From: Paul
Date: Sat, 09 Mar 2002 22:16:00 +0000
Subject: Re: [TSSP] short H/D
Hi Terry, I'm looking for unequivocal experimental evidence that the current in a resonating coil can exceed the base current by quite a noticeable amount in some geometries. This would provide an experimental answer to Antonio's question to me from the pupman list today: > Can Les be greater than Ldc? I think so, the math says so, net of my errors. Also we see a reference in Breit, 1921. But I think in order to drive the point home it would be nice to have the current profile of a very short h/d coil, or a flat spiral. We can predict quantitatively for neither reliably, until the caps program can handle those dielectrics, but qualitatively the raised current max ought to be there and in the case of a flat spiral should be quite high, eg 140% of rim current. Experimental confirmation would validate our determination of the effects of the coil's internal capacitance, ie we would be observing the circulating current through that internal capacitance and witnessing its contribution to the total EMF. Even your h/d=2.92 coil should have an elevated current max, but it will be less than 5% higher than Ibase, so not a very clear effect. In my h/d=1.36 it should be a max 15% above Ibase, which is more noticeable. Need to get down to h/d=1 for a really clear current profile. The current max should be well up towards the center of the coil, say 35% or 40% height, with a current some 20% or 25% higher than Ibase. I posted some notes to pupman describing very roughly how to go about it, I'll paste them in here... [starting with a coil wound with several series taps] Initially no resistors inserted, the series taps are closed by links. And we don't go anywhere near the coil with a meter - the coil must remain negligibly perturbed by the measurements. Insert a resistor into one of the links and measure Q. Now remove the resistor and replace the shorting link. Insert a variable resistor into the base lead, and adjust this to get the same Q. Repeat for every tapping point. The loss at the tapping point x is proportional to I(x)^2 Rt where I(x) is the current at x, and Rt is your tapped-in resistance. To obtain the same loss, ie equal Q, with a resistance in the base lead, you need Ibase^2 Rb = I(x)^2 Rt. So, for each tap x, you can work out the profile I(x)/Ibase = sqrt(Rb/Rt). The taps and tap resistor need to be physically quite small, so that you don't disturb the capacitance by more than a percent or so. Check this by monitoring Fres throughout the proceedings. Bart wrote: > If so, would ideal resistor positioning be something like > every 10% of the radius? Not quite. You could distribute the series taps more appropriately, bearing in mind the expected profile, so perhaps more taps in the outer half. Also, scatter them around the circumference, to reduce their mutual capacitance. This method is potentially very accurate, because there are no probes or wires near the coil, and any error in your ohmmeter cancels out in the Rb/Rt. You just need a reliable, but not a calibrated, Q indicator. Firing a low frequency square wave into the coil might do, so that you can adjust Rb to get the same Fres ringdown time as you got with Rt. Choose an Rt that roughly halves the Q from its original value - that wont alter the current profile much, so long as the original Q is fairly decent to begin with. end of paste. All in all, a tricky measurement needing a specially wound coil which would have little other use because of the taps. To begin with, the point about Les being higher than Ldc for these coils could be demonstrated by a direct measure of Les: measuring base current and top volts simultaneously, then Les = |Vtop|/|Ibase|/(2.pi.Fres) I'm pretty sure I can defend the predicted current profiles just on theoretical grounds, especially thanks to your voltage profile work. But folk do like to see those unequivocal experimental confirmations. -- Paul Nicholson, --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.