it would be like playing a violin 

 with a saw instead of a bow. 



What we want is not a sudden starting and stopping of the 

 current, but a continuous rise and fall of the current, correspond- 

 ing in every gradation and inflexion to the motion of the air 

 agitated ly the voice of the speaker. 



Prof, Graham Bell has recounted the many unsuccessful 

 attempts which he made to produce undulatory currents instead 

 of mere intermittent ones. He had, of course, to give up alto- 

 gether the method of making and breaking contact. Every 

 method involving impact of any kind, whether between electric 

 contact pieces or between the sounding parts of the instalment, 

 introduces discontinuity of motion, and therefore precludes a 

 faithful reproduction of speech. 



In the ultimate form which the telephone in his hands assumed, 

 the electric current is not merely regulated but actually generated 

 by the aerial vibrations themselves. 



The electric principle involved in Bell's telephone is that of 

 the induction of electric currents discovered by Faraday in 1831. 

 Faraday's ow n statement of this principle has been before the 

 scientific world for nearly half a century, but has never been im- 

 proved upon. 



Consider first a conducting circuit, that is to say, a wire which 

 after any number of convolutions returns into itself. Round such 

 a circuit an electric current may flow, and will flow if there is an 

 electromotive force to drive it. 



Consider next a line of magnetic force, such a line as you see 



here made visible by sprinkling iron filings on a sheet of parafiin 

 paper. This line, as Faraday also first showed, is a line return- 

 ing into itself, or, as the mathematicians would say, it is a closed 

 ciurve. 



Now, if there are two closed curves in space, they must either 

 embrace one another so as to be linked together, or they must 

 not embrace each other. 



If the line of force as well as the circuit were made of wire, 

 and if it embraced the copper circuit, it would be impossible to 

 unlink them without cutting one or other of the wires. But the 

 line of force is more like one of Milton's spirits, which cannot 



" In their liquid texture mortal wound 

 Receive, no more than can the fluid air." 



Now, if the copper circuit or the lines of force move relatively 

 to each other, then in general some of the lines of force which 

 originally embraced the circuit will cease to embrace it, or else 

 some of those which did not embrace it will become linked 

 with it. 



For every line of force which ceases to embrace the circuit 

 there is a certain amount of positive electromotive force, which, 

 if unopposed, will generate a current in the positive direction, and 

 for every new line which embraces the circuit there is a negative 

 electromotive force, causing a negative current. 



In Bell's telephone the circuit forms a coil round a small core 

 of soft iron fastened to the end of a steel magnet. Now lines of 

 magnetic force pass more freely through iron than through any 

 other substance. They will go out of their way in order to pass 

 through iron instead of air. Hence a large proportion of the 

 lines of force belonging to the magnet pass through the iron core, 

 and, therefore, through the coil, even though there is no iron 

 beyond the core, so that they have to complete their circuit 

 through air. 



But if another piece of soft iron is placed near the end of the 

 core it will afford greater facilities for lines which have passed 

 through the core to complete their circuit, and so the lines 

 belonging to the magnet will crowd still closer together to take 

 advantage of an easy passage through the core and the iron 

 beyond it. If then the iron is moved nearer to the core, there will 

 be an increase in the number of such lines, and, therefore, a 

 negative current in the circuit. If it is moved away there will be 

 a diminution in the number of lines, and a positive current in the 

 circuit. This principle was employed by Page in the construc- 

 tion of one of the earliest magneto -electric machines, but it was 

 reserved for Prof. Bell to discover that the vibrations of a tinned 

 iron plate, set in motion by the voice, would produce such 

 currents in the circuit as to set in motion a similar tinned plate 

 at the other end of the line. 



It will help us to appreciate the fertility of that germ of science 

 which Faraday.first detected and developed if we recollect thatyear 

 after year he had employed the powerful batteries and magnets and 

 delicate galvanometers of the Royal Institution to obtain evidence 

 of what he all along hoped to discover — the production of a current 

 in one circuit by a current in another, but all without success, 

 till at last he detected the induced current as a transient pheno- 

 menon, to be observed only at the instant of making or breaking 

 the primary circuit. 



In less than half a centtuy, and by the aid of no second Fara- 

 day, but in the course of the ordinary growth of scientific princi- 

 ple?, this germ, so barely caught by Faraday, has developed on 

 the one hand into the powerful ciurents which maintain the illu- 

 mination of the lighthouses on our coasts ; and on the other, into 

 these currents of the telephone which produce an audible effect, 

 though the engine that drives them is itself driven by the tremors 

 of a child's voice. 



Prof, Tait has recently measured the absolute strength of 

 these telephone currents. He produced them by means of a 

 tuning fork vibrating in front of the coil of the transmitter. 

 Before the transmitted note ceased to be audible at the other 

 end of the line he measured by means of a microscope the ampli- 

 tude of the vibrations of the fork. 



He then placed a very delicate galvanometer in the circuit and 

 found what deflexion was produced by a measiu-ed motion of the 

 fork. 



Finally he measured the deflection of the galvanometer pro- 

 duced by a small electromotive force of known magnitude. 

 He thus found that the telephone currents produced an audible 

 effect when reversed 500 times a second, though their strength 

 was no greater than what a Grove's cell would send through a 

 million megohms, about a thousand million times less than the 

 currents used in ordinary telegraphic work. ; 



