Helmholtz coils ...

... theory and practice

Hi,

I just built a small Helmholtz coil and see a discrepancy between theory and practice.

The calculation using two different fornulas have as result the exact same value in Tesla or µT. Below in the attached images.

Now measuring with a magnetometer of a cell phone gives me a more or less 50% real value compared to the calculated theoretical value. Measured with tw different phones. You can also see that the substraction (changin the polarity of the input voltage) of the geomagnetic field gives me plus/minus the same value.

Any explanation why so ?

Thanks in advance for comments

regards Rainer

Hi,

I made some more test with the phone in a different position and not centered but resting on the inner diameter

I thought from what I read that the strongest field is in the center or did I misunderstand that ¿?

to do your measurements you will want to use a magnetometer probe, not a cell phone. a cell phone has other materials inside it that can add errors to your readings. also, how are you regulating the current through the coils?

rsfoto said:

I thought from what I read that the strongest field is in the center or did I misunderstand that ¿?

Click to expand...

no, the most uniform field is in the center, and the fields increase in intensity with proximity to the actual coils:

unclejed613 said:

to do your measurements you will want to use a magnetometer probe, not a cell phone. a cell phone has other materials inside it that can add errors to your readings. also, how are you regulating the current through the coils?

Click to expand...

Thanks and I am using a regulated power supply

unclejed613 said:

no, the most uniform field is in the center, and the fields increase in intensity with proximity to the actual coils:

View attachment 136395

Click to expand...

Thanks

Hi,

BTW it looks like I misunderstood the formula for calculation.

A single coil has in my case 9 turns and 60mm diameter. I input 18 turns and I guess that is why I have the double plus/minus the errors.

After correcting my calculation the measured value was more or less the calculated value plus / minus

After making some more test theory and practice do fit with of course the normal errors we have as amateurs and simple measuring methods like our cell phones

Now I bought some sensitive bipolar hall sensors and will built some measuring devices.

Nothing serious really just having fun.

Thanks for your time