15 Dec
2008
15 Dec
'08
6:47 p.m.
On Dec 15, 2008, at 5:21 PM, Tony Duell wrote:
I rmember seeing (and alas failing to obtain) an
partically mechanical
digital voltmeter. The circuitry was a servo system, similar to the
ones used in chart recorders. The motor drove a mechanicla turns
counter, coupled to an accurate multi-turn helical potentiometer, which
provided the feedback to the servo amplifier (it replaced the
slidewire in th chart recorder implementation). THe operation is
obvious, apply an input voltage, the motor turns until the feedback
from the pot balances the input votlage, the turns counter then
effectively shows where the pot is
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.
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.
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.
Cheers,
Chuck