Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . ismethod insert one limb all the way in the spool; turn on thecurrent; tap it; turn off the current; remove this limb, andinsert the other limb in the opposite end of the coil andrepeat the operations. You will now have a magnet, thestrength of which, until the saturation point of the steel isreached, will depend on the current strength, and the numberof turns of wire on the spool. 15. Magnetisation by an Electromagnet.—A bar electro-magnet (i. e., the above spool and its iron core)
Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . ismethod insert one limb all the way in the spool; turn on thecurrent; tap it; turn off the current; remove this limb, andinsert the other limb in the opposite end of the coil andrepeat the operations. You will now have a magnet, thestrength of which, until the saturation point of the steel isreached, will depend on the current strength, and the numberof turns of wire on the spool. 15. Magnetisation by an Electromagnet.—A bar electro-magnet (i. e., the above spool and its iron core) may be sub-stituted for the permanent steel magnets used in the previousmethods for making a magnet, or two bar electromagnetsconnected by a piece of soft iron forming a horseshoe elec-tromagnet may be used (see Fig. 11.) In this case the cur-rent is passed through one spool in one direction, and thenthrough the other spool in the opposite direction, when the MAGNETISATION. 9 free ends of the core will have a N and S-pole. One-half ofthe bar to be magnetised can now be stroked over one pole, >^T3?4g^.. Fig. 11.—Magnetisation by an Electromagnet. beginning at its middle point and following the direction ofthe dotted lines in Fig. 11, stroke on all sides, and then strokethe other half of the bar on the other pole. 16. Making Permanent Steel Magnets.—The artificialmagnets in paragraphs 2 and 11 are called permanent mag-nets because they retain their magnetism permanently, to acertain extent, after magnetisation. Some qualities of steelwhich possess good machine tool properties are not adapted Fig. 12.—Round Bar Magnet. to making good permanent magnets. Steel containing acertain percentage of manganese cannot be magnetised, whilesome brands of cast steel, spring steel, and mild plate steelare readily magnetised, but do not retain their magnetism 10 PRACTICAL ELECTRICITY.
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