Transformer.
In alternate current lighting the induction coil by which the primary
current with high initial electro-motive force is caused to produce a
secondary current with low initial electromotive force.
A typical transformer consists of a core of thin iron sheets. The
primary is of comparatively thin wire and often of ten or more times as
many turns as the secondary. The latter is of thicker wire. Where the
ratio of 10 to 1 as regards number of turns in the primary and secondary
obtains, the initial E. M. F. of the secondary is one-tenth that of the
primary circuit.
The cores are laminated, as described, to avoid the formation of
Foucault currents.
The counter-electro-motive force of the transformer when the secondary
circuit is open, prevents any but the slightest current from passing
through the primary. In proportion as the secondary is closed and its
resistance diminished, as by lighting more lamps in parallel, the
counter-electro-motive force of the transformer falls and more current
passes through the primary.
Fig. 339. FERRANTI'S TRANSFORMER.
The economy of the apparatus is in the fact that counter-electromotive
force reduces current through a conductor without absorbing any energy.
A resistance coil cuts down a current, but absorbs energy equal to the
current multiplied by the potential difference between the terminals of
the coil. This electric energy is converted into heat energy and is
wasted. But the counter-electromotive force of a transformer is exerted
to reduce current without production of heat and with little waste of
energy. This is one of the advantages of the alternating current system
of distribution of electric energy.
The object of a transformer being to secure safety to the person or to
life by the separation of the high potential primary or street circuit,
and the low potential house circuit, any contact of the two circuits in
the converter is a source of danger. Special care should be taken to
ensure absence of leakage, as it is termed. Mica or other insulation is
sometimes employed to prevent the wires from coming in contact by
piercing or sparking with the core and with each other.