The lines of force of the galvanic field are, indeed, circles or curves
which inclose the conducting wire, and their number is proportional
to the strength of the current. In the figure, where the current is
supposed to be flowing up the wire (shown by the dark arrows), the
little arrows show the direction in which a free north pole would be
urged round the wire;[1] a south pole would, of course, be urged round
the wire in the contrary direction. Now, though when we look at the
telegraph wires, or at any wire carrying a current of electricity, we
cannot _see_ these whirls of magnetic force in the surrounding space,
there is no doubt that they exist there, and that a great part of the
energy spent in starting an electric current is spent in producing these
magnetic whirls in the surrounding space. There is, however, one way of
showing the existence of these lines of force; similar, indeed, to
that adopted for showing the lines of force in the field surrounding a
magnet. Pass the conducting wire up through a hole in a card or a plate
of glass, as shown in Fig. 5, and sprinkle filings over the surface.
They will, when the glass is gently tapped, arrange themselves in
concentric circles, the smallest and innermost being the best defined
because the magnetic force is strongest there. Fig 6 is an actual
reproduction of the circular lines produced in this fashion by iron
filings in the field of force surrounding an electric current.

[Footnote 1: It will not be out of place here to recall Ampere's
ingenious rule for remembering the direction in which a current urges
the pole of a magnetic needle. "Suppose a man swimming in the wire with
the current, and that he turns so as to face the needle, then the north
pole of the needle will be deflected toward his left hand."]

[Illustration: Fig. 4]