Some years later Graham Bell, Professor of Vocal Physiology in the University of Boston, turned his attention to the electric transmission of speech, probably being led to do so from his experiments in teaching the deaf and dumb. His apparatuses shown at an exhibition in Philadelphia in 1876, consisted of a tube having one end open for speaking into, and the other closed by a tightly stretched membrane to which was attached a very light steel bar magnet. The vibrations set up in the membrane by the voice made the little magnet move to and fro in front of the poles of an electro-magnet, inserted in a battery circuit, thus inducing currents of electricity in the coils of the latter magnet. The currents produced in this way varied in direction and strength according to the vibratory movements of the membrane, and being transmitted along a wire they produced similar variations in current in another electro-magnet in the receiver. The currents produced in this manner in the receiver set up vibrations in a metal diaphragm in front of the magnet poles, and so the words spoken into the transmitter were reproduced. Since the year 1876 the telephone has developed with remarkable rapidity, and an attempt to trace its growth would involve a series of detailed descriptions of closely similar inventions which would be quite uninteresting to most readers. Now, therefore, that we have introduced the instruments, and seen something of its principle and its early forms, it will be most satisfactory to omit the intermediate stages and to go on to the telephone as used in recent years. The first telephone to come into general use was the invention of Graham Bell, and was an improved form of his early instrument just described. A case or tube of ebonite, which forms the handle of the instrument, contains a steel bar magnet having a small coil of insulated wire at the end nearest the mouthpiece of the tube, the ends of the coil passing along the tube to be connected to the line wires. Close to the coil end of the magnet, and between it and the mouthpiece, is fixed a diaphragm of thin sheet-iron. A complete outfit consists of two of these instruments connected by wires, and it will be noticed that no battery is employed.