Originally Posted by
bigal1941
"Tap Changing" ....... I have no idea what he meant
Tap Changing was an old expression for resistance control (of variable voltage), a semi-mechanical device in those days. They came in various sizes, little ones for small scale, up to huge ones in electric railway locomotives from the driver's main power control. Nowadays such control is all-electronic with thyristors etc.
If you had an old car then variable speed wipers were an example of such a control, as you moved the switch round through the various positions it was "tapping" different positions on a resistor, to apply different voltages to the wiper motor.