16 Dec
2008
16 Dec
'08
9:57 a.m.
On 16 Dec 2008 at 12:31, Dave McGuire wrote:
"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
I remember running into those in school in Electrical Measurements
Lab. Wall galvanometers, standard resistances, slidewires...
In the mill we used a wooden box that everyone called a
"galvanometer", but was really a potentiometer bridge (in the
classic, not the common electronic usage sense) bridge with a Weston
cell for setting reference voltage from a dry cell and a book of
tables correlating temperature with the (rotary) slidewire position.
You measured ambient temperature with your pocket thermometer to get
the cold junction reference and worked from there, applying
millivoltages to the recorder/controller. I believe the box was also
made by L&N. Every instrument was to be calibrated (and the case
cleaned and waxed) once each month. Very welcome relaxing work in
contrast to the everyday hot, filthy stuff.
I haven't the faintest idea how things are done nowadays.
Cheers,
Chuck
(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