My return to Classic Cmp - San Diego, software archive, etc.

Chuck Guzis cclist at
Mon Dec 15 20:47:42 CST 2008

 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.


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