clock going counter-clockwise

Discussion in 'General Science & Technology' started by Avatar, Mar 25, 2005.

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  1. Avatar smoking revolver Valued Senior Member

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    Hello all!

    Here's the case:

    My mother today took my joungest sister to some sol-fa lessons (a home teacher). So, it was a regular flat of an old lady. After some hour she watched at a clock that was on a shelf. Clock was a regular, battery run alarm clock with dials. She noticed it had stopped and it stayed so for some minutes, then before her eyes (the clock was infront of her) it started counting time counter-clokwise. She could clearly see that, because it had a second dial (or what you call those). And it continued to count time backwards for some 15 minutes, all the time she stayed there. Then the lesson was over and she left.
    She asked me what could be the explanation and I don't know, so I'm asking you. What could have caused that?
    There was no computer in the room and nothing apart from the usual except a few piano strokes now and then.

    Maybe a low battery? An sms of a cell-phone? Piano?

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    Hope you can clear this up!

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    p.s. To ghosts busters and exorcists: sorry, this isn't a question for you
     
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  3. plexus Registered Senior Member

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    Low battery.
     
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  5. Avatar smoking revolver Valued Senior Member

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    Why should it cause a clock to go backwards? What's the physics/mechanism behind that?
    I want an explanation, not a relevation, please.

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  7. plexus Registered Senior Member

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    I know it is because of low battery from experience: my own kitchen clock does that; once the battery is changed, it acts normal again.
    What's inside the clock? Those wheels? Perhaps there's a clue.
     
  8. invert_nexus Ze do caixao Valued Senior Member

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    I suspect metaphor run amuck coupled with a propensity for young girls to suffer from hysteria.
     
  9. Iris Registered Senior Member

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    You can buy novelty clocks that run backwards. Are you sure it wasn't running backwards all the time, and she just happened to notice it 15 minutes before the end of the lesson?
     
  10. Avatar smoking revolver Valued Senior Member

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    I am sure.
    Also see plexus' post.
     
  11. Trilobyte Registered Senior Member

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    In that type of battery-mechanical clock it works using a round shaped permanent magnet and a solenoid coil (just a densely wound copper coil). The solenoid is pulsed 'on' in a short burst and stays off for a second, then makes a pulse in the next second that is opposite in polarity of current flow through the coil (and so opposite in magnetic polarity) to the first pulse. The electrical pulses are created by an astable pulse generator circuit. The round permanent magnet has a gear fixed to it and sits loosely in a cup-shaped pit.When the electromagnetic solenoid is pulsed it attracts the furthest edge of the permanent magnet and forces it to make a half turn towards it then stop then stay there when the field is removed.(meanwhile the gear has moved the second hand round by one second and the hour and minute hand by the equivalent using small plastic gears). When the current direction through the magnet is in the reverse direction for the next pulse, the field is reversed and so now repels the end it previously attracted (and attracts the other edge of the permanent magnet) forcing it to make another half turn, rotating the same way, and moving the hands round by another second. The ticking noise mostly comes from the rattle of this little round permanent magnet flipping over once each second.

    The key point(for reverse rotation), I think, is with the location of the poles of the permanent magnet. I have not properly checked but I think to ensure that the permanent magnet only rotates in the required direction the poles must not actually be at either side of the diameter but sightly off around the circumference (the same way a horse shoe magnet can be bent - but only enough to create that effect).

    When the battery is low, the solenoid is weaker and the permanent magnet doesn't complete a half turn ( at random every so often ) then in the next second this misalligned starting position must cause it to flip back in the opposite rotation direction which then goes on as the new normal direction.

    Actually,thinking about it, the polar misallignment is not necessary (although it could be a useful improvement!). More likely when the permanent magnet flips over (it is rather fast of you watch it) the momentum keeps it turning very slightly before stopping and this gives enough of an angle to bias the next flip to turn the same direction. Again with low battery, weaker solenoid and so there is not enough momentum and instead of being off centre slightly in the correct direction it is off centre and biased towards turning backwards or flipping one way then the other so that the second hand never makes any progress.

    My GOD that took far longer to explain than to think of!!
     
    Last edited: Mar 26, 2005
  12. Trilobyte Registered Senior Member

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    By the way just for a better mental picture, the cylindrical permanent magnet is normally about 1/2 cm tall and about the same diameter.
     
  13. Avatar smoking revolver Valued Senior Member

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    I thank thee for this explanation though.

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  14. Odin'Izm Procrastinator Registered Senior Member

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    I agree thats interesting information never knew how clocks worked untill now... and im being serious.

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  15. Nomadd22 Registered Senior Member

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    Maybe the clock was Australian.
     
  16. Odin'Izm Procrastinator Registered Senior Member

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  17. spidergoat pubic diorama Valued Senior Member

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    The clocks at the National Institute of Science and Technology (a government agency) ran backwards sometimes. They had dials, but all of them were coordinated from a central clock (that's where they have an atomic clock). The one in my dad's office ran slow, but on the hour, it reset itself. So, maybe the old lady's clock was set up the same way. I've seen some clocks at the Sharper Image that do that too.
     
  18. Thersites Registered Senior Member

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    Don't know if they're still made, but there used to be clocks which ran counter-clockwise on sale in the UK: the numbers were arranged the opposite way to an ordinary clock's too. They were partly a novelty, partly a way of making people think and look twice when they checked the time.
     
  19. Avatar smoking revolver Valued Senior Member

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    spidergoat.. an old lady has a clock coordinated to an atomic clock?! wtf you've been drinking?!

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    Trilobyte explained everything perfectly
     
  20. neil cox Registered Senior Member

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    Some people who live in cheap flats, spend big money for strange things. I've seen atomic clocks advertised for as little as $30/USA. The better ones would not run backwards more than one second, except to make a daylight time correction/probably at 2 am once per year.
    A gag clock is possible with even more elaborate behavior, but more likely the magnet explanation is correct. Several other designs have been sold in recent decades. Neil
     
  21. CANGAS Registered Senior Member

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    It would be rare to have any two batteries that would run down exactly in unison even on identical load. If the clock ran on two batteries, then one could reach zero voltage while the other still had enough voltage to run rhe clock. However, the hot battery could then reverse-charge the dead one, depending on the circuitry of the two. Then, when the hot one goes dead, the reverse-charged one could run the clock motor backward. I'm not just guessing. I'm personally familiar with situations of a weak battery reverse-charging a dead battery.
     
  22. Oli Heute der Enteteich... Registered Senior Member

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    I've got one of that ilk. Wonderful for when you have post-pub guests and you ask them to check the time

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  23. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    Do you know why clocks go "clockwise"? I give hint to answer:

    Be a stick in the mud and spin with the Earth.
     
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