just given concern the loss of field or the
shortening and apparent disappearance of the magnetic lines or
circuits, as giving rise to the self-induction or increased potential
on breaking. Where the energizing current is slowly cut off or
diminished the energy is gradually transferred to the wire in
producing elevation of potential during the decrease; and the collapse
and cutting of the wire by the collapsing circuits or lines is then
only more gradual.

Let the current be returned to the wire after disappearance of
magnetism, and the lines again seem to emanate from the wire and at
the same time cut it and produce a counter potential in it, which is
the index of the abstraction of energy from the circuit, and its
storing up in the form of elastically strained lines of magnetism
around the conductor. The effect is that of self-induction on making
or upon increase of current, the measure of the amount being the
energy stored in the magnetic circuits which have been extended or
opened up by the current. The greater the current and the shorter the
path for the lines developed around the axis of the conductor, the
greater the energy stored up. Hence, a circular section conductor has
the highest self-induction, a tube of same section less as its
diameter increases, a flat strip has less as its width increases and
thickness diminishes, a divided conductor much less than a single
conductor of same shape and section. Separating the strands of a
divided conductor increases the length of magnetic paths around it,
and so diminishes the self-induction. A striking instance of this
latter fact was developed in conveying very heavy alternating currents
of a very low potential a distance of about three feet by copper
conductors, the current being used in electric welding operations.

The conductors were built up of flat thin strips of copper for
flexibility. When the strips were allowed to lie closely together, the
short conductor showed an enormous self-induction, which cut down the
effective potential at its ends near the work. By spreading apart the
strips so as to lengthen a line around the conductor, the
self-induction could be easily made less than 35 per cent. of what it
had been before. The interweaving of the outgoing and return conductor
strands as one compound conductor gets rid almost entirely of the
self-inductive effects, because neither conductor has any free space
in which to develop strong magnetic forces, but is o