16 Dec
2008
16 Dec
'08
9:31 a.m.
On Dec 15, 2008, at 9:47 PM, Chuck Guzis wrote:
In the early days of my youth (pre computer), I worked
as an
instrumentation technician. Many chart recorders, such as L&N
Speedomax, used a "slidewire" that resembled a single-turn wirewound
potentiometer. But the Honeywell Brown Electronik recorders used a
multi-turn helical pot about the size of a coffee mug. I never
understood why they were referred to as "slidewires" and not
"helipots". It was not uncommon for the line operators to steal the
silver bead used as a contact in those.
"Slidewire" is a term that dates back to the 1800s. A length of
wire whose resistance per unit of length is accurately known is
stretched between two binding posts alongside a ruler, and a tap can
be slid along it to make contact at various points. Here are two of
mine:
http://www.neurotica.com/misc/slidewires.jpg
(yeah, I haven't dusted in there in a while)
They also used to steal the gold support wires in the
galvanometers
in L&N Micromax recorders. There was an interesting device--could
run for a month on a single dry cell, provided you kept the drive
mechanism wound (worked by periodically clamping the galvanometer
needle and mechanically sensing which way to move the pen to bring
the bridge back into balance). Except for those support wires,
rugged as a cast-iron toothbrush.
Neat stuff. I have a big collection of L&N equipment here, but it
all predates their mechanized recorders. They actually seem to be
more rare than the older stuff.
The thefts were really odd considering that these
recorders were
connected to an array of Pt-Pt+10% Rh thermocouples, each about 4 ft.
long. The metal in one of those was probably worth almost as much as
a bucket of those silver beads. Yet they were never filched.
I think the types of people who would lift that stuff from work
are the ones who wouldn't have any clue about Pt.
-Dave
--
Dave McGuire
Port Charlotte, FL