=129. Field-Magnets; Armature; Commutator.= What we need then, to produce an induced current by a rotary motion, is a strong magnetic field, a rotating coil of wire properly placed in the field, and some means of leading the current from the machine. [Illustration: Fig. 145.] [Illustration: Fig. 146.] If a loop of wire, Fig. 140, be so arranged on bearings at its ends that it can be made to revolve, a current will flow through it in one direction during one-half of the revolution, and in the opposite direction during the other half, it being insulated from all external conductors. This agrees with the experiments suggested in § 127, when the current generated in a coil passed in one direction during its motion _toward_ the strongest part of the field, and in the opposite direction when the coil passed _out_ of it. A coil must be cut by lines of force to generate a current. A current inside of the machine, as in Fig. 140, would be of no value; it must be led out to external conductors where it can do work. Some sort of sliding contact is necessary to connect a revolving conductor with outside stationary ones. The magnet, called the _field-magnet_, is merely to furnish lines of magnetic force. The one turn of wire represents the simplest form of _armature_. Fig. 141 shows the ends of a coil joined to two rings, X, Y, insulated from each other, and rotating with the coil. The two stationary pieces of carbon, A, B, called _brushes_, press against the rings, and to these are joined wires, which complete the circuit, and which lead out where the current can do work. The arrows show the direction of the current during one-half of a revolution. The rings form a _collector_, and this arrangement gives an _alternating current_. [Illustration: Fig. 147.] In Fig. 142 the ends of the coil are joined to the two halves of a cylinder. These halves, X and Y, are insulated from each other, and from the axis. The current flows from X onto the brush A, through some external circuit, to do the work, and thence back through brush B onto Y. By the time that Y gets around to A, the direction of the current in the loop has reversed, so that it passes toward Y, but it still enters the outside circuit through A, because Y is then in contact with A. This device is called a _commutator_, and it allows a constant or _direct current_ to leave the machine. [Illustration: Fig. 148.]