Then with a range of incandescent globes eg 15W, 25W, 40W, 60W,75W,100W can bring things into service by watching the brightness of the globe. As they dim down as the caps reform, put in the next globe up in wattage. If they don't dim, time to stop.

Definitely nice to have a built-in indicator, but you might need a lifetime supply of incandescent bulbs - they are going off the market.

Perhaps a single 15w bulb could be put in series with a switchable bank of resistance coils like the sewing-machine pedal, to provide the indication without the man-handling.

Maven

What does the commercial variac do?
Has it been considered whether or not a cheaper
variac that is safe can be produced by a hobbyist that is also cheaper?

A technical description is here:

http://en.wikipedia.org/wiki/Rotary_variable_differential_transformer.

It's a simple but clever idea, splitting the flux between two secondary coils in proportion to how the rotating core (rotor) is positioned against the fixed stator. One of the secondaries is the output voltage, the other secondary is its complement.

In theory, you might be able to kludge one using components of a suitable electric motor. Given the dangerous voltages involved, I would be inclined not to try it.

To build a proper one from scratch would no doubt cost more than buying one. Since the whole point is to provide full-wave linear transformation, I think any switching solution using PWM would need some clever engineering to re-create a clean, full sine-wave across the whole desired voltage range.

Maven

Photo uploaded.

The Variac that we know now is somewhat simpler in operation. It is a toroidal transformer with a sliding brush to provide the output, so there is only one winding.
This provides a variable output with a current rating determined by the transformer spec. As it is an autotransformer, there is no isolation between input and output, so one has to be aware of this when using.
The device referred to by Maven is generally used as an angular position transducer, normally with low power outputs and used for instruments of some sort.
Harold

Thanks for clarifying. So the sliding brush acts as a variable tap, according to how many windings of the toroid are in circuit?

Maven

So my Mum upgraded her lights, I got an airbag safety some time ago that can be earthed,
and I have my variac / work light.

Image Link

I suppose it would pay to get some bulbs while they are still being produced.