Page:Elementary Text-book of Physics (Anthony, 1897).djvu/385

§ 309] the machine: this current passes through the field magnets and increases the strength of the magnetic field. This in turn reacts upon the armature, and the current rapidly increases until it attains a maximum due to the fact that the magnetic field does not increase proportionally to the current which produces it. Such a machine is called a dynamo-machine. By suitable arrangements of the conductors which lead the current from the machine, either direct or alternating currents may be obtained.

The induction coil, or Ruhmkorff's coil, consists of two circuits wound on two concentric cylindrical spools. The inner or primary circuit is made up of a comparatively few layers of large wire, and the outer, or secondary, of a great number of turns of fine wire. Within the primary circuit is a bundle of iron wires, which, by its magnetic action, increases the electromotive force of the induced current in the secondary coil. Some device is employed by which the primary circuit can be made or broken mechanically. The electromotive force of the induced current is proportional to the number of windings in the secondary coil, and as this is very great the electromotive force of the induced current greatly exceeds that of the primary current. The electromotive force of the induced current set up when the primary circuit is broken is further heightened by a device proposed by Fizeau. To two points in the primary circuit, one on either side of the point where the circuit is broken, are joined the two surfaces of a condenser. When the circuit is broken, the extra current, if the condenser be not introduced, forms a long spark across the gap, and so prolongs the fall of the primary current to zero. The electromotive force of the induced current is therefore not so great as it would be if the fall of the primary current could be made more rapid. When the condenser is introduced, the extra current is partly spent in charging the condenser, the difference of potential between the two sides of the gap is not so great, the length of the spark and consequently the time taken by the primary current to become zero is lessened, and the electromotive force of the induced current is proportionally increased.