battery, and the primary current directly
proceeding from it.

Now, Faraday's discovery was, that this
galvanic or primary current, at the moment
when it begins to flow, and again at the
moment when it ceases to flow, produces a
secondary or induced, and perfectly
independent current, in another conductor
wound around the first, but not in contact
with it. At the moment when the primary
current begins to flow, the induced current
passes in the same direction with it; but
at the moment when the primary current
ceases to flow, the induced current passes
in the opposite direction. Instead of being,
as in the primary current, continuous,
the induced current is only momentary;
and, in order to produce it at pleasure,
it is necessary to have some contrivance
by which to cut off and to restore the
primary current as often as may be
desired. As often as it is cut off, the
reverse induced current passes; as often as
it is restored, the direct induced current
passes. The instrument used for this
purpose is called a break, or contact breaker.
It is placed in a gap in the primary or
galvanic circuit, communicating with one
extremity of the gap, and capable of being made
to touch the other extremity also. When it
touches, it is said to "make" contact, and,
when it ceases to touch, it "breaks" contact.

Not only does the magnet, like the
primary current, induce electricity, but a piece
of soft iron is rendered magnetic during
the passage of a primary current through
a coil of wire surrounding it. If the iron
be massive, it retains its magnetic quality
for a few moments after the galvanic
current ceases; but, if it be of small bulk, it
gives up its magnetism immediately.

In the manufacture of a "coil" for the
display of induced electricity, all the
foregoing facts are taken into account. The
centre, or core, of the coil is formed of a
bundle of soft iron wire. Around this is
wound the wire for the primary current,
and around this again the wire for the
secondary current. When the ends of the
primary wire are connected with the two
poles of a galvanic battery, the core of
iron wires becomes a core of magnets, and
hence assists the primary current in
inducing electricity in the secondary wire.
When the ends of the primary wire are
disconnected from the battery the core ceases
to be magnetic, and the withdrawal of the
magnet assists the cessation of the primary
current in again inducing electricity in the
secondary wire.

The largest induction coils hitherto made
have been about a foot or fifteen inches in
length, by about four inches in diameter.
Seven miles has been about the extreme
limit of length of the secondary wire; and
nine inches the greatest length of spark
that could be obtained. With these figures
as standards of comparison, we approach the
"monster coil" now under consideration.

In this, the central core of iron wires is
composed of pieces each five feet long, and
the thickness of knitting needles, the whole
core being five inches in diameter. The
primary wire is of copper, thirty-seven
hundred and seventy yards in length. The
secondary wire is also of copper, and is one
hundred and fifty miles in length. The
rods of the core are separated from one
another, or insulated, by being wound
round with cotton, and the primary wire
is covered in a similar manner. The
secondary wire is covered with silk; and
all these coverings are required in order to
force the current to keep within each wire,
or to pass along its length, instead of
escaping from it laterally to contiguous
turns of the spiral. The whole apparatus
is enclosed within cylinders of vulcanite,
and is mounted on strong supports,
themselves similarly covered. The ends of the
secondary wire issue one from each
extremity of the coil, and are connected to
"terminals," one of which is a point, and
the other a polished disc of metal. They
stand on movable columns in front of the
coil; and the wires, when necessary, can
be detached from the terminals, and
attached to any other apparatus that may be
required. When the primary wire is
connected with a powerful galvanic battery,
and contact is made, the core becomes a
bundle of magnets, and this bundle
combines with the primary wire to induce an
electric current in the secondary wire.
When contact is broken, the primary
current ceases to flow, the core loses its
magnetism, and an electric current is again
induced in the secondary wire. If the
terminals be not too far apart, this induced
current leaps across the space between them
in the form of a visible spark or flash.

There is yet another piece of subsidiary
apparatus, called the condenser. This
consists of a number of small sheets of
insulated tinfoil, connected together, and with
the primary wire, to which they form a sort
of loop circuit. The condenser is supposed
to afford a safety-valve, or reservoir of
space for the primary current, and a
security against any injury being done to the