Originally Posted by K8ERV
Yep, from Wikipedia
Of the many projects they worked on, their very first financially successful product was a precision audio oscillator
, the Model HP200A
. Their innovation was the use of a small incandescent light bulb
(known as a "pilot light") as a temperature dependent resistor
in a critical portion of the circuit, the negative feedback loop which stabilized the amplitude of the output sinusoidal waveform. This allowed them to sell the Model 200A for $54.40 when competitors were selling less stable oscillators for over $200. The Model 200 series of generators continued until at least 1972 as the 200AB, still tube-based but improved in design through the years.
One of the company's earliest customers was Walt Disney Productions
, which bought eight Model 200B oscillators (at $71.50 each) for use in certifying the Fantasound surround sound
systems installed in theaters for the movie Fantasia
Reality is that which, when you stop believing in it, doesn’t go away.