I have got lots of big caps and nothing to do with them. Is there anything fun I can use them for?

They are 259MFD @ 220V motor star capacitors.

Can I build something to make them blow up?

 

If you just plug them in to the mains they will make good smoke bombs.
If you want them to explode, you will need higher voltage.
Perhaps use one in a voltage doubler circuit and see if the second capacitor explodes before the first one runs out of smoke.

You could use a microwave over transformer to hit them with about 2,000 volts.

 

I remember once when I was in a store cupboard at school, when I came across some 12v capacitors. I called my friend over, clipped no less than 12 pp3 batteries together, and touched the leads the wrong way. Don't worry, I was safe and used a crocodile clip to hold the cap.

Kabobs!

Fun times...

 

If you just plug them in to the mains they will make good smoke bombs.

I have put mains in these before and they do nothing. I say nothing, but when you give them to people to play with once you have charged them up ... it's entertaining.

As for overvoltage. I had a Jacobs ladder running of a 1.5k neon sign transformer and I put one of these across the ladder to see if I could boost it up a bit. The ladder stopped working and the cap did nothing. The transformer did have some sort of protection circuit on it, I guess that is possibly why it all went duff.

What is the most reliable way of blowing up a cap?

 

I have put mains in these before and they do nothing.

Maybe you didn't wait long enough...or maybe I see them smoke because they are in series with a motor. The motor inductance increases the current through the capacitor. That's the answer. Add an inductor to nullify the capacitive reactance. 40 millihenries should do the job...or tape an ounce of Symtec to each capacitor and insert a firecracker.

But cearially, a microwave transformer should work.

 

Add an inductor to nullify the capacitive reactance.

I have a selection of inductors but please explain in a bit more detail as to how to wire up the circuit.

 

Put 40 millihenries in series with the capacitor and connect power.
The inductor must be able to survive a few dozen amps for a few seconds and not saturate the core, therefore the core is air.

L = [mean radius times number of turns]squared / 9 mean radius + 10 length
I wonder if that's in inches or millimeters...
You can google it.

Air core inductors are also used in speaker crossover circuits, but the wire is too small.
or you could just grab a hundred foot roll of 12 gauge house wiring and connect that up as an inductor.

 

I remember once when I was in a store cupboard at school, when I came across some 12v capacitors. I called my friend over, clipped no less than 12 pp3 batteries together, and touched the leads the wrong way. Don't worry, I was safe and used a crocodile clip to hold the cap.

Kabobs!

Fun times...

My first job was making power supplies. For entertainment we would take a cap on extremely long leads and snake them along the floor to someone else's bench to set them off.

Next job someone stayed late to wire a 1/2W carbon resistor from under someone's bench back to his, hiding all the wiring. Next day while his neighbor was troubleshooting a new supply he gave the resistor a bit of overcurrent. When he heard the moan and the switch go off he turned the power off... several minutes of inspection by his neighbor showed no problem with the supply, so he continued with his testing, until the voltage on the resistor was re-applied.

BTW, here in the states we call those things alligator clips.