The spring wasn't very strong and, as I remembered it, the plunger was a
steel rod about 12-15 mm diam. It had a rounded end, although the
initial prototype had a gramophone needle (remember them?) projecting a
few mm, but this was removed before it was put into service. The force
was insufficient to cause physical damage: the reaction was more one of
shock! It was amusing to see the students filing into the lab and tippig
over their stools to cheack whether there was a mechanism in the
woodwork, before sitting down. We got the prof with it once!

Brian

Ingemar Hernefjord (KC/EMW) wrote:
>  One of them was modified with a
> spring-loaded plunger that came up through the slot when the seat was
> depressed with a load of about 40 kg or more, purely mechanical.
>
> ....OUUUUUCH!!!...was the thing pointed?...
>
> /Ingemar
>
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