>An intrinsically safe system does what it does by sticking to
>that part of the laws of physics that make sparks impossible.

or encloses them in a way that will not allow the spark to reach the outside 
environment.

I remember a colleague telling of using a dial telephone at a local bulk oil 
depot, the dial made a rapid return to the rest position, which then clamped 
the seal around the shaft, then the dial mechanism could be heard doing its 
controlled speed return as it operated the digit counting switch. 

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