THE JOKE SHOP

What bites and what does not

What bites is energy, not volts. The biggest spike in this whole mission is the one the kickback pen makes: 396 volts across a relay coil, measured, for about half a millisecond. The energy behind it is half of 0.25 henries times 17.96 milliamps squared, which is 40 microjoules. That is a snap and nothing else, and it is the same class as the piezo out of a lighter.

Now the other end. A camera flash capacitor is 100 microfarads charged to 300 volts, and half of C times V squared makes that 4.5 joules — about a hundred and ten thousand times the coil. Mains is worse again, because it is not energy-limited at all: it will supply amps for as long as you are holding on. Neither is in this kit and neither is coming. Nothing in the Joke Shop stores more than the 8.5 millijoules in a 470 microfarad capacitor at six volts, and nothing in it runs off anything but the 9 volt block.

So the rule the shop runs on: a gag may be loud, sudden, startling and annoying. It may not store energy. Every build in this mission carries its own Safe and Not safe line, and the three that are out — a capacitor bank, a flash unit, anything on mains — are out for the reason above, not because somebody is being careful.

One build named in the shop book is missing from this mission on purpose. The piezo finger zap is coming with the crystal lab: it needs a piezo element, and the bench has not got one yet.