causes the needle to point North ordinarily, so there must also
be magnetism around the wire when it is carrying a current.
From this we can conclude that any wire, carrying a current, has
magnetic properties and that around a live wire there is a ma-
entice field consisting of lines of force.
Further experimenting would show that the magnetic field,
consisting of lines of force, gets bigger as the current increases,
disappears or collapses when current stops, and reverses if the
current is reversed. (See Fig. 8(b).)
In our study of Radio, it will be necessary sometimes to
know the direction of the lines of force in a magnetic field of a
wire carrying current, without having a compass to show their
direction. This will not be difficult if you learn the lines of force
around a wire carrying current are always clockwise if you consider
Fig. 8
the current flowing away from you. Here is a good way
to get a clear idea of this and fix it firmly in your mind. Clinch
your right-hand, then bend back your thumb as far as you can.
Consider that your thumb is pointing in the direction of the
current flow, then your fingers will point in the direction of the
lines of force. (See Fig. 9.)
If a wire is made up into a coil, and a current sent through
it, there will be produced a concentrated magnetic field such as
shown in Fig. 10(a). This is because each turn of wire has its
own lines of force which combine with the lines of force of other
turns of wire in the coil.
All together these lines of force form a single magnetic field
for the entire coil. The field thus produced is exactly similar to
that produced by a magnet, for it has polarity; (N and S poles).
