# Measuring low Electromagnetic field

1. Jan 18, 2016

### msarker

Hi, I am working on a project where I've to make an electromagnet and very very low amount of current (micro amp to nano amp) will be supplied to the electromagnet. And finally have to measure the magnetic filed with a sensor.

My question, is it feasible to generate measurable magnetic field with this low amount of current?

2. Jan 18, 2016

### Karmaslap

Assuming you're using a solenoid shape which is longer than it is wide (and not all that wide) you can estimate the strength of the field in the center of the solenoid with B = μNI/L. As you can see you will have a very, very small field with micro or nano amps even with a high number of turns. You should be able to easily estimate the strength of the field with this equation knowing what geometry you will be using. Remember that you want your answer in Gauss and not Tesla.

The Earth's magnetic field is .3 to .6 Gauss (Sourceyperphysics) so your field will need to be larger relatively just to be able to simply measure it. If you estimate that you will generate a field that is "close" to this value then you can calculate the strength of the field by the deflection of a compass needle or other small magnet of known magnitude.

3. Jan 19, 2016

### msarker

thank you so much for your reply. I've used the equation of magnetic field along the axis of the solenoid. I've to measure the field at a distance (>=2 mm) from one edge of the solenoid. at this distance the field get reduced than the center of the solenoid.
Increasing number of turns is not good for low current that also it increases the diameter of the solenoid. I've bought a DC milligaussmeter (-2000 milligauss to 2000 milligauss). this milligaussmeter can detect small deflection.
I've only one option to use magnetic core with high magnetic permeability to increase the field strength.
I've bought bunch of core but none of them has high magnetic permeability. I've even bought an expensive alloy rod from GoodFellow. Their website says that it has magnetic permeability more than 50,000 but I've got very low (around 10_based on my experiment).
Based on my calculation, If I use magnetic permeability more than 50,000 than I will have decent measurable magnetic field at a certain distance.
What I understand that, high magnetic permeability value will be multiplied with the value of magnetic field that I've calculated for without core. Am I missing something here?
And also why it is hard to find a magnetic core with high magnetic permeability?

Thank you for your time and consideration.

4. Jan 19, 2016

### jim hardy

5. Jan 20, 2016

### Karmaslap

If increasing your number of turns is no good, can you decrease the diameter of the air gap? That will also give you a stronger field. Materials with high magnetic permeability won't multiply your field- they will "guide" it to some extent so you can more easily measure it. Any ferromagnetic core will also roughly double your field (below saturation, which you are) to measure.

Most materials do not have a high permeability. Pure iron does (~10,000 maybe for 99.9%pure iron? Not sourced.) and then special materials such as Mu-metal or Vanadium Permedur. I know you can order small specially shaped pieces of Permedur online, but I think it has a permeability of only 20-30,000. Mu metal is higher, I think.

https://en.m.wikipedia.org/wiki/Permeability_(electromagnetism)

Turned to wikipedia. You might be ablw to get mu-metal or any of the other materials (those permeabilities are insane. I didnt think any material was over 50,000) but I'm not sure how much it will help. What WILL help you measure the magnetic field more easily is crafting the high-permeability material into a cone-shape, with the wide end at the entrance of the solenoid. The "tip" of the cone will then "output" a concentrated flux which is much easier to measure- in essence the flux lines are being forced together as they struggle to stay inside the highly permeable material instead of the surrounding air. Then by geometry the magnitude of the field you measure, times the area of the tip of the cone, divided by the area of the base of the cone is the magnetic field in the center of the solenoid (or close, not all of the flux will "travel" inside the cone, some will escape.

It would be a lot easier to increase current, or the number of turns, though, but this core geometry tip could help you measure.

6. Jan 20, 2016

### Jeff Rosenbury

You may do several (4) orders of magnitude better on your Gaussmeter. A SERF (spin exchange relaxation free) magnetometer may help. It has some drawbacks which may be unacceptable though.

7. Jan 20, 2016

### jim hardy

msarker - read up on "magnetic circuit"

Only if the entire path is high permeability, which air is not, do you reap the benefit.