heroes of the telegraph-第45节
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ng his researches； he soon found that he had discovered a prin… ciple of wide application； and that it was not necessary to confine his experiments to wires； since any substance which conducted an electric current would answer the purpose。 All that was necessary was that the materials employed should be in contact with each other under a slight but definite pressure； and； for the continuance of the effects； that the materials should not oxidise in air so as to foul the contact。 For different materials a different degree of pressure gives the best results； and for different sounds to be transmitted a different degree of pressure is required。 Any loose； crazy unstable structure； of conducting bodies； inserted in a telephone circuit； will act as a microphone。 Such； for example； as a glass tube filled with lead…shot or black oxide of iron； or 'white bronze' powder under pressure； a metal watch…chain piled in a heap。 Surfaces of platinum； gold； or even iron； pressed lightly together give excellent results。 Three French nails； two parallel beneath and one laid across them； or better still a log… hut of French nails； make a perfect transmitter of audible sounds； and a good microphone。 Because of its cheapness； its conducting power； and its non…oxidisability； carbon is the most select material。 A piece of charcoal no bigger than a pin's head is quite sufficient to produce articulate speech。 Gas…carbon operates admirably； but the best carbon is that known as willow…charcoal； used by artists in sketching； and when this is impregnated with minute globules of mercury by heating it white… hot and quenching it in liquid mercury； it is in a highly sensitive microphonic condition。 The same kind of charcoal permeated by platinum； tin； zinc； or other unoxidisable metal is also very suitable； and it is a significant fact that the most resonant woods； such as pine； poplar； and willow； yield the charcoals best adapted for the microphone。 Professor Hughes' experimental apparatus is of an amusingly simple description。 He has no laboratory at home； and all his experiments were made in the drawing…room。 His first microphones were formed of bits of carbon and scraps of metal； mounted on slips of match…boxes by means of sealing…wax； and the resonance pipes on which they were placed to reinforce the effect of minute sounds； were nothing more than children's toy money boxes； price one halfpenny； having one of the ends knocked out。 With such childish and worthless materials he has conquered Nature in her strongholds； and shown how great discoveries can be made。 The microphone is a striking illustration of the truth that in science any phenomenon whatever may be rendered useful。 The trouble of one generation of scientists may be turned to the honour and service of the next。 Electricians have long had sore reasons for regarding a 'bad contact' as an unmitigated nuisance； the instrument of the evil one； with no conceivable good in it； and no conceivable purpose except to annoy and tempt them into wickedness and an expression of hearty but ignominious emotion。 Professor Hughes； however； has with a wizard's power transformed this electrician's bane into a professional glory and a public boon。 Verily there is a soul of virtue in things evil。
The commonest and at the same time one of the most sensitive forms of the instrument is called the 'pencil microphone；' from the pencil or crayon of carbon which forms the principal part of it。 This pencil may be of mercurialised charcoal； but the ordinary gas…carbon； which incrusts the interior of the retorts in gas…works； is usually employed。 The crayon is supported in an upright position by two little brackets of carbon； hollowed out so as to receive the pointed ends in shallow cups。 The weight of the crayon suffices to give the required pressure on the contacts； both upper and lower； for the upper end of the Pencil should lean against the inner wall of the cup in the upper bracket。 The brackets are fixed to an upright board of light； dry； resonant pine… wood； let into a solid base of the same timber。 The baseboard is with advantage borne by four rounded india…rubber feet； which insulate it from the table on which it may be placed。 To connect the microphone up for use； a small voltaic battery； say three cells （though a single cell will give surprising results）； and a Bell speaking telephone are necessary。 A wire is led from one of the carbon brackets to one pole of the battery； and another wire is led from the other bracket to one terminal screw of the telephone； and the circuit is completed by a wire from the other terminal of the telephone to the other pole of the battery。 If now the slightest mechanical jar be given to the wooden frame of the microphone； to the table； or even to the walls of the room in which the experiment takes place； a corresponding noise will be heard in the microphone。 By this delicate arrangement we can play the eavesdropper on those insensible vibrations in the midst of which we exist。 If a feather or a camel…hair pencil be stroked along the base… board； we hear a harsh grating sound； if a pin be laid upon it； we hear a blow like a blacksmith's hammer； and； more astonishing than all； if a fly walk across it we hear it tramping like a charger； and even its peculiar cry； which has been likened； with some allowance for imagination； to the snorting of an elephant。 Moreover it should not be forgotten that the wires connecting up the telephone may be lengthened to any desired extent； so that； in the words of Professor Hughes； 'the beating of a pulse； the tick of a watch； the tramp of a fly can then be heard at least a hundred miles from the source of sound。' If we whisper or speak distinctly in a monotone to the pencil； our words will be heard in the telephone； but with this defect； that the TIMBRE or quality is； in this particular form of the instrument； apt to be lost； making it difficult to recognise the speaker's voice。 But although a single pencil microphone will under favourable circumstances transmit these varied sounds； the best effect for each kind of sound is obtained by one specially adjusted。 There is one pressure best adapted for minute sounds； another for speech； and a third for louder sounds。 A simple spring arrangement for adjusting the pressure of the contacts is therefore an advantage； and it can easily be applied to a microphone formed of a small rod of carbon pivoted at its middle； with one end resting on a block or anvil of carbon underneath。 The contact between the rod and the block in this 'hammer…and…anvil' form is； of course； the portion which is sensitive to sound。
The microphone is a discovery as well as an invention； and the true explanation of its action is as yet merely an hypothesis。 It is supposed that the vibrations put the carbons in a tremor and cause them to approach more or less nearly； thus closing or opening the breach between them； which is； as it were； the floodgate of the current。
The applications of the microphone were soon of great importance。 Dr。 B。 W。 Richardson succeeded in fitting it for auscultation of the heart and lungs； while Sir Henry Thompson has effectively used it in those surgical operations； such as probing wounds for bullets or fragments of bone； in which the surgeon has hitherto relied entirely on his delicacy of touch for detecting the jar of the probe on the foreign body。 There can be no doubt that in the science of physiology； in the art of surgery； and in many other walks of life； the microphone has proved a valuable aid。
Professor Hughes communicated his results to the Royal Society in the early part of 1878； and generously gave the microphone to the world。 For his own sake it would perhaps have been better had he patented and thus protected it； for Mr。 Edison； recognising it as a rival to his carbon…transmitter； then a valuable property； claimed it as an infringement of his patents and charged him with plagiarism。 A spirited controversy arose； and several bitter lawsuits were the consequence； in none of which； however； Professor Hughes took part； as they were only commercial trials。 It was clearly shown that Clerac； and not Edison； had been the first to utilise the variable resistance of powdered carbon or plumbage under pressure； a property on which the Edison transmitter was founded； and that Hughes had discovered a much wider principle； which embraced not only the so…called 'semi…conducting' bodies； such as carbon； but even the best conductors； such as gold； silver； and other metals。 This principle was not a mere variation of electrical conductivity in a mass of material brought about by compression； but a mysterious variation in some unknown way of the strength of an electric current in traversing a loose joint or contact between two conductors。 This discovery of Hughes really shed a light on the behaviour of Edison's own transmitter； whose action he had until then misunderstood。 It was now seen that the particles of carbon dust in contact which formed the button were a congeries of minute micro… phones。 Again it was proved that the diaphragm or tympanum to receive the impression of the sound and convey it to the carbon button； on which Edison had laid considerable stress； was non…essential；
