The early telephones used a device similar in principle to a modern receiver for both transmitter and receiver. When used as a transmitter,however,a telephone receiver is not very sensitive,because the amount of energy in the human voice is so small that the sound output from the distant receiver is almost inaudible over even short telephone lines. This is due to the losses which occur in the conversion to electrical energy,losses on the line,and losses in the reconversion to acoustical energy.
300 amd 400 series PMG telephones use a more sensitive and efficient type of transmitter called a carbon transmitter . This consists,basically,of many grains of carbon called carbon granules,packed between two carbon electrodes mounted in a small chamber which hes insulated sides so as not to short-circuit the electrodes. One electrode is fixed to the back of the chamber;the front electrode is attached to the centre of a light,flexible diaphragm. The resistance of the transmitter is the resistance of the carbon granules between front and back electrodes.
Principle of operation:When a sound wave strikes the diaphragm,the varying pressures in the adjacent air particles cause it to vibrate. Acoustical energy is converted into mechanical energy in the vibration of the diaphragm.
A compression moves the diaphragm end front electrode inwards (Fig. lb). The increased mechanical pressure “packs”the carbon granules closer together,and this increases the number of granules in contact with each other. Thus,the area of contact is increased and this reduces the contact resistance of the carbon between the electrodes.
Conversely,a rarefaction moves the diaphragm and front electrode outwards (Fig. lc),and the transmitter resistance increases.
When a battery is connected between the transmitter electrodes in an electric circuit,a D.C. flows through the conducting carbon granules. The variations in transmitter resistance vary or regulate the current in the circuit. Thus,a small value of acoustical energy can vary a comparatively large value of electrical energy supplied by the battery. With an efficient transmitter,this variation may be over 1,000 times greater than the original acoustical energy.
The carbon transmitter,therefore,does not directly convert acoustical energy into electrical energy,but can be considered to be an amplifier es it effectively amplifies or enlarges the acoustical energy supplied to it. An amplifier does not violate the energy conservation law,as this law applies only to energy conversion,and not to one form of energy controlling another form.