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Friday, 29 July 2011


Late one night, half a lifetime ago when I was a student, I was repairing an old valve oscilloscope rescued from a skip at the back of Imperial College.  I decided that I'd had enough and that it was time for bed.  I poured a bowl of cornflakes and set it on the top of the bedside fridge (Peltier, so silent) that had the milk in for the morning, and went to sleep.

At about three a.m. I was woken by a crunching noise.  I turned on the light to find my nose about 20 centimetres from the nose of a mouse holding a cornflake in both hands.

The mouse didn't stay long.  I spent a few minutes chasing it round the room, until it ran into the scope, which still had its casing off.

I then had a really good idea: I turned the scope on.  After a few moments it had warmed up, and I clapped my hands.  I heard scampering noises from inside the works.  Another clap, another scamper, then a squeak followed by silence.

I could see the corpse lying in the wiring.

I then had a really bad idea: I reached in to extract the mouse.

I found myself on the opposite side of the room.

Either directly, or via the mouse, I had touched the final anode.  I estimate that this must have been at about 6,000 volts DC.  I was fortunate that I hadn't instead touched the main positive power line at around 300 volts; that might well have finished me.  The final anode supply was, of course, severely current-limited.

I said I found myself on the other side of the room, and that is exactly what the experience was.  I remember the shock, and I hadn't lost consciousness at all.  But, because what passes for my brain hadn't told my muscles to move, that brain deduced that an external agency had thrown me across the room.  In reality I had jumped, but my brain said thrown.

The next day I got to thinking, like all inventors, about better mousetraps.

Now, as a mousetrap, an oscilloscope is perhaps over-complicated.  But its principle is simplicity itself.

So I took a square of cardboard about 30 centimetres on one side and glued a 10 centimetre disc of aluminium cooking foil in the middle.  Around this I glued an annulus of aluminium foil with a 2 centimetre gap between it and the disc.  I put a dollop of peanut butter (crunchy) in the middle of the disc, wired the two pieces of aluminium across the mains, set the trap on the floor, left a note saying "Beware Electric Mousetrap" for my flatmates, and went back into IC to do a day's research for my PhD.

On my return that evening there was another dead mouse.  This time I turned off the mains before I picked it up to dispose of it.  It was a bit cooked...

But, now I'm older, I don't think I would use HT electricity in a mousetrap.

Better would be a strong gamma source such as 60Co in a small depleted uranium labyrinth with no line-of-sight from the source to the outside.  A DU cup dropped over the source should allow you to put peanut butter in and to take dead mice out...

1 comment:

Chris Rosenbaum said...

OW! You must have been fortunate for low current or very brief contact...8A DC is enough to result in asystole (flatline).

Actually I've been trying to find your E-mail address because of some interesting ideas I had...first, I am glad that you open-sourced the RepRap system. Second, when I saw the video on YouTube about the RepRap, I got to thinking about businesses built around the concept...granted the system is open source, but businesses could make money off of labor in constructing subassemblies.

Third, and this is one thing I'd like to "open source," the recycler machine. This is something that while open-sourced as far as design, will need some hard manufacturing owing to the materials tolerances.

The thing is...what is the maximum tolerance of ABS plastic? An example...this is something I heard from a Legoland rep...(I live in Carlsbad, CA) that a Lego Block can sustain a METRIC TON of compressive force. However, there exists a machine that deal with that.

It's called the BlendTec Total Blender.

The design is simple. It's the manufacture that's the tricky part. Wash the stuff, shred it down, dry it, empty the feed into the hopper which empties into an extruder, which churns out the filament again.

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