The invention of the microphone was a result of the development of the telephone. As a technology was developed to transmit sound over a distance as an electrical signal, methods of converting sound into electrical impulses were required. The very first microphones were bulky, awkward and performed poorly. They used liquid mercury as a variable resistor, and tended to muffle out sound. A more sensitive and mobile solution was needed.
In 1881 Henry Runnings devised a transmitter wherein the variable resistance medium consisted of a mass of finely divided carbon granules held between two conducting plates. Between a metal diaphragm, and a parallel conducting plate, and a mouthpiece, is a chamber filled with fine granules of carbon. The diaphragm, and the plate, form the terminals of the transmitter, and the current from the battery must therefore flow through the mass of granular carbon.
When the diaphragm is caused to vibrate by sound waves, it is brought into more or less intimate contact with the carbon granules and causes a varying pressure between them. The resistance offered by them to the current is thus varied, and the desired undulations in the current produced. This transmitter, instead of having one or a few points of variable contact, is seen to have a multitude of them.
It can carry a larger current without heating, and at the same time produce greater changes in its resistance, than the forms previously devised, and no ordinary sound can cause a total break between the electrodes. These and other advantages have caused this type in one form or another to largely displace all others.
At first the practice was to put the transmitter, together with the receiver and battery, directly in circuit with the line wire, With this arrangement the changes produced in the resistance by the transmitter were small in comparison with the total resistance of the circuit, especially in the case of a long line, and the changes in current were therefore small. Edison remedied this difficulty by using an induction coil in connection with the transmitter.
The induction coil used then and now is made as follows : Around a core formed of a bundle of soft iron wires is wound a few turns of comparatively heavy insulated copper wire. Outside of this, and entirely separate from it, is wound another coil consisting of a great number of turns of fine wire, also of copper, and insulated. The transmitter, together with the battery, is placed in a closed circuit with the coarse winding of a few turns, while the fine winding of many turns is included directly in circuit with the line wire and the receiving instrument.
The coarse winding is usually termed the primary winding, because it is associated with the primary source of current, the battery; while the fine winding is usually termed the secondary winding, because the currents flowing in it at the transmitting station are secondary, or induced currents. In coils of this kind the coarse winding, is almost invariably termed the primary for the above reason, although many conditions exist in electrical work, and in telephone work, where the high resistance winding is in reality the primary coil.
