This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1901 Excerpt: ...the degree of subdivision of the core. For straight (or bar) electromagnets fine iron wire may be used for the core, while in horse-shoe magnets the core may be built up of iron stampings. If stampings are used the eddy currents may be further decreased by covering the sheets with shellac, or by interposing layers of insulating material, such as paper, mica, etc., and thus decreasing the electrical conductivity of the core. The oxide on the surface of the iron is usually relied upon for insulating purposes. If the current flowing through the coil of an electromagnet varies periodically the resultant magnetic flux also varies periodically. The armature is then acted upon by a varying force which passes successively from zero to a maximum, and then back to zero. For orrc complete cycle of current or magnetic flux the armature has two maximums of attraction. For a current of low frequency, the armature wiil vibrate with a frequencydouble that of the current. As the current frequency increases the vibrations of the armature will be less manifest, and it will chatter against its stop, and finally, for higher frequencies the armature will be attracted without any chattering. The point at which the vibration and chattering cease depends largely upon the inertia of the armature. The polarized type of electromagnet can be made to respond to a very small alternating current when the armature is placed in a biased position. When current flows in two parallel conductors in the same direction there is an attraction between the two conductors, and a repulsion if the currents are flowing in opposite directions. Consider two conductors conveying alternating currents which are in phase with one another, as indicated by Fig. 33. There is a varying attraction between the cond...
