[Boatanchors] Weston Sensitrol Relay

[Jim Wiley]

One use for the Sensitrol relay was in long-lines telephone equipment, 
commonly known as "carrier" telephone.  When transmitted over wire 
lines, as the outside air temperature changed, so did the conductivity 
of the wires themselves.  This change affected the amount of attenuation 
that the open wire line offered, thus requiring constant monitoring of 
the signal levels and adjustments to the equipment to keep signal levels 
approximately constant.  Carrier telephone systems typically transmitted 
between 4 and 24 individual voice channels per pair of wires, depending 
on the vintage of the equipment.  Along with the voice channels, there 
was also a "pilot" channel.  The pilot carrier, (a single frequency) was 
monitored at the receiving end, and a portion of the signal was detected 
and sent to a metering circuit, not unlike the "S" meter on a 
communications receiver. 


A Sensitrol relay had sets of contacts that were attached to the moving 
coil movement, typically a "low" set and a "high" set.  When the meter 
reached one of these limits, the contacts would close, and a signal 
would be sent to the gain adjustment system, typically a motor driven 
attenuator.  The motor would adjust the attenuator so that the pilot 
signal remained more or less constant, and by implication, the whole 
"group" of telephone channels.   Thus, from the users point of view, the 
levels remained constant.   Some Sensitrol relays contained magnetic 
contact points, so they would remain closed ("latched") until manually 
reset.  These were typically used on alarm systems to warn the station 
technicians when something went outside of expected limits, as for 
example when a wire broke. 


To illustrate how low the loss of open wire transmission lines could be, 
consider that Western Electric "J" carrier could go 50 miles or so 
between repeater (amplifier) stations.   In Winter months, the cold 
weather lowered the loss (lowered the resistance of the wires) so much 
that would bypass the repeaters entirely, and could go a hundred miles 
or more  without  a repeater!   The WECO type J carrier used frequencies 
up to about 110 KHz.     We also used the wires directly for voice  
telephone circuits.  This was "under" the carrier frequencies,  and was 
used for "farmer" lines (rural telephone circuits), and the 
technician's  "order wire" - a circuit they used for maintenance.  
During  maintenance line-ups, we could talk between repeater stations, 
in some cases for hundreds of miles using just station battery (48 
volts),  carbon microphones, an ordinary telephone headsets. 


- Jim, KL7CC






n2lxm at juno.com wrote:
> Happy Thanksgiving All,
>
>
>         During my cleaning up of the Shop/shack( Yea I know, Sacrilege )
> today I came across an 1 - 0 - 1 microamp Sensitrol Really. I have seen
> these in the pass, this one was in a box of stuff given to me by a fellow
> club member who has moved out state.  I would like to know how one would
> use one. The back of the meter has five(5) connection studs and a long
> tube with a Blue and Red wire coming out of it. The case is stamped MAR 2
> 1953, so this old girl is over fifty Seven years old. Any information
> would be helpful. Hope Everyone had a great Thanksgiving.
>
>
>
>                                 Jeff N2LXM
>                   AMATEUR RADIO OPERATOR
>                                 AMERICAN
>                                   PATRIOT
>                   Proud Father of two US MARINES
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