Re: Kevin's distinguished relative?

[Dan Strassberg <dan.strassberg@worldnet.att.net>]

At 08:21 PM 4/19/98 -0400, you wrote:
>While going through old issues of the Boston Traveler (or Traveller <g>), I
>found a page 1 story from July of 1922 that speaks about a certain James A.
>Vahey, an inventor and radio engineer for the Edison Electric Company who is
>lavishly praised for his latest invention, a triangular-shaped device which
>locates points where electric circuits are broken.  The Traveler says this
>man has "through his invention, [done] more to maintain electrical service
>by locating cable breaks and short circuits than any other individual in the
>country."  Kevin, is he related to you, and did he leave you any money?!!!

I don't know whether or how the 1922 device worked, but such devices surely
exist today--and they work. They aren't triangular in shape, however.
They're called time-domain reflectometers (TDRs), and for fiber-optic cables
they're called optical time-domain relectometers (OTDRs). They work by
sending a very short-duration pulse down the line. An open circuit or short
circuit or other impedance discontinuity causes a reflection, which returns
to the sending end of the cable, where the instrument is located. The
propagation speed down the cable is an appreciable fraction of the speed of
light and can be determined with good precision. Since you know how long it
takes for the signal to reach the discontinuity and its reflection to
complete the round trip, you can locate the discontinity with considerable
accuracy and also determine its nature (that is, an open- or short circuit).

Now, power-distribution wiring is particularly challenging for TDRs because
such wiring does not have the tightly controlled impedance of wires used for
transmitting signals. The twisted-pair copper wiring used for telephony is
the most common example of controlled-impedance signal wiring (and, alas,
one of the worst examples, although telephone wiring still has more closely
controlled impedance than power wiring.) Despite the challenges, TDRs can
and do work on power wiring.

This is probably way more than you ever wanted to know, Donna, but I still
thought you might be interested.

- -------------------------------
Dan Strassberg (Note: Address is CASE SENSITIVE!)
ALL _LOWER_ CASE!!!--> dan.strassberg@worldnet.att.net
(617) 558-4205; Fax (617) 928-4205

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