They showed these at my school. Some images like the beer can mortar, the man running with a wobbly resonating plank, and a pyrotechnic pellet firing gun are indelible on the mind of my cohort. At least the easily excitable boys.

In one of these shows there's a demonstration of magnetic tape. As far as I remember they attach a coil of wire to a loudspeaker and demonstrate that you can get sound from wiggling a magnet near the coil, and magnetism by shouting at the speaker.

They then literally file some iron filings on to some sticky tape and move it past the coil as someone shouts into the speaker. They then do the same thing again, but this time the sound of shouting comes out if the speaker.

Even when I'm writing this now I'm thinking, "well of course it works, that's obviously the whole principle", but I'm sure I only feel that because I saw the demonstration.

His companion Rex Garrod also stars, and was one of the greatest ever competitors of UK Robot Wars (with Cassius I/II).