A nice transformer demo:

Several aspects of transformers are made particularly clear to students who see the act of winding a secondary coil on the core of a primary which is already connected to an ac source. It is easy to demonstrate the transformer by placing a coil-bulb combination over the core of a solenoidal ac electromagnet.

The usual procedure is to use a prewound secondary. However, the importance of the number of turns on the secondary is demonstrated much more vividly if the secondary is wound turn-by-turn on the core of the (“plugged-in”) primary.

A bulb connected to the two ends of the growing coil will become brighter with each new loop added.

This demonstration can provide data for a sample calculation using the turns ratio. In one approach, the secondary voltage may be estimated to be approximately 6 Vrms when a 6-V flashlight bulb appears to be as bright as it does when connected to a 6-V battery.

From this voltage, the known primary voltage and the (counted) number of secondary turns, the number of primary turns can be computed (assuming an ideal transformer). A more careful analysis using voltmeters and ammeters will reveal that this transformer differs considerably from ideal behavior. For example, the primary current may be several amps when the secondary current is zero.

This non-ideal behavior can lead to valuable discussions of several concepts including energy conservation and eddy currents.