That French clock in the sitting room that has not worked for a long time now. A clock sold in Le Havre, an antique Leon Hatot thing, it has been hanging there.
Open the case and give the pendulum a nudge. It swings, and then it stops swinging rather abruptly. Try it again, and the swinging pendulum shows the same behaviour, slowing to a standstill with unnatural speed. Normally it takes some time for a pendulum to slow, this one acts as if there is something retarding it. Return to the breakfast coffee and wonder about that.
The only thing that could affect a magnetic pendulum like that would be an electrical current on the electromagnet. And then only if it is timed exactly wrong, a positiv impulse given where a negative one would be required. So instead of applying force to the pendulum to keep it moving it does exactly the opposite.
Get up and take the clock down from the wall, dislodging a layer of dust from the top of the case in the process. Take out the battery, and put it back in the other way round. Remove the remaining dust from the case and hang the clock up again. Give the pendulum a nudge, and see that it now behaves more like a normal pendulum, still slowing, but not stopping abruptly. Consider this for a few minutes, and then look at the pendulum from the side through the glass sides of the casing, and see that the electromagnetic coil on the pendulum is not centred on the fixed bar magnet. The clock is hanging with a rearward inclination. There are small screws to adjust this, but it is still off centre. Move the top of the clock away from the wall, sliding along the hook to the limit. Now the two magnets are more or less centred.
Give the pendulum a nudge, and now it goes on for a long time.
Getting up early and making a cup of coffee and drinking it and stupidly looking at a clock and wondering why it does not work has a positive effect after all.
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