| Physics majors of the nineteen fifties and earlier will
remember long hours spent balancing alternating current bridges. The detector
was a pair of headphones, and the EMF was a 1000 Hz oscillator similar
to the one at the left. This is a General Radio Type 813-A Audio Oscillator,
listed in the 1939 GR catalogue at $34.00 -- the 400 Hz model was $36.00.
Inside was an electrically maintained tuning fork, driven by a DC power
supply in the 4.5 to 6 V range. Output impedances of 50, 500 and 5000 Ohms
This instrument is in the Greenslade Collection in Gambier, Ohio.
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