. Robison's Manual of radio telegraphy and telephony for the use of naval electricians . held in place on a cylindrical drum or ringand the whole is called an armature. If this ring is made of iron, thestrength of the magnetic field is much increased, because the iron affordsa path for the lines of force from one pole to the other and therebylessens the distance through which they have to pass in the air. (Seeart. 25.) MANUAL OF GADIO TELEGRAPHY AND TELEPHONY. 31 The tendency to current flow in C D created by cutting lines of forceis called the electro-motive force in C D (see art, 3), and is
. Robison's Manual of radio telegraphy and telephony for the use of naval electricians . held in place on a cylindrical drum or ringand the whole is called an armature. If this ring is made of iron, thestrength of the magnetic field is much increased, because the iron affordsa path for the lines of force from one pole to the other and therebylessens the distance through which they have to pass in the air. (Seeart. 25.) MANUAL OF GADIO TELEGRAPHY AND TELEPHONY. 31 The tendency to current flow in C D created by cutting lines of forceis called the electro-motive force in C D (see art, 3), and is found todepend on the number of lines cut in a given time, so that the fasterC D revolves, and the stronger the magnetic field, the greater the electro-motive force and the greater the current produced in any given , if the current induced in C D, instead of all flowing through E F,is divided, so that part of it flows around the core of the electro-magnet(fig. 21), this current can take the place of that produced by the batteryin A B and the battery can be dispensed Fig. 21. 38. In art. 6 it is stated that wrought or soft iron can be magnetized,but only retains its magnetism while under the influence of the magnet-izing force. Steel or hard iron once magnetized retains its magnetiza-tion permanently and special means to demagnetize it are required. Itis found that electro-magnets with soft-iron cores can be made morepowerful (i. e., will give a stronger field) than if the cores are of steel,and that electro-magnets with either kind of core can be made to givemuch stronger fields than any permanent magnet. Also, that soft-ironcores retain a very small part of their magnetism and polarity when thecurrent is broken, so that, if the magnet poles between which C Drevolves are made of the most efficient material (wrought iron or mildsteel containing no phosphorus), when C D stops they still retain theirpolarity in a slight degree. When C D starts to revolve again, t
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