The microphone has been so named from its power of increasing sounds resulting from mechanical vibrations transmitted by solid substances, and thus rendering audible such ordinarily inaudible sounds as a fly's footsteps on the stand of the instrument. Its action is thus described by Prof. James Blyth.
In the microphone transmitter, as usually employed in circuit with a battery and a bell telephone, are essentially 2 pieces of carbon resting lightly against each other, through which the current passes. That the instrument may work effectively, 2 things are requisite: first, that the carbons be always in contact, or at least sufficiently near for the current to pass between them; and secondly, that they may not be pressed together so tightly as to prevent any motion of the one relatively to the other. This state of things is sufficiently well described by the term " loose' contact." To understand the action of the microphone, it is necessary to find out what effects are taking place at the loose contact when the instrument is acted upon by sonorous waves. These are twofold: first, the effect produced by the sound waves (that is, the variations of density due to the condensations and rarefactions of the air), which pass directly through the air when they arrive at the loose contact; - and secondly, the effect produced by tremors set up in the entire instrument, wooden supports, and carbons together, by the sound waves which strike against it and are thereby stopped.
For distinction, the first of these may be called the air effect, and the second the tremor effect. To isolate the air effect, it is obviously necessary either to fix the carbons rigidly in their supports, so as to avoid any motion of the one relatively to the other, or to use a strong current and place them just clear of contact with each other.
Fig. 85 illustrates how this may be done : a, 6, c are 3 blocks of brass firmly fixed to a heavy wooden sole - plate. To the top of a is soldered a piece of brass tube A, about 2 in. long and 5/8 in. bore. To the top of 6 is soldered a piece of similar tube A, about 4 in. long. Through c passes a fine screw s worked by a milled head m. A piece of carbon - rod e is fixed firmly into h, and has a hole J in. in diameter drilled through its centre. A long piece of carbon /, pointed at one end, passes tightly through the tube k, and can be moved backwards and forwards by the screw s. A piece of indiarubber tube l is passed over the left end of the tube A, and to this is attached a mouthpiece n. By means of the wires x and y soldered to the carbon rods, they are put in circuit with the battery d (20 Grove's cells) and the telephone t, which must either have a small resistance, or be placed in a separate circuit from that containing the battery, so as to be acted upon inductively.
When the carbon / is screwed tightly into the hollow of e, the circuit is completely closed, and no sound uttered into n is heard at t. But when / is drawn gradually back until small electric arcs are seen to pass between / and e, every sound uttered into n is loudly and distinctly reproduced in the telephone t. Here is clearly only the air effect acting, and that solely upon the small electric arcs passing the carbons. It is somewhat difficult to get the sounds to last for any length of time, in consequence of the arc distance soon getting too great for the current to pass, and requiring re - adjustment. When the arc begins and ends, a sharp click is heard in the telephone; but in the interval during which the arc lasts, the sounds are distinct.
As far as the tremor effect is concerned, it is obvious that the microphone action must depend either (1) upon the variation of resistance due to variation of pressure, or (2) to variation in the extent of surface contact due to the elastic yielding of the carbons under pressure.
To test the first of these causes, Prof. Blyth made experiments on the effect of pressure upon the specific resistance of carbon. For this purpose he took a short length of carbon rod, and soldered wires to it at a short distance from each end. By means of these wires the resistance of the carbon rod was balanced in the Wheatstone bridge. Pressure was then applied by means of a lever to the carbon in a longitudinal direction. No appreciable variation in the resistance was observed even under considerable pressure; and it only be came manifest when the pressure was sufficient to bend or crush the carbon. Similar experiments, with the same result, have been made by Prof. Thompson. Hence it can hardly be believed that variation of specific resistance due to pressure can have the slightest effect in producing the microphone action.
To test the second cause above mentioned - that is, the variation of resistance due to variation in the extent of surface contact due to elastic yielding under pressure - Prof. Blyth experimented as follows: - In the apparatus already described, he replaced the tubular carbon by a finely - pointed piece, so as to have 2 fine points exactly opposite each other. The resistance of the points was balanced in the bridge in the usual way. Pressure was then applied by a known number of turns or parts of a turn of the fine screw, and the change of resistance was noted. The screw was then brought back to former position, and the pressure relieved so as to allow the elasticity of the carbon to act and restore the points to their first condition. It is obvious that if the change of resistance were due merely to elastic yielding, it should now be the same as before. This was found not to be the case. From the gritty nature of the carbon, the points of contact were perpetually changing, and hence the variation, of resistance produced in this way obeyed no regular law.
From this irregularity it is impossible to conclude that this cause could explain the transmission of musical sounds, far less articulate speech.