# Helmholtz Coil

## Homework Statement

Hi,

I've got a presentation on an experiment we did using the Helmholtz coil and I'm starting to run dry on material. As some additional applications I found that you can:

- Use the coil set up to cancel external interference (Shield other experiments? Not sure about that one.)

- Produce a more stable and uniform field when a third coil is added around the outside.

I'm struggling to find anything regarding these that may be worth talking about. Just wondering if anyone has a very brief, qualitative description of how these two effects work. It's just a small presentation so I don't want to delve into the mathematical depths; especially having gone through all the data...

Many thanks

## The Attempt at a Solution

One of the properties of Helmholtz coils (two coils face-to-face, separated by their own diameters, if I remember correctly) is that they produce an extremely uniform field at the midpoint of the line joining their centres. This was invaluable for people doing magnetic resonance, because it meant that a the whole of a sample placed at that point would experience the same field, and therefore the same resonance condition, to very high precision. Electromagnets for EPR, and (in the old days before superconducting magnets) NMR, were designed in that way, with Helmholtz coils wrapped around the pole pieces, and the sample and its associated EM radiation source mounted in the middle of the gap between the poles. (I'm using the past tense, but it may still be common practice.) The poles could be up to about 25 cm in diameter, though 15 cm or so was probably more common. For EPR a common field would be 0.35 T, and, judging by the sharpness of some of the resonance lines, the fields were uniform to a few ppm over as much as a couple of cm.

Not quite what you asked, but perhaps of interest.

CricK0es
Yeah but you gave me something else to throw in xD So thank you

rude man
Homework Helper
Gold Member
Don't know about shielding. Doubt it. But - per wikipedia:
To improve the uniformity of the field in the space inside the coils, additional coils can be added around the outside. James Clerk Maxwell showed in 1873 that a third larger-diameter coil located midway between the two Helmholtz coils can reduce the variance of the field on the axis to zero up to the sixth derivative of position. This is sometimes called a Maxwell coil.