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Article by the late Stephen Tazewell OA, reprinted from January 1987 edition of the newsletter.

Thomas A. Edison, in 1878, became the first to produce a successful telephone transmitter of the variable resistance type. Edison developed his instrument on the then acknowledged principle 'that the increase of pressure between two conductors in contact produces a diminution of their electrical resistance'. Edison found this especially so in employing carbon as a conductor. His experiments disclosed the great variation in resistance under pressure.

After much experimentation in design he decided on what was known as the 'button' transmitter. This device consists of a diaphragm clamped to the iron case of an iron cap in which the ebonite mouthpiece is screwed. Pressing against the centre of the mica

This forms a vibrating cover to the chamber with ebonite sides which contains carbon or lamp black. The amount of pressure on the carbon is adjusted by a set screw in the back of the device. The terminals for the battery and line are connected to the platinum plate and the body of the device. Sound waves hitting the diaphragm causes it to vary its pressure on the carbon thus producing corresponding variations in the resistance of the circuit containing the battery and originally a Bell receiver.

The spoken sound thus causes a pulsatory current which in turn passes through the receiver with reproduction at good strength. The 'button' transmitter produced a very much greater output than Bell's magneto type transmitter.

Not content with the improved transmitter, Edison devoted continued experiments to further increase the efficiency of the instrument over long distance lines. This led to the use of the induction coil in which the transmitter and battery are connected to the primary or low side of the coil whilst the line and receivers are connected in the secondary or high winding of the coil.

As already stated, the carbon transmitter produces variations of resistance in the circuit in which it is coupled. The greater the proportional variation to the resistance in the circuit, the louder the sound from the receiver. The signal will increase as the resistance of the circuit apart from the transmitter decreases. Naturally it follows that the greater the resistance of the line, the greater will be the loss in efficiency and the weaker the reproduction in the receiver.

This is better explained by the following examples. Consider the resistance of a transmitter to be 5ohms and the variation of resistance on a certain note is 1ohms. Assuming the circuit is 15ohms resistance, the variation will represent 1/20 or 5% of the total resistance. On the other hand, consider the case of a long line of 1000ohms, a variation of only 1/1000 part or 0.1% would be obtained. This would be a very feeble result.

Should the resistance of the transmitter be increased to 100ohms and the variation increased by the same proportion to 20ohms than in a 1000ohms circuit there would only be a variation of 1/50 or 2% which is 20 times better. In practical terms this cannot be obtained.

By making use of Edison's induction coil, the resistance of the transmitter circuit can be kept low and the relative variation can be made very wide. With the secondary coil of greater turns the current induced in it by the variations in the primary coil will have a high EMF (voltage) and will overcome a considerable amount of the resistance in a long line and give greater output from the receiver.

There were a great number of transmitters available in the early years, however the Edison 'button' had a fairly long and efficient run over a great number of years. It was finally adopted and extensively used in the Bell telephone. Its most significant successor was A.C. White's solid back transmitter which is to be found in many later instruments and was a most reliable and efficient device.

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