Electricity meter

 

It struck me yesterday that I'd never bothered to find out how an electricity meter works. I'd guessed that there must be some electromagnet thing going on with the rotating disc. Perusal of the Internet suggests that the disc is part of an induction motor. The disc is non-magnetic but there are electric coils above and below it. These act as electromagnets. Because the supply is alternating, the strength of the magnetic flux through the coils, and thus through the disc, varies. The changing flux induces an emf in the disc which causes eddy currents to flow by Faraday's Law. By Lenz's Law the act as small electromagnets themselves which repel from the coil electromagnets, turning the disc. The bit I have not yet understood is the way that this is wired to make the speed of the disc proportional to the energy supplied. Oh, and I also need to look up 3-phase again. It's been nearly 3 decades since I had a working understanding.

A clock with a crutch

 

I was asked to fix this clock because it was stopping after a few minutes every time it was set going. The problem was that the pendulum was not swinging evenly from side to side. You could tell by listening to the tick-tock. The tick and the tock weren't even. I understand that this means it doesn't swing far enough on one side to pick up the escapement cog. That means that the mechanism can't give the pendulum a push using the energy from the spring wind-up mechanism. Hence the pendulum loses energy and stops. The crutch mechanism appears to be a second part on the back of the pendulum that can be bent out of line to even up the swing even if the clock is not quite level.

How high is that plane? (part 2)

Here's the little finger method applied to the aeroplane. I suspect that my finger might be slightly nearer than previously measured due to the constraints of taking the photograph but would block out no more than 1.2 degrees of my vision if it were 60cm away from the camera. The plane is a quarter the size (I measured a big photograph) so the plane subtends 0.3 degrees of my vision. which is 0.005 radians. If the plane is 60 metres long in real life it is 11 km above me, about 7 miles. This photograph was taken near Oxford. It may be different in Wigton where the planes are preparing to head out over the Atlantic.

My little finger measures angles

A couple of weeks ago I posted about estimating the height of a plane by trying to estimate the angle that it subtends (blocks out) in my vision. I was looking at an astronomy website earlier and I was shocked to note that the Moon and the Sun only block out 0.5 degrees each. The plane didn't look as big as the Sun so I must have overestimated the angle. The site suggested using my little finger to help measure angles. My little finger is 15mm across and held out like this is 700mm from my eye. Hence the angle subtended is 15/700 = 0.021 radians or 1.2 degrees. So the full Moon or the Sun would only occupy half as much of my vision as my little finger. I will test this out as soon as possible and re-estimate the angle that the plane blocks out.

Back to the pylons at Saltholme

 

We went back to Saltholme. Here's a picture of the pylons above the reedbed. Last week I was musing that the smaller pylons were closer together than the bigger pylons (I know this seems obvious but I do like to measure things.). So this week I was driven at 80km/h along the straight road beside the pylons. I timed 11 seconds between the smaller pylons. That would be 244m apart -so my map was correct and they are really 250m apart. And I verified that the bigger pylons were 16 seconds and thus about 350m apart.

Red sky in the morning

 

I was taught "Red sky at night, shepherd's delight; red sky in the morning, shepherd's warning". Well, it's a very red sky this morning. This means that a frontal system is coming in from the Atlantic and it will rain later. At dawn and at dusk, the Sun is low in the sky. The heavier scattering by dust particles allows only the longer wavelength redder rays through. We don't notice much until it reflects off the clouds. The Sun rises in the east but Atlantic fronts come from the west, hence the redder light readily reflects off them. Have a nice day and enjoy the rain later! :)

Root mean square

 

I lit a small bulb using two separate power supplies. The yellow battery pack provided a d.c. supply and the other source was low voltage a.c.

 

 

I connected both sources to the oscilloscope. The flat line is the d.c. and the wave is the a.c. They both provide the same energy because they lit the bulb by the same amount. So the flat line must show the average voltage of the wave. This type of average is called a root mean square. Since the true mean of such a wave would be zero because it is as much negative as it is positive, we have to square it before we average it to get rid of the negatives. Then the root it back to normal size.

 

 

In fact, the flat line is lower than it should be so the two sources can't quite have lit the bulb equally.