4, where instead of tufted
groups we have a sort of magnetic whirl to represent the lines of force.
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.
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