Wimsha

Wimsha, a bearded instrument-maker, works a pair of glass discs while a blue spark crackles between them.

The Physics

In the early 1880s James Wimshurst, a London shipwright and amateur experimenter, perfected an influence machine: two insulating discs carrying metal sectors, turned in opposite directions on one axle. Neutralising brushes and collecting combs let a tiny starting charge on one disc induce opposite charge on the other, over and over, so the charge grows with each turn. No rubbing is involved, so unlike older friction machines it works reliably in damp weather and starts itself.

Q grows by induction each turn

• Q: charge stored on the collectors, usually Leyden jars (C)
• V = Q/C: voltage across the spark gap (V)

The energy comes from your arm: you do mechanical work turning the discs against the electrostatic attraction of the charges.

Honest caveat: The spark length depends on humidity, disc size and the jars; typical classroom machines give a few centimetres, tens of kilovolts.

On the sticker

Wimsha leans over two large glass discs on one axle, with curved arrows showing them turning in opposite directions and a cyan spark at the centre. Printed text: WIMSHA with the line INDUCTION. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.