THE ENGINEER
DEC. 6, 1878
EDISON’S TELEPHONIC AND
ACOUSTIC RESEARCHES.
(Continued)
THE MICROPHONE.
The device of using several pieces of the semi-conductor instead of one was early tried by Mr. Edison. He found, in general, that the loudness of the sound was increased by thus multiplying the number of contact surfaces, but also that the articulation was impaired. Instruments of this nature have since become known as microphones, though it is not probable that faint sounds were ever augmented through their agency so that they could easily be recognised at a distance from their source.
Fig.9 shows one of the first forms invented by Mr. Edison, April 1st, 1877. Four pieces of charcoal are used, C C, &c., each supported by an upright spring, as at S and S’. The piece of charcoal nearest the diaphragm impinges upon a disc of carbon, which is fastened to the center of the diaphragm. The primary wires of an induction coil are attached to the diaphragm and the spring S. The circuit is then completed through the semi-conductors. Other forms are shown in Figs. 10 and 11. The former has two carbons separated by a plate of metal. The latter has three contiguous pieces of carbon. Fig.12 (devised Sept. 21, 1877) illustrates a microphone, having ten plates of zinc, a mixture of dextrine and lamp-black having been previously worked into the pores.
In Fig.13 (devised June 7, 1877) fifty discs, D, with iron protoxidised on the surface, are shown enclosed in a glass tube.
A novel form of transmitter, used by Mr. Edison in his experiments, is shown in Fig.14 (devised Aug. 12,1877). The semi-conductor is a collection of small fragments of cork covered with plumbago. It can be used with or without a diaphragm. The instrument shown in Fig.15 (devised Aug. 24, 1877) acts both as transmitter and receiver, the latter fact being discovered by Mr. Charles Batchelor, Mr. Edison’s assistant. The solid carbon of the transmitter is here replaced by silk fibres covered with graphite. Its action as a receiver is probably due to the attraction of parallel currents, the volume of the whole being contracted during the passage of a current through F.
In the accounts which have been published of experiments with the microphone the statement has frequently been made that minute sounds are actually magnified by it, in the same sense that minute objects are magnified by the microscope. A little reflection will show, however, that there is no real analogy in the action of the two instruments. The sound that is heard in the receiving instrument of the microphone when a fly is walking across the board on which the transmitter is placed is not the sound of the fly’s footsteps, any more than the stroke of an electric bell, or sounder, is the magnified sound of the operator’s fingers tapping lightly, and it may be, inaudibly on the key. This view of the subject readily explains why the microphone has failed to realize the expectations of many persons who, upon its first exhibition, enthusiastically announced that by its aid we should be able to hear many sounds in nature which hitherto remained wholly inaudible.
SHORT CIRCUITING TELEPHONES.
A number of the telephones invented by Mr. Edison may be classed together as short circuiting, or cut out telephones. The principle on which they act might thus be briefly stated:- In vibrating the diaphragm cuts from the circuit resistances, which are proportional to the amplitude of the vibrations. A transmitter constructed upon this principle is shown in Fig.16 (devised March 20, 1877). A lever, L, of metal, vibrating in a vertical plane, rests at one end upon a strip of carbonised silk, C, which is part of the primary circuit of the induction coil, I.
In the course of its vibrations the lever cuts from the circuit parts of the silk, the current passing temporarily through the lever. Another, acting on the same principle, but differing considerably in construction, is shown in Fig.17 (devised August 21,1877).
A fine wire, W, of high resistance, is wrapped around a cylinder in a spiral groove. The wire forms part of the primary circuit of the coil, C. A spring, S, of metal, in the form of an ellipse, is fastened at one side to the diaphragm, while the other side presses against the uninsulated wire upon the cylinder. The diaphragm in moving towards the right flattens the spring, making it impinge on a greater number of convolutions than it would if the motion were in the opposite direction. The resistance of the circuit depends, therefore, upon the position of the center of the diaphragm. The disadvantage of this arrangement is that either a whole convolution or none at all is suppressed from the circuit, rendering the current rather more intermittent than pulsatory.
In Fig.18 (devised October 21, 1877), a similar spring rests upon a narrow strip of metal on the surface of a glass plate. The film is shown in perspective at F, and consists of a fine strip of the silvered surface of a mirror, the rest of the burnished metal having been removed. The action of this instrument is similar to that of the instrument shown in Fig.16.
Still another form of short circuiting telephone is shown in Fig.19 (devised November 1, 1877). A spiral spring, W, is wrapped around a cylinder, the diaphragm pressing against the last turn, so that in vibrating the convolutions approach or recede from each other. A very slight motion of the diaphragm is sufficient to cause the first few coils to come together; and, in general, the number of coils that thus touch each other is dependent upon the amplitude of the diaphragm’s motion. The wire is included in the primary circuit of an induction coil, so that the resistance of the circuit fluctuates as the diaphragm vibrates. This wire has also been used as the primary of the induction coil itself with better results.
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