On laser spirit levels

A few weeks ago I picked up a cheap laser spirit level (£4.99) at a motorway service station, as an impulse buy. It was a good buy, because unlike the laser pointers (costing three or four times as much) you can buy to highlight parts of your presentation, it produces a line rather than a spot.

However, today I found a proper DIY job for it. Arrange a series of picture up the staircase. It dutifully generated the require line, and after a little tweaking I got it parallel with the dado rail, and hence (by trusting inference) with the stairs themselves. It was easy to measure regular intervals along the line and mark for the picture-hooks. Great!

Unfortunately, the pictures hang from cords, and it is impossible to tie the cords to precisely the same length, so despite the preparation, the overall array is a mess. Or was, until I spent goodness knows how long moving picture hooks and re-tying cords and testing by eye to get it right.

There's a moral here, related to Ashby's law of requisite variety ;

The most potent element of the system is the one you can't measure

It's certainly true of teaching.

That's what makes it so much fun!