From: Paul
Date: Tue, 17 Oct 2000 17:16:02 +0100
Subject: [TSSP] V profile by a gradient detector
I suggested: > >To avoid the problem of probe capacitance to earth, you > >could measure the differential voltage, ie the gradient, > >which does not require a grounded reference wire to go > >into the coil field, ie > > > > | | > > | | > > | | <--- > > | | | > > | | | > > | | [V-probe] ----> fibre > > | | | > > | | | > > | | <--- > > | | > > | | > > | | > > | | > > | | > > > >Not to scale! The two probe points would be fairly close, say > >an inch or two. The V-probe reports the difference voltage. You > >then slide the probe-pair up the coil to produce a chart of > >dv/dx, which you then integrate for V. Terrell W. Fritz wrote: > The voltage transducer is a fairly big shielded metal box. > I fear it would not work well like this. Yes, the risk is that the probe will short-circuit the very gradient its trying to measure. Another problem is that the gradient will be quite sensitive to the gap between coil and probe tips. I guess the same is true of any close-in V probe, the coil-probe gap and the detector input C will form a potential divider. I'm tempted to have a go at this myself. I have a large coil available, 1.6 by 0.6 metres, so a differential sensor will be 'small' alongside it. The problem I think will be maintaining a uniform coil-probe separation. A dielectric block between coil and probe tips might do the trick, so that the thing can just be pressed against the coil and held by tape. One thing's for sure, the added internal capacitance by this method will perturb the system much less than the external capacitance imposed by a probe referenced to earth. -- Paul Nicholson, Manchester, UK. --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.