On 28-Oct-2012 15:46:09 Dan wrote:
On Sun, 28 Oct 2012, Chris Green wrote:
Yes, I'm conscious of the grounding problem. While the boat wiring (the batteries are on a boat) is quite well behaved in having a single earth point to the hull etc. the length of the wires to the voltages being measured are such that there may be problems.
If the "grounding problem" you're worried about is magnetic pickup, then one traditional solution is:
- start with a wire twice as long as you need
- bend it round a fixed support at its half-way point
- sticky-tape both ends of the wire to the chuck of a power drill
- turn the power drill on for a few seconds
- cut the wire at the bend.
Hey presto, you've got signal and ground wires in a twisted pair.
HTH, Dan
Hmmm, I doubt that will work! The reason being that, with the ends taped to the chuck, it would not be possible for "counter-twist" to build up along each arm of the wire. So, when the doubled/twisted wire is released from its constraints, it would: a) Tend to wrap round itself; b) When this wrapping is undine (essential for extended use), the twist which had been imparted by the power drill would undo itself.
To test this theoretical doubt, I have just taken a 3m length of USB cable, doubled it, hooked the halfway point over a door-handle, then, holding the free ends firmly together (to emulate being taped to the power drill) I rotated the free end pair until there was plenty of apparent twist along the doubled cable.
I then unhooked the doubled end. Result: exactly as predicted above!
Has anyone successfully used Dan's method in practice?
The real way to impart a true twist would be to double the wire round the fixed support and then, starting at the support and working outwards away from the support, twist the wire, allowing the counter-twist to build up (with the free ends twisting correspondingly). Industrial twisting/spinning (e,g, in rope making) would have the strands being fed in from separate bobbins, which would counter-rotate.
(Now that I check at: http://en.wikipedia.org/wiki/Rope-making#Styles_of_rope_construction it would seem to conform the above).
Best wishes to all, Ted.
Best wishes to all, Ted.
------------------------------------------------- E-Mail: (Ted Harding) Ted.Harding@wlandres.net Date: 28-Oct-2012 Time: 17:46:58 This message was sent by XFMail -------------------------------------------------