# Re: Classic coil

```Tesla List wrote:
>
> >From Hans.Grimstad-at-maxware.no Sun Dec  1 22:28:46 1996
> Date: Sun, 1 Dec 1996 23:34:05 +0100
> To: tesla-at-pupman-dot-com
> Subject: Classic coil
>
> Hello everyone !
>
> I have been rereading my reprint of Teslas lecture to the Institution of Electrical
> Engineers in London. On page 15, he describes a small bipolar coil. Each of the
> terminals of the coil is connected to a circle, one is 80 cm in diameter, the other is
> 30 cm in diameter. During operation of the coil, the discharges between the
> terminals produce a "luminous sheet" with an area of about 0.43 square metre. He
> states that he in earlier experiments, using bigger circles had covered an area of
> more than one square metre.
> This coil has 2 primaries with 96 turns in each, and two secondaries with 260 turns in
> each. When both the primaries and the secondaries are connected in series, this
> gives a ratio of conversion of about 1:2.7.
>
> It seems that a lot of people are designing coils with much bigger conversion ratios
> (1:67 for a coil with 1000 windings on the secondary and 15 on the primary). I would
> say that Teslas results with this coil are quite impressive. Why the big secondaries
> in "modern" coils ?
>

Hans -

The main reason for "big" secondaries is to get high inductance
values for the secondary coil. In a Tesla coil, this inductance plays
a big part in determining output voltage -- it's not a simple function
of turns ratios. (My large coil has only about 250 turns in it's
secondary, yet cranks out a 72" spark.)

Also, the length of the secondary is a consideration when you take
into the fact that the output spark has a tendency to go where it isn't
wanted, ie: down the secondary. A taller coil form withstands a higher
output voltage.

Hope that clears up your question.

- Brent
```