Fig. 12.—Magnetic lines of force.
Experiment 5.—If a few iron filings (A, [Fig. 12]) be scattered evenly on a sheet of paper (C, [Fig. 12]) and a horse-shoe magnet (B, [Fig. 12]) approached from beneath, the filings arrange themselves in a curious design, which really maps out the magnetic lines of force. Now, if this experiment is repeated whilst using the bell-bobbin in place of the permanent horse-shoe magnet, and a current passed round the coils, the same disposition of the filings ensues, showing that the bobbin’s iron cores have been magnetized. In [Fig. 13], A is the filings; B, the wire from negative terminal battery; C, the sheet of paper; and D, the wire from positive terminal.
Experiment 6.—“Flax” wire, whose core consists of about forty fine copper filaments stranded together, may often be had in scrap lengths at electricians’ shops, as it finds wide employment in lighting installations. The silk and rubber insulation should be ignited and allowed to burn, any residue being carefully wiped off with rag, after which two lengths of, say, three strands apiece are separated from the wire core. These are very flexible, so that when attached to the battery terminals and magnet wires the bobbin, being extended as shown by a silk strand, is able to revolve freely in any direction. As a matter of fact, whilst the current is flowing the bobbin sets itself north and south like any other magnet—a very ordinary performance, the reader may remark! But, on the other hand, if the flexible wires be changed over so that the one previously connected to the battery’s positive terminal is now connected to the negative and vice versa, with the result that the current travels round the bobbin coils in an opposite direction, the electro-magnet swings half a turn, and comes to rest with its pole that was towards the north now pointing south. So that the bobbin resembles a weathercock, except in so far as it changes with the current instead of the wind ([Fig. 14]).
Fig. 13.—The filings arrange themselves in a curious design.
Fig. 14.—An electrical weathercock.
Experiment 7.—The number “7” has been regarded among races of men as peculiarly fortunate. Perhaps happily, therefore, it falls to this experiment, which, indeed, is rather the construction of new than the arrangement of old apparatus. The magnetic properties of a bell-bobbin may be utilized in constructing a primitive electro-motor. Decapitate four 11⁄2-inch nails, and, having bent 1⁄8-inch of both ends of each at right angles, mount them equal distances apart round the circumference of a thread reel ([Fig. 15]).
| A. | Nail ready to drive into reel. |