=125. Various forms= of telephones are shown in Figs. 134, 135, 136. Fig. 134 shows a form of desk telephone; Fig. 135 shows a common form of wall telephone; Fig. 136 shows head-telephones for switchboard operators. [Illustration: Fig. 136.] CHAPTER XVII. HOW ELECTRICITY IS GENERATED BY DYNAMOS. =126. The Dynamo=, _Dynamo-Electric Machine_ or _Generator_, is a machine for converting mechanical energy into an electric current, through electromagnetic induction. The dynamo is a machine that will convert steam power, for example, into an electric current. Strictly speaking, a dynamo creates electrical pressure, or electromotive force, and not electricity, just as a force-pump creates water-pressure, and not water. They are generally run by steam or water power. [Illustration: Fig. 137.] =127. Induced Currents.= We have already spoken about currents being induced by moving a coil of wire in a magnetic field. We shall now see how this principle is used in the dynamo which is a generator of induced currents. [Illustration: Fig. 138.] Fig. 137 shows how a current can be generated by a bar magnet and a coil of wire. Fig. 138 shows how a current can be generated by a horseshoe magnet and a coil of wire having an iron core. The ends of the coil are to be connected to an astatic galvanoscope; this forms a closed circuit. The coil may be moved past the magnet, or the magnet past the coil. [Illustration: Fig. 139.] [Illustration: Fig. 140.] [Illustration: Fig. 141.] [Illustration: Fig. 142.] Fig. 139 shows how a current can be generated by two coils, H being connected to an astatic galvanoscope and E to a battery. By suddenly bringing E toward H or the core of E past that of H, a current is produced. We have in this arrangement the main features of a dynamo. We can reverse the operation, holding E in one position and moving H rapidly toward it. In this case H would represent the armature and E the field-magnet. When H is moved toward E, the induced current in H flows in one direction, and when H is suddenly withdrawn from E the current is reversed in H. (See "Study," Chapter XXV., for experiments.) [Illustration: Fig. 143.] =128. Induced Currents by Rotary Motion.= The motions of the coils in straight lines are not suitable for producing currents strong enough for commercial purposes. In order to generate currents of considerable strength and pressure, the coils of wire have to be pushed past magnets, or electromagnets, with great speed. In the dynamo the coils are so wound that they can be given a rapid rotary motion as they fly past strong electromagnets. In this way the coil can keep on passing the same magnets, in the same direction, as long as force is applied to the shaft that carries them. [Illustration: Fig. 144.]