In a message dated 31/07/00 15:58:41 GMT Daylight Time, ted@.......... writes:

> The analogue is an electrical circuit in which the values
>  of the components are arranged to give them a response to electrical
>  vibrations that can be compared with the mechanical properties of the
>  building.  An ingenious photo-electric device then converts the record of
>  an earthquake into a varying electrical current which can be passed through
>  the analogue, which is [then] studied by the techniques of electrical
>  engineering."
>  
>  Would anyone have any idea how this "ingenious photo-electric device" for
>  converting paper traces into voltage signals might work?     

Hello Ted,

    I think that the earthquake records studied would probably have been 
single traces produced specially for the purpose, rather then trying to read 
overlapping traces on a drum. There were two technologies around which could 
have converted the trace into an electrical signal. 

    Using a black line on a roll of paper, you drew the illuminated paper 
through the reader at the proper speed and used a servo motor driving two 
photocells on a cross traverse to follow the black line. You used the servo 
potentiometer voltage as the output.

    The other method was to print a negative of the trace on photographic 
film, so that the trace line was clear against a black background. Then you 
illuminated one side of the film and drew it across a photopotentiometer. 
This is a linear device with a cermet strip resistance down one edge, a CdS / 
CdSe bridge strip in the center and a conductive strip on the other edge. You 
put a voltage across the resistance and the bridge of CdS underneath the 
clear trace conducts and transfers the corresponding voltage from the 
resistor to the conductive strip. You don't need a high frequency response 
for earthquake signals. From memory, the best resolution that you could get 
with photopotentiometers was in the 1 micron region. I don't know if you can 
still buy them?

    Regards,

    Chris Chapman
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