What it is and the problem it solves
HP is a 1939-founded company that built its first success on an audio oscillator solving amplitude instability in lab-grade signal generation.
How it works
HP’s first successful product, the HP 200A, used a small incandescent light bulb as a temperature-dependent resistor in a feedback circuit to stabilise audio oscillator output amplitude.
What works
The HP 200A worked because the bulb’s resistance rose with temperature, reducing gain as output increased—creating passive, analog, real-time amplitude regulation without active components or complex calibration.
What does not
The sources do not establish that HP solved digital computation, miniaturisation, mass-market consumer electronics, or software platforms in 1939. Its initial scope was narrow: electronic test and measurement hardware.
What it changes
It changed how engineers built stable, repeatable audio-frequency sources—shifting from hand-tuned, drift-prone circuits to thermally self-regulating designs using off-the-shelf bulbs.
Is it worth your time
Yes—if you work with precision analogue signal generation or early test equipment design, its thermal-resistor feedback method remains a concrete case study in low-cost amplitude control.
