Why the primary is a flat spiral?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Shaun Epp by way of Terry Fritz <twftesla-at-qwest-dot-net>" <scepp-at-mts-dot-net>

I did a demonstration to a group of Ham Radio people I know this evening and
it went really well.  I demostrated the spark show, burning / stripping a
CD, tube light lite from a distance and on top of the coil.  They where all
impressed, I even recieved  a round of applause   : ^ ) .  One woman asked
me why the primary coil was flat and I was somewhat stumped.  I just
explained that its used to couple the energy into the the secondary.  Now
I'm wondering what to tell the next person, something to do with coupling?

What other stage tricks do others do?