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THE POPULAR EDUCATOR. 



carried to a telephone in a distant room, in which case the 

 ticking of it will b most plainly heard. Another favourite 

 experiment, and one which at the time of its publication made 

 somewhat of a popular excitement, is to listen to the foot- 

 falls of a fly. This can be managed thus : A match-box is 

 furnished with an opening, covered with fine muslin. In this 

 prison the fly is confined, and will naturally walk over every 

 part of it in the endeavour to find a loophole for escape. Its 

 " tramp," although quite inaudible to the unaided ear, is 

 plainly heard in the attached telephone. 



With a little trouble these sounds can be made audible to a 

 roomful of people. The telephone should be supported in this 

 case on an upright rod, and its end should be furnished with a 

 cone of cardboard like a huge trumpet-mouth. If the instru- 

 ment be carefully adjusted, the touch of a feather upon the 

 carbon block will be heard all over a large room, and will be 

 magnified into a loud rasping noise. 



But a good microphone will not only transmit noises, but will 

 make a most admirable carrier of the human voice indeed, 

 many most successful telephone transmitters are upon the 

 microphonic principle. One of the first recorded experiments 

 in this direction was described in a country newspaper, and 

 we cannot better demonstrate the capabilities of the instru- 

 ment in this respect than by quoting the paragraph referred to. 

 " Last Sunday a microphone was placed in the pulpit of a 

 chapel in this town, and connected by a private telegraph line 

 running by with the residence of a gentleman over a mile distant. 

 Every part of the service was distinctly heard at the gentle- 

 man's house, with the exception of a few words rendered 

 indistinct by the preacher becoming a little excited and 

 shaking the microphone, the presence of which he never 

 dreamt of, or he might have been somewhat nervous at the 

 idea that his sermon was being conveyed away he knew not 

 whither. So faithfully did the microphone do its work, that 

 the chapel- keeper was heard to close the doors after service, 

 walk up the aisle, and up the pulpit steps, in conversation with 

 some one else. During the week, experiments have been made 

 in the schoolroom of another chapel, and the singing of the 

 scholars was transmitted and retransmitted over a number of 

 telegraph lines with remarkable clearness. The idea is about 

 to be put to practical use, the gentleman already referred to 

 having given instructions that his house should be connected 

 with that of a gentleman near, in order that an invalid may 

 hear the service from one of the churches in the town." 



As already indicated, loose contact-points between metallic 

 conductors will give the microphonic phenomena, but the 

 carbon arrangements described are on the whole more satis- 

 factory, and certainly are more easily arranged. But recently, 

 Mr. J. Munro has been experimenting with metallic conductors, 

 and has obtained remarkable results. In fact, he has produced 

 on this principle first-class transmitting instruments. One of 

 these we will describe, for it can easily be made, and gives 

 capital results. It is called the " Lyre Telephone." It takes 

 its name from the fanciful shape of its frame, which is formed 

 of a U-shaped piece of mahogany. In the hollow centre of this 

 frame is stretched, by means of two wires at the top and 

 bottom, a square piece of metallic gauze. Immediately in front 

 of this fixed gauze plate is another one of slightly smaller size, 

 which is hinged at the bottom, and rests on the top against the 

 other one. The pressure between the two can be regulated by 

 a weight sliding upon a rod fixed at right angles to the 

 movable gauze plate. 



Another convenient form of microphone is that which acts 

 without an independent battery, for it represents a battery cell 

 in itself. It resembles in its arrangement that form described 

 above. But instead of there being two little scooped-out 

 blocks of carbon in which the ends of the pencil rest, as in 

 the first kind of microphone described, there is only one. This 

 supports the upper end of the carbon pencil, the lower end 

 resting upon a plate of carbon. Underneath this carbon plate, 

 which may be three or four inches square, is an amalgamated 

 plate of zinc, the two elements being separated by a piece of 

 blotting-paper which has been soaked in very dilute sulphuric 

 acid. The whole arrangement rests upon the base board, and 

 can readily be prepared for use by putting the wet blotting- 

 paper in position. 



When Professor Hughes brought his researches before the 

 Royal Society some years ago, he said that it was impossible to 



indicate the applications or effects of his discovery, because 

 the whole question had been studied with crude materials, and 

 scarcely sufficient time had elapsed to enable him to consider 

 its ultimate uses. As a telephone transmitter he did not claim 

 that the microphone was superior to others already in use, but 

 he considered it far more sensitive than any ho had heard, and 

 indicated that its power could be increased by multiplication 

 of transmitting contacts in quantity or intensity ; the loudness 

 being at present limited by the capability of the receiver. The 

 materials at his disposal, and the arrangement of them, had 

 not yet been sufficiently studied. He merely wished to show 

 that it was possible to transmit clear and intelligent articulate 

 speech, and to render audible sounds which hare hitherto been 

 inaudible, by the mere operation of sonorous vibrations upon 

 the conducting power of matter. With regard to his theory of 

 the microphone, he reminded his hearers that if sand be 

 sprinkled upon a membrane such as a drum-head it would 

 be jerked up and thrown away from that membrane under the 

 influence of sonorous vibrations. If by mechanical means it 

 were prevented from being jerked away, either by being con- 

 tained in a tube, or by being kept down by weight or pressure, 

 the tendency of the movement of the vibrating body would be 

 alternately to compress and relieve from pressure the substance 

 so held upon it, and in so doing vary its resistance. This 

 explanation was given merely as an analogy, and to illustrate 

 the explanation of the phenomena. 



We will now proceed to review some of the applications of 

 this most interesting discovery, when it will be readily seen 

 that the microphone is no mere toy, but is of practical import- 

 ance as an instrument of physical research. In certain surgical 

 operations, and notably in the case of bullet-wounds, it is of 

 first importance to ascertain the exact position of a foreign 

 body before proceeding to remove it. With an ordinary probe 

 this can sometimes be effected without much difficulty ; in 

 other cases the patient is put to much pain and inconvenience. 

 Now, if the searching-probe be furnished with a microphonic 

 attachment and a telephone receiver, the surgeon, or his 

 assistant, will hear a loud click in the instrument directly the 

 bullet is touched by the probe. Without the microphone it 

 can easily be passed over without recognition. 



We may here mention another form of probe for hidden 

 bullets, which was invented many years before the microphone. 

 It is effective ,in action, but its accessories of a battery and 

 electric bell, with connecting wires, are, perhaps, rather too 

 formidable for the sick-room. It consists of two wires, forming 

 part of the probe, their ends almost touching. Directly the 

 bullet is touched by these wires they are bridged over by the 

 metallic mass, and the circuit is completed, so that the bell 

 rings. 



To return to Professor Hughes' microphone. Hardly twelve 

 months had elapsed since he first made his discovery public, 

 when he handed to Dr. B W. Richardson a very clever and 

 useful little instrument:, founded upon its principle, which he 

 called the " Audiometer." Its use is to test the hearing of 

 any particular person. When we remember the enormous 

 prevalence of deafness even among one's circle of friends, 

 together with the very few cases where recovery is attained, 

 the importance of a means of measuring from day to day the 

 progress of the malady under medical treatment will be at 

 once conceded. The Audiometer places in the physician's 

 hands the means of doing this, for he can tell by it how many 

 degrees of hearing capability his patient has gained or lost 

 since his last visit. 



To describe this instrument is not so easy as to understand 

 its principle from a rapid glance at the thing itself, for it is most 

 simple in construction. A horizontal bar is furnished at each 

 end with a flat coil of wire, one containing about 6 yards and 

 the other 100 yards of wire. These coils are fixed in their 

 places. Between them, and sliding 1 on the bar, so that its 

 position can be readily changed, is another coil of 100 yards of 

 wire. The bar constitutes a scale, divided into 20 centimetres, 

 and each of these parts is again subdivided into 10, so that 

 the hearing can be tested from a maximum of 200 units to 

 zero. The sliding coil is really an induction coil, its ends 

 being connected with a telephone, which the patient holds to 

 his ear. The fixed coils are in connection with two Leclanche 

 cells, and a microphonic key that is to say, a little contrivance 

 which the operator can depress, and so make a microphonic 



