We have now to consider the means used for conveying the current,
continuous or alternating, to the lamps we intend to use. The leads for
the electric current, which correspond in some measure with the pipes
which convey gas, are made of copper wire, as pure as can be obtained,
covered with some insulating material to prevent the escape of the
current through contact with other conductors. The size of the wire is
regulated according to the amount of current which is to be conveyed; it
will do no harm to have it of twice the required section, but if it is
of less than the required section, it will offer so much resistance to
the passage of the current, that it will probably get fused in a very
short time. If the lead attached to one terminal of the dynamo comes
back to the other terminal without there being any lamps in the circuit,
or other means of making use of the current, it is said to be short
circuited, and if the dynamo is kept going something must give out very
soon. The two leads must therefore never be connected with one another,
except by a lamp or other resistance, and the manner in which the lamps
are placed, and the size of the leads, depend upon the relative tension
and quantity of current and the kind of lamps to be used. If the current
is to be used in arc lamps it is usual to have a high E.M.F., which
allows of the leads being of small section; but if it is to be used in
incandescent lamps it is found more convenient to have a low E.M.F., and
as this implies a large quantity of current, the leads have to be of
large section.
Arc lamps usually require to be placed in series, that is to say, in
such a manner that the current, after leaving the dynamo, passes through
each lamp in succession. The E.M.F. required in this case is the sum of
the E.M.F. for each lamp, the quantity required being the same as for
one lamp. This accounts for the high E.M.F. used in arc lighting and
the small size of the wire for conducting the current. Incandescent
lamps can be either in series or parallel, and frequently the two
systems are combined. To explain the meaning of having lamps parallel,
we will suppose the two leads from a dynamo to be taken along a wall,
parallel to one another, and about six inches apart, ending at the end
of the wall, but not connected in any way. If we then place lamps at
intervals between the two leads, connecting one loop of each to the
upper lead, and the other to the lower lead, by means of small copper
wire, these lamps are said to be all parallel. In this arrangement the
current required is the sum of the quantity necessary for each lamp, but
the E.M.F. is the same as that required for one lamp of the same kind.
As we therefore require to send a large quantity of current through the
leads at a small pressure or E.M.F., these leads must be of large
section. In the above arrangement each lamp may be turned on or off
separately without affecting the others. Sometimes two or more lamps are
placed in groups between the parallel leads; these are then in series
with regard to one another, and can only be turned on or off two or more
at a time, in other words, one group at a time. If our dynamo is
producing a current of 100 volts E.M.F. when working at its proper
speed, and our lamps are 100-volt lamps, we shall be able to turn each
lamp on or off separately; but if we want to put in 50-volt lamps, we
must place two together, and we shall then have to turn them on or off
two at a time. I am supposing that in both cases the lamps require the
same quantity of current, though of different E.M.F.
To prevent the lamps being spoilt by the current being too strong
through a sudden increase in the speed of the dynamo, as also to prevent
the leads getting fused, and perhaps setting fire to the casing, it is
usual to have safety fuses in various parts of the circuit. These are of
different kinds, but a typical one consists of a small lead wire, large
enough to carry the normal current, but which fuses when the current is
too strong, and at once interrupts its passage. The lamps in the same
portion of the circuit are then extinguished and so saved from
destruction, and cannot then be lighted again until the fuse is renewed,
which, however, can be done with ease.