Transformers come in many styles and sizes. Neon sign transformers
are distinguished by being current limited and center grounded. Center
grounded simply means that the center of the transformers output winding
is electrically connected to ground. This results in both output terminals
being hot with respect to ground. Current limited means that when the transformer's
output is shorted, only a limited amount of current will flow. Current
limiting is achieved by the use of a tightly coupled, magnetically saturable
(usually iron) core. The iron core provides resistance to the fluctuating
magnetic field which, in turn, impedes the current flow. Impedance, like
resistance, is measured in Ohms and has the same effect on current flow,
namely current(Amperes) = potential(Volts)/impedence(Ohms).
Pole pigs are not current limited, shorting the output will allow such
large currents to flow that the windings will melt (usually vaporizing
the oil filling and bursting the transformer case). To use a pole pig in
Tesla coil service, some sort of current limiting must be added, usually
in series to the input of the pole pig. Both resistive (electric heating
elements) and inductive (arc welders) limiting has been used.
