Electricity

by W. H. McCormick

Fig. 31.—Diagram of Microphone Transmitter.

By means of this power of magnifying vibrations a microphone transmitter can be used on a line of tremendous length, where an ordinary Bell transmitter would be utterly useless. The general features of this transmitter, [Fig. 31], are a diaphragm and a block of carbon separated slightly from one another, the intervening space being filled with granules of carbon. These are enclosed in a case of ebonite having a mouthpiece in front and two terminals behind, one terminal being connected with the carbon block and the other with the diaphragm. From these terminals wires are led to a battery and to the receiver, which is of the Bell type. The current has to pass through the carbon granules, and the movements of the diaphragm when set in vibration by the voice vary the pressure upon the granules, and in this way set up variations in the current. Carbon dust also may be used instead of granular carbon, and then the instrument is called a “dust transmitter.”

Fig. 32.—Combined Telephone Transmitter and Receiver.

It is usual to have a transmitter and a receiver on one handle for the greater convenience of the user. The arrangement is shown in [Fig. 32], and it will be seen that when the user places the receiver to his ear the transmitting mouthpiece is in position for speaking. The microphone with its carbon dust is placed at A, just below the mouthpiece, and the earpiece or receiver B contains a little magnet and coil with a diaphragm in front, so that it is really a Bell instrument. A little lever will be noticed at C. This is a switch which brings the transmitter into circuit on being pressed with the finger.

It is now time to see something of the arrangement and working of telephone systems. As soon as the telephone became a commercially practicable instrument the necessity for some means of inter-communication became evident, and the telephone exchange was brought into being. The first exchange was started in 1877, in Boston, but this was a very small affair and it was run on very crude lines. When one subscriber wished to communicate with another he had to call up an operator, who received the message and repeated it to the person for whom it was intended; there was no direct communication between the various subscribers’ instruments. As the number of users increased it became necessary to devise some system whereby each subscriber could call the attention of an operator at the central station, and be put into direct communication with any other subscriber without delay; and the exchange system of to-day, which fulfils these requirements almost to perfection, is the result of gradual improvements in telephone methods extending over some thirty-five years.

When a subscriber wishes to telephone, he first must call up the operator at the exchange. Until comparatively recently this was done by turning a handle placed at the side of the instrument. This handle operated a little dynamo, and the current produced caused a shutter at the exchange to drop and reveal a number, just as in the electric bell indicator, so that the operator knew which instrument was calling. As soon as the operator answered the call, the shutter replaced itself automatically. The signal to disconnect was given in the same way, but the indicator was of a different colour in order to prevent confusion with a call signal. These handle-operated telephones are still in common use, but they are being replaced by instruments which do away with handle-turning on the part of the subscriber, and with dropping shutters at the exchange. In this latest system all that the subscriber has to do is to lift his telephone from its rest, when a little electric lamp lights up at the exchange; and when he has finished his conversation he merely replaces the telephone, and again a little lamp glows.

We must now see what happens at the exchange when a call is made. Each operator has control of a number of pairs of flexible cords terminating in plugs, the two cords of each pair being electrically connected. The plugs rest on a shelf in front of the operator, and the cords pass through the shelf and hang down below it. If a plug is lifted, the cord comes up through the shelf, and it is drawn back again by a weight when the plug is not in use. Two lamps are provided for each pair of cords, one being fixed close to each cord. The two wires leading from each subscriber’s instrument are connected to a little tube-shaped switch called a “jack,” and each jack has a lamp of its own. When a subscriber lifts his telephone from its rest a lamp glows, and the operator inserts one plug of a pair into the jack thus indicated, and the lamp goes out automatically. She then switches on her telephone to the caller and asks for the number of the subscriber to whom he wishes to speak; and as soon as she gets this she inserts the other plug of the pair into the jack belonging to this number. By a simple movement she then rings up the required person by switching on the current to his telephone bell.

Here comes in the use of the two lamps connected with the cords. As long as the subscribers’ telephones are on their rests the lamps are lighted, but as soon as they are lifted off the lamps go out. The caller’s telephone is of course off its rest, and so the lamp connected with the first cord is not lit; but until the subscriber rung up lifts his instrument to answer the call, the lamp of the second cord remains lit, having first lighted up when the plug was inserted in the jack of his number. When the second lamp goes out the operator knows that the call has been responded to, and that the two subscribers are in communication with each other. Having finished their conversation, both subscribers replace their instruments on the rests, whereupon both lamps light up, informing the operator that she may disconnect by pulling out the plugs.