### Author Topic: Ampere experiment.  (Read 3276 times)

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#### NickPerry

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##### Ampere experiment.
« on: October 21, 2016, 08:46:58 pm »
Ok, I'm trying to find an example of an actual way to measure 1 amp (NOT using a pre-known measurement tool like a multi meter)

according to Wikipedia the procedure goes as follows:

-get 2 infinity long wires.
-Place them in a vacuum.
-space them 1 meter apart.
-attach a scale to them to measure how much attraction force they exert.
-pass a current through both of them (same current, in the same direction, so I guess hook the 2 wires up in series?)
adjust the current until the scale reads 2 x 10 ^ 7 Newton meters of force. (no clue where they got this number, if you know please let me know)

now you have the baseline of 1 amp, and can continue to count the electrons to get a coulomb, and calculate out what 1 volt/ohm/watt is.

but I can't seem to find anywhere where they do this experiment (lots of people making electromagnets, but no one figuring out the actual ampere's running through the system using that magnetic attraction)

any thoughts on this?

#### TimFox

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##### Re: Ampere experiment.
« Reply #1 on: October 21, 2016, 08:53:24 pm »
In practice, this is done by winding two coils with accurate dimensions and calculating the force per ampere between them using your equation and the integral calculus.
These two coils are then used in a balance, similar to a weighing scale, comparing the force between the stationary and dangling coil with the gravitational force on the usual masses on the other side of the beam.
See  http://physics.kenyon.edu/EarlyApparatus/Electrical_Measurements/Current_Balance/Current_Balance.html

« Last Edit: October 21, 2016, 08:56:33 pm by TimFox »

#### NickPerry

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##### Re: Ampere experiment.
« Reply #2 on: October 21, 2016, 08:57:35 pm »
so basically a watt balance in reverse?

makes sense, though I'm not sure how one converts an infinity long wire to that of a finite length coil. but I guess that's why we have mathematicians.

#### Rerouter

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##### Re: Ampere experiment.
« Reply #3 on: October 21, 2016, 09:02:52 pm »
the wires can be any length, what you will be measuring is the repelling force of the magnetic field that current generates,

Here is what you are currently using for the math
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/wirfor.html

While you should probably be using force per unit length as per here
http://theory.uwinnipeg.ca/physics/mag/node10.html

This effect can be measured even centimeters apart, just the closer they are the easier it is to see,
I would not have through a vacuum to be necessary, just still air,
and its 2x10^-7 Newtons of force produced by 1A flowing through long wires spaced 1m apart,

still, if you get them much closer that force increases, but there is a golilocks zone where your wire diameter appears small compared to the spacing, and the spacing small to the wire length
https://en.wikipedia.org/wiki/Amp%C3%A8re%27s_force_law

#### NickPerry

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##### Re: Ampere experiment.
« Reply #4 on: October 21, 2016, 09:06:28 pm »
Thank you!
that's some very useful information!

Time to crunch some numbers & build an experiment ^.^

Thanks again!

#### Brumby

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##### Re: Ampere experiment.
« Reply #5 on: October 22, 2016, 02:10:52 am »
.... though I'm not sure how one converts an infinity long wire to that of a finite length coil. but I guess that's why we have mathematicians.

Mathematicians handle infinitely long wires as easily as EEs can handle a soldering iron.

Being capable in both is fun.

#### IanB

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##### Re: Ampere experiment.
« Reply #6 on: October 22, 2016, 02:31:50 am »
In case it is ambiguous, the force between parallel wires is the force per unit length. The total force between a pair of infinitely long wires would of course be infinite...

The reason for saying the wires should be "infinitely" long is to ensure a uniform magnetic field and avoid any end effects where the magnetic field becomes distorted.
I'm not an EE--what am I doing here?

#### vk6zgo

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##### Re: Ampere experiment.
« Reply #7 on: October 22, 2016, 06:42:32 am »
In case it is ambiguous, the force between parallel wires is the force per unit length. The total force between a pair of infinitely long wires would of course be infinite...

The reason for saying the wires should be "infinitely" long is to ensure a uniform magnetic field and avoid any end effects where the magnetic field becomes distorted.

Of course,if they are infinitely long,they may traverse regions which cause such distortion of the magnetic field.
No matter,they are in "free space".

In very early times,one alternative specification was how much mass of metal a current could deposit in a given time,in an electroplating bath.

Smf