How does a clip-on ammeter measure current without battery power?

[sophiecentaur]

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??

 

[The Electrician]

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."

 

[zoki85]

As the eBay description says, "Indicates approximate current and direction of flow when clipped to cables."

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)

 

[sophiecentaur]

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)

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.

 

[NascentOxygen]

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.

 

[sophiecentaur]

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

 

[jim hardy]

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.

 

[sophiecentaur]

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.

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.

 

[OmCheeto]

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.

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.

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.

Friedrich Drexler (1858 - 1945)
To his later regret, in 1884 he did not patent his invention of electrotechnical metres – today known as moving-iron instruments.

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 Earth's 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 Earth's magnetic field, and a black line drawn in, at 45°
And duct tape, of course.

Discharging:

Charging:

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.

 

[jim hardy]

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

 

[OmCheeto]

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

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:

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?

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.

 

[OmCheeto]

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

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:

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?

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.

 

[dlgoff]

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.

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

And why do the Earth's magnetic field lines have a negative slope where I live?

Because you don't live on the equator? Which way is north in your pic?

 

[OmCheeto]

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

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)​

Because you don't live on the equator? Which way is north in your pic?

To the right.

 

[dlgoff]

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"
...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.

Holy crap. Must be number 1.

Anyway,

Magnetic declination or variation is the angle on the horizontal plane between magnetic north (the direction the north end of a compass needle points, corresponding to the direction of the Earth's magnetic field lines) and true north (the direction along a meridian towards the geographic North Pole).

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

Magnetic dip, dip angle, or magnetic inclination is the angle made with the horizontal by the Earth's magnetic field lines. This angle varies at different points on the Earth's surface. Positive values of inclination indicate that the magnetic field of the Earth is pointing downward, into the Earth, at the point of measurement, and negative values indicate that it is pointing upward.

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

 

[OmCheeto]

Holy crap. Must be number 1.

Anyway,
http://en.wikipedia.org/wiki/Magnetic_declination
http://en.wikipedia.org/wiki/Magnetic_dip

After some more experimenting with the magnetic dip, I've decided that this is a homework problem. Good grief!

 

[NascentOxygen]

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?

How can you be sure you have that assemblage balanced at its CoG?

 

[OmCheeto]

How can you be sure you have that assemblage balanced at its CoG?

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.

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

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

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.

 

[dlgoff]

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.

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

What about those boats. Still have one?

 

[OmCheeto]

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

What about those boats. Still have one?

One? My FIVE boats are made of wood.

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

 

[dlgoff]

One? My FIVE boats are made of wood.

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

Probably don't need a compass or an ammeter but cool all the same.

 

[NascentOxygen]

Appears to be permanently maintained on a floating charge.

 

[OmCheeto]

hmmmm... I rebuilt my ammeter, and discovered that I don't know how to balance the system.

How on Earth did those people in the olden days figure this out!

 

[sophiecentaur]

Has this thread lost its way a little. All the discussion has been about a wire with a magnet next to it. Surely the iron yoke is the important thing.
I wonder how relevant the Earth's magnetic field is, to this when you have a massive iron yoke around the wire and a gap with the compass in it. I have a feeling that what's needed is a small (compass style) magnet which will fit inside the gap - sprung to lie parallel with the flat ends of iron U. For the best calibration, the magnet must be small and to fit well within the local magnetic circuit. The field due to the current in the wire will be around the yoke and will cause the magnet to deflect against the spring according to the sense and amplitude of the current. I have a feeling that the reluctance path across the gap will be so much higher than the path through the 'back' of the yoke that the Earth's field will have little effect on the magnet itself.
I think it's a sort of common mode / differential mode thing, with the Earth's field only affecting the differential mode.

PS After a few weeks, the boat may be watertight - if the reason it sank is that the boards had shrunk. I have a friend who had an old clinker built boat and he used to let it sink at the beginning of the season and then it was ok.

 

[RonL]

One? My FIVE boats are made of wood.

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

Better put a lock on that beauty, someone might try to steal it.

 

[OmCheeto]

Has this thread lost its way a little. All the discussion has been about a wire with a magnet next to it. Surely the iron yoke is the important thing.

I've looked an many diagrams, and can't figure out how that works, which is why I went with Old Jim's description.

I wonder how relevant the Earth's magnetic field is, to this when you have a massive iron yoke around the wire and a gap with the compass in it. I have a feeling that what's needed is a small (compass style) magnet which will fit inside the gap - sprung to lie parallel with the flat ends of iron U. For the best calibration, the magnet must be small and to fit well within the local magnetic circuit. The field due to the current in the wire will be around the yoke and will cause the magnet to deflect against the spring according to the sense and amplitude of the current. I have a feeling that the reluctance path across the gap will be so much higher than the path through the 'back' of the yoke that the Earth's field will have little effect on the magnet itself.
I think it's a sort of common mode / differential mode thing, with the Earth's field only affecting the differential mode.

I just redid the experiment, with the conductor vertical, and came up with the following:

dist(m)__amps____gauss
0.006____1.7_____0.57
0.014____3.4_____0.49
0.036____6.1_____0.34
0.051____8.3_____0.33

At least I'm getting closer. The vertical component, per Don's computer, should be 0.49 gauss.

I'm calling this, close enough for government work.

PS After a few weeks, the boat may be watertight - if the reason it sank is that the boards had shrunk. I have a friend who had an old clinker built boat and he used to let it sink at the beginning of the season and then it was ok.

I started a new thread.

 

[dlgoff]

I just redid the experiment, with the conductor vertical, and came up with the following:

dist(m)__amps____gauss
0.006____1.7_____0.57
0.014____3.4_____0.49
0.036____6.1_____0.34
0.051____8.3_____0.33

At least I'm getting closer. The vertical component, per Don's computer, should be 0.49 gauss.

I'm calling this, close enough for government work.

Excellent.
So the boat diversion didn't hold up your objective:

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

 

[jim hardy]

Sorry if i took an oversize leap of imagination...

Sophie's description has to be right on. Torque on a magnet opposing torque from a hairspring is surely how they work.
Will disassemble one when get back to Arkansas and post a picture...

I can relate that my gauss meter and my ammeter show about same miniscule deflection when rotated wrt Earth's field.

 

[jim hardy]

by the way i too love wood boats.
Circa 1986 I stumbled across a builder in Canada who still made wood and canvas boats
and brought home to the Fla Keys one of his "Marquette" 16' canoes

http://nor-west.ca/en/

but i felt guilty scraping it across oyster beds in Florida Bay so sold it to a nature photographer who uses it in North Florida's spring fed rivers.. it's been in several calendars now.
Might well be the only one of his boats to have seen latitude 25...

sorry for the digression ... but you know; they just don't make nostalgia like they used to.

 

[sophiecentaur]

Dogs and wooden boats - I feel the same about them both. I love to look at and play with other people's. They are both too much like hard work to own - I'd end up resenting them and that wouldn't be good. My boat is GRP and she takes enough hours of maintenance every year as it is. At least I can neglect her a bit now and again and she doesn't rot and her boards don't shrink when she's dry. I look after my daughter's dog now an then. Great fun but she goes home at the end of the week.