In July 1939, a Stanford graduate student named William R. Hewlett filed US Patent 2,268,872, "Variable Frequency Oscillation Generator." It issued January 6, 1942. The patent came out of his EE master's thesis under Frederick Terman. Its cleverness hides inside one absurdly humble component: a small incandescent lamp.

The problem Hewlett attacked: engineers needed clean audio-frequency sine waves to test amplifiers, microphones, telephone lines, and radios. Commercial audio oscillators in 1939 used heterodyne (beat-frequency) architectures — two RF oscillators mixed down to audio. They were bulky, drifty, and expensive (≈$400, roughly $9,000 today).

Hewlett wanted to use a Wien-bridge RC network — much simpler, tunable with a two-gang variable capacitor, no inductors. But Wien-bridge oscillators had a chronic problem: to sustain oscillation you needed loop gain of exactly 3. A hair less and oscillation died; a hair more and the amplifier clipped, producing distortion. Component tolerances and tube aging made "exactly 3" impossible.

Hewlett's fix was gorgeous. He placed a tiny tungsten-filament lamp in the negative-feedback leg of the amplifier. When oscillation started to grow, more current flowed through the filament; the filament heated; its resistance rose; negative feedback increased; gain fell back to exactly what was needed. When oscillation shrank, the filament cooled, resistance dropped, gain rose. A slow thermal loop enforced amplitude with almost no distortion — a mechanical-thermal analog of what we now call automatic gain control (AGC).

The economics were startling. Hewlett and his college friend David Packard built the resulting instrument — the HP Model 200A — in a rented garage at 367 Addison Avenue, Palo Alto, in 1939. They priced it at $54.40 (Hewlett later joked the odd number was because it sounded like a "cheap" number). Their first big customer: Walt Disney Studios, which bought eight upgraded HP 200B units in 1940 to calibrate the Fantasound multi-channel audio system used in the theatrical release of Fantasia.

That garage is now California Historical Landmark #976 — officially "Birthplace of Silicon Valley." Hewlett-Packard grew from that oscillator into one of the defining companies of the 20th century, its founding "HP Way" management culture copied across the Valley.

Modern relevance is direct and pervasive:

• AGC is everywhere. Every cellular receiver, Wi-Fi radio, hearing aid, and podcast normalization filter uses automatic gain control. Hewlett's lamp is the pedagogical ancestor of the JFET, LDR, and DSP-controlled attenuators that do the job today.
• Low-distortion sine sources still use Wien-bridge + soft-nonlinearity AGC. Audio Precision test gear, Krohn-Hite generators, and countless op-amp reference designs (see the classic Texas Instruments SLOA060 app note) descend directly from figure 2 of Hewlett's patent.
• Slow-loop nonlinear stabilization — using a component whose parameters drift on a longer timescale than the signal itself — is now standard in RF power amplifiers, laser diode drivers, and even PLL charge pumps.
• The startup archetype. Two engineers, a garage, one clever patent, one anchor customer (Disney), no VC. That template — later mythologized by Jobs and Wozniak two miles away — starts here.

The whole patent is 4 pages. The insight fits on a napkin. And it built a $100 billion company plus an industry.