Mailbox: why does my digital watch drift?

This is the part of the show where we read out your correspondence. First up,Mark Jacobsen is pondering how a tank moves forward on its continuous track...

Chris - Right well, this is very similar to the way that a digger on tracks actually moves. Inside those tracks, there's a cog on each end of the digger, and that cog engages with a groove on the underside of the track, and as the cog moves around, and the same way as a bicycle crank moves the bicycle chain, it actually advances the track and because the track is basically being picked up at one end and laid down at the opposite end, the digger rolls forward inside the track it's leaning forward, and the enormous surface area that you've got with those tracks against the ground means you get an enormous amount of traction, which is why things like big heavy earth moving machinery and tanks use them.

Adam - Brilliant. And meanwhile Nev asks: Why does the time on my digital wristwatch always end up being five minutes ahead within three months? I always turn it back but it still jumps ahead. Every digital wristwatch I have ever had did this.

Chris - Yeah I'm with you Nev, happens to me as well. The reason is that lots of these watches use crystals, they actually use a piezoelectric crystal. Basically what you do is you apply a small electric current to the crystal. The crystal is very finely shaped so that it vibrates when you put the electricity in, and it vibrates a bit like a miniature tuning fork and it does it, usually in those crystals 32,768 times a second, which means you can use that to work out roughly how fast the watch is ticking, but it does that at a certain speed which is determined by the temperature, and if you warm the watch up it will vibrate a bit faster. If you cool it down it will vibrate a bit slower, and because most people's watches are not very well insulated and they’re not a constant temperature, there is a wandering of that vibrational frequency of the crystal. So the watch timekeeping wanders. Really high quality watches and timepieces do two things; one, they insulate the crystal to stop the temperature variations happening so much, and also they incorporate machinery to correct for temperature variation but cheap watches like you’re £2.50 one you get from a Christmas cracker don't do that. So you just have to put up with the fact you're gonna be either very early or very late for you’re meeting pretty quickly.