# [TCML] Tesla coil EM Gauss readings

Mon Feb 25 07:07:10 MST 2008

```Mddeming at aol.com wrote:
>
> In a message dated 2/24/08 8:50:55 P.M. Eastern Standard Time,
>
>
> You  can also calibrate your probe with a pair of Helmholtz coils.. Two
> parallel coils separated by their radius (i.e. 10 cm diameter separated
> by 5 cm).  It has an area in the middle that's fairly uniform field,  and
> you can calculate the field by physics, if you know the current.   You
> put an AC current in, stick your probe in, and go to  it.
>
>
>
>
> Hi Jim,
>
> This is absolutely correct, but what percentage of newbies (or others)  would
> have any idea what the required physics calculations are?
>
> Matt D.
>
>
<grin>Why those that take the first hit on Google for Helmholtz coil
(which happens to be the wikipedia entry):
http://en.wikipedia.org/wiki/Helmholtz_coil

B= (4/5)^1.5 * mu0 * n * I /R
mu0 is 4piE-7
n is number of turns
I is current

B= 8.9918E-7 * n * I / R

by the way, if you want a field comparable to that of Earth (1 Gauss
around 1E-4 tesla) with 1m diameter coils, it takes about 1 amp through
56 turns on each coil.  If you assume perfect coupling between the
coils, that works out to about 32 mH of inductance, which is about like
a typical woofer, so bear that in mind when designing your drive circuit.
```