Transformers are bi-lateral, they operate both ways.
However, when using your transformer as a step-up, you must not apply more than 12 volts to the primary winding, which will result in no more than 240 volts on the secondary.
If you connect the 12v winding to a 240v supply, there will be a flash, bang and a lot of sparks.
At best the supply fuses will blow, at worst you will burn the place down.
Transformers are bi-lateral, they operate both ways.
However, when using your transformer as a step-up, you must not apply more than 12 volts to the primary winding, which will result in no more than 240 volts on the secondary.
If you connect the 12v winding to a 240v supply, there will be a flash, bang and a lot of sparks.
At best the supply fuses will blow, at worst you will burn the place down.
I think I tried it; a step down hooked to a step up to make an isolation transformer. It worked terribly.
Maybe it's the impedance being multiplied by N^2, plus the wire resistance.
taking 240 mains, turning it down to 12v with a variac, sticking it into the 12v secondary of a tranny and getting 240v back out seems an unusual and as mentioned above very wasteful exercise
You can put the primary in series with a 240v 10w bulb and the source and see what comes out of the secondary. Cautiously increase the bulb wattage for more output. This is more of a current source feed than the voltage source presented by the Variac.