Page:Popular Science Monthly Volume 18.djvu/510

494 which are wound a number of insulated coils. Radial pieces connected with the end of the wire of one coil and the beginning of the next conduct the currents to the commutator cylinder, when they are taken off by brushes. The armature is mounted upon a shaft and revolves between the poles of an electro-magnet, the arrangement being that shown in Fig. 16. When the machine is used on an electro-motor, the current enters the armature by the brushes and is alternately changed in direction by the commutator, so that there is attraction as the armature approaches and repulsion as it leaves the poles of the field magnet.



Besides machines constructed primarily to generate currents, but which may be used as electro-motors as well, there are various others designed especially for the latter purpose. The commonest form of these is a set of electro-magnets arranged radially around the armature, which is rotated under the influence of the changing polarity, or the magnetization and demagnetization of the field-magnets.

One of the best of the machines constructed for motor purposes, and one that has received high praise from competent electricians, is that of M. Marcel Deprez. It consists of a compound permanent horseshoe magnet with a Siemens armature between its poles. This latter is simply a soft-iron cylinder with two longitudinal grooves in which insulated copper wire is wound, the poles being the faces of the cylinder between the coils. It is placed with its axis parallel to the arms of the magnet instead of across them as is ordinarily done, and to this arrangement is due its greater power, as the whole strength of