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How does ammeter work?

  1. Oct 30, 2014 #1

    sophiecentaur

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    This is not promoting an advert - just look and see!
    There is a 'clip on' ammeter for sale on eBay and I can't imagine how it works. It claims to work by clipping it round a cable and the scale shows +/- 30A DC.
    I spent good money on a Hall Effect clamp meter when it looks like I could have spent a whole lot less on one of these. Plus, it doesn't seem to need a battery. I guess the error could be in the "DC" claim. Perhaps you have to see how far the needle moves, momentarily at switch on??
     
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  3. Oct 30, 2014 #2

    The Electrician

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    It's a moving iron ammeter.

    At the very end of this page:

    http://en.wikipedia.org/wiki/Ammeter

    under the sub-heading "Application" you'll find:

    "A portable hand-held clamp-on ammeter is a common tool for maintenance of industrial and commercial electrical equipment, which is temporarily clipped over a wire to measure current. Some recent types have a parallel pair of magnetically soft probes that are placed on either side of the conductor."

    As the eBay description says, "Indicates approximate current and direction of flow when clipped to cables."
     
  4. Oct 30, 2014 #3
    And how well the measurement approximation works determines the class of the instrument. Class 2.5 is less expensive than class 0.5.
    I'm a big fan of analogue electrical instruments, but one must be careful not to buy digital that "pretends" to be analogue one (have seen some funny examples)
     
  5. Oct 30, 2014 #4

    sophiecentaur

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    I guess that several Amps is needed for the instrument to register. It will be a matter of having as light a movement as possible, consistent with being too lively and 'wobbly'. The turning effect due to the B field will not be much.
    I am very happy with my Hall Effect device - a bit of technology well worth waiting for. A good analogue instrument is very expensive and can be a joy to use. I remember lusting after a 'new model' AVO, when I was a lot younger and I frequently borrowed one from my work. Electronics is so good these days and so cheap - for home and 'constructor grade' use.
     
  6. Oct 31, 2014 #5

    NascentOxygen

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    When I was a teenager I salvaged a squat, flat ammeter from the dash instrumentation of an old car wreck in a local dump. The meter had no terminals, just two U supports on its rear (pressed out of the sheet metal that formed its case) and through which the heavy gauge conductor threaded snuggly. I cut off 6" of the cable supporting the meter when I took it home in case that cable material meant something special to the meter. I was astonished that it should function with no electrical connection, and in next to no time I had flattened my giant 6V dry cell battery demonstrating the meter's amazing operation.

    I'm inclined to believe it was centre-reading, too.

    Yes, sophie, as I recall it was indeed a wobbly needle! Well underdamped. It was of German manufacture, as I recall. Amazing inventors, those Germans.
     
  7. Oct 31, 2014 #6

    sophiecentaur

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    Thanks for that info. The field round a current carrying wire can certainly be enough to upset a compass but it's another matter to use it actually to measure the actual current. It confirms my view that it probably is as much use as a chocolate teapot, for my purposes. :p
     
  8. Nov 1, 2014 #7

    jim hardy

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    I have two such meters, one an antique similar to what Nascent described.
    Basicaly it's a compass that deflects in presence of field, as you described. It is thirty amps full scale.
    The other is a much newer Gauss meter. Earth's field will barely deflect needle when I swing it from due North to due South. ( I forget how many gauss full scale)

    They appear identical except for the wire guides on back of ammeter and it's 1930's font on meter face.
     
  9. Nov 2, 2014 #8

    sophiecentaur

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    [QUOTE="jim hardy, post: 4899873, member: 327872"]I have two such meters, one an antique similar to what Nascent described.
    Basicaly it's a compass that deflects in presence of field, as you described. It is thirty amps full scale.
    The other is a much newer Gauss meter. Earth's field will barely deflect needle when I swing it from due North to due South. ( I forget how many gauss full scale)

    They appear identical except for the wire guides on back of ammeter and it's 1930's font on meter face.[/QUOTE]

    I should have known!! :D
    Not an item I would expect to find regularly used these days. But, before Hall Effect, a lot better than nothing.
     
  10. Nov 2, 2014 #9

    OmCheeto

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    I've been scratching my head as to how these things work, even after several hours of research. About the only thing I figured out was who invented this device.

    In the interest of science, I just built one, based on Jim Hardy's comments.

    I'm still not sure how exactly it works, as the data seems a bit scattered.
    Here's my setup:

    One current carrying conductor, aligned with the earths magnetic field
    A set of 4 neodymium magnets, self-clamped onto the end of a thread, suspended from a stick
    One 4 x 6 inch card, with blue lines aligned with the earths magnetic field, and a black line drawn in, at 45°
    And duct tape, of course.

    Discharging:
    pf.2014.11.02.0932.discharging.at.45.degrees.jpg

    Charging:
    pf.2014.11.02.0933.charging.at.90.degrees.from.dischargeing.jpg

    data:
    distance: measured with a nonmetallic caliper
    amps: measured with a digital multimeter
    gauss: derived, from adding everything together(might be wrong)

    dist(m)__amps____gauss
    0.11_____6.1_____0.11
    0.028____3.1_____0.22
    0.016____2.7_____0.34


    Given, that there are 3 magnetic fields: one circular(conductor), one linear(earth's), and one, somewhat in-between(magnets!), I can only state that I was very happy to be not off by an order of magnitude.
     
  11. Nov 2, 2014 #10

    jim hardy

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    OM now that is Neat !!!

    I see your magnet rotated.......

    My antique ammeter is very handy for qualitative measurements under the car hood (Bonnet?).

    Old jim
     
  12. Nov 2, 2014 #11

    OmCheeto

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    Neater still, is after my nap, I went back and looked at my setup, and decided I wasn't qualified to tie my own shoes.

    I'd started the experiment at the lower amperages, and decided that the distances were so close, that I didn't have to worry about stray magnetic fields. Unfortunately, that stuck in my head, as the distances got really big. Big mistake:

    pf.2014.11.02.1907.idiot.and.his.magnets.jpg

    The big black conductor made a fairly decent half loop, so my equation for the magnetic field generated by a straight wire was invalid.

    I just redid the experiment. The results were much better.

    dist(m)__amps____gauss
    0.058____9.5_____0.33
    0.037____6.1_____0.27
    0.023____3.3_____0.29

    0.008____1.7_____0.43

    Of course, you can't get rid of stray fields, but you can estimate them:

    dist(m)__amps____gauss
    0.300____9.5_____0.06
    0.600____9.5_____0.03


    Tomorrow, I will cut up some tuna fish cans, and see if I can't contain some of the fields.

    SCIENCE!!!!!!!!

    ps. I'm still trying to figure out why, when I'm told that the earth's flux is ≈0.54 gauss where I live, I come up with a smaller number. And why do the earth's magnetic field lines have a negative slope where I live?

    pf.2014.11.02.2006.earths.magnetic.tilt.lines.at.Oms.house.jpg

    And should I modify the experiment to compensate for that?

    pps. The image was taken with a perfectly level level. I was the one askew.
    ppps. Borek was correct. There is dirt on my lens.
     
  13. Nov 2, 2014 #12

    OmCheeto

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    Neater still, is after my nap, I went back and looked at my setup, and decided I wasn't qualified to tie my own shoes.

    I'd started the experiment at the lower amperages, and decided that the distances were so close, that I didn't have to worry about stray magnetic fields. Unfortunately, that stuck in my head, as the distances got really big. Big mistake:

    pf.2014.11.02.1907.idiot.and.his.magnets.jpg

    The big black conductor made a fairly decent half loop, so my equation for the magnetic field generated by a straight wire was invalid.

    I just redid the experiment. The results were much better.

    dist(m)__amps____gauss
    0.058____9.5_____0.33
    0.037____6.1_____0.27
    0.023____3.3_____0.29

    0.008____1.7_____0.43

    Of course, you can't get rid of stray fields, but you can estimate them:

    dist(m)__amps____gauss
    0.300____9.5_____0.06
    0.600____9.5_____0.03


    Tomorrow, I will cut up some tuna fish cans, and see if I can't contain some of the fields.

    SCIENCE!!!!!!!!

    ps. I'm still trying to figure out why, when I'm told that the earth's flux is ≈0.54 gauss where I live, I come up with a smaller number. And why do the earth's magnetic field lines have a negative slope where I live?

    pf.2014.11.02.2006.earths.magnetic.tilt.lines.at.Oms.house.jpg

    And should I modify the experiment to compensate for that?

    pps. The image was taken with a perfectly level level. I was the one askew.
    ppps. Borek was correct. There is dirt on my lens.
     
  14. Nov 3, 2014 #13

    dlgoff

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    Earths field can be effected by local anomalies. Here's NOAAs Geomagnetic Calculators.

    Because you don't live on the equator? Which way is north in your pic?
     
  15. Nov 3, 2014 #14

    OmCheeto

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    I've been playing with my magnets for quite some time, and never researched why they pointed consistently downward, in the northern direction.

    Here's the result of your calculator for where I live:

    Date_____________________2014-11-03
    (+E|-W ) Declination_____15° 37' 16"
    (+D|-U) Inclination______67° 27' 56"
    Horizontal Intensity_____20,278.9 nT
    (+N|-S) North Comp_______19,529.9 nT
    (+E|-W) East Comp_________5,460.6 nT
    (+D|-U) Vertical Comp____48,874.5 nT
    Total Field______________52,914.6 nT


    I have no idea what any of that all adds up to, other than the total field is very close to what I saw earlier.
    Since I live near the 45th parallel, do I subtract 45° from the inclination?

    I always assumed one of two things:
    1. There's a nickel-iron meteorite in my back yard, or
    2. I'm sitting on top of 2 kilometers of semi-ancient basalt. (17 to 6 million years ago)​

    To the right.
     
  16. Nov 3, 2014 #15

    dlgoff

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    Holy crap. Must be number 1.

    Anyway,

    http://en.wikipedia.org/wiki/Magnetic_declination

    http://en.wikipedia.org/wiki/Magnetic_dip
     
  17. Nov 3, 2014 #16

    OmCheeto

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  18. Nov 5, 2014 #17

    NascentOxygen

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    How can you be sure you have that assemblage balanced at its CoG?
     
  19. Nov 5, 2014 #18

    OmCheeto

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    Balanced at the center of gravity? Obviously it isn't/wasn't.* The string was vertically supporting the magnets, so the fulcrum was at the top. (another reason why people should be tying my shoes)

    I gave up after 30 minutes of trying to balance the magnets @ 67.5° with a horizontal supporting string. Buckycubes are only 4mm on the side, and eyeballing such things, at my age, are, well, fruitless.

    As for the tilt of the magnetic field:

    I'm sure I'd seen the images before, but it never really sank in.

    pf.2014.11.05.1707.global.magnetic.dip.jpg

    ------------------------------------------------------------------------------

    * I thought of that also. So I swapped the magnets. It only made things worse:

    pf.2014.11.05.1818.it.only.made.things.worse.jpg

    After a lifetime of looking at compasses pointing north, it was quite the shock to see that north was 2 feet in front of me.
     
  20. Nov 5, 2014 #19

    dlgoff

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    So the meteorite is smaller than you thought? I like your perseverance.

    What about those boats. Still have one?
     
  21. Nov 5, 2014 #20

    OmCheeto

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    One? My FIVE boats are made of wood.

    rosi_still_floats.JPG

    All in just in good as shape as this little lady. :)
     
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