English: An early vacuum tube astable multivibrator oscillator, being used with a tuning fork to calibrate a wavemeter, France, around 1920. The Abraham-Bloch multivibrator (small box at left), a two-tube cross-coupled oscillator circuit invented in 1918 by French engineers Henri Abraham and Eugene Bloch, was called a multivibrateur because its square wave output contained many harmonic components, in contrast to ordinary sine wave oscillators. Harmonics up to 150 times the fundamental frequency could be detected. The photo shows an Abraham-Bloch multivibrator being used to calibrate a "wavemeter" (center). Wavemeters were instruments that measured frequency with a precision tuned circuit. They were used to measure the frequency of early radio transmitters to certify that they were broadcasting on their correct government-mandated frequency in compliance with law.

The multivibrator oscillator (left) was first set to exactly 1000 Hz using a precision tuning fork frequency standard (small device in front of oscillator). The oscillator output and a signal from the tuning fork were applied to a detector and audio amplifier (right). The nonlinear detector tube mixed the two signals, producing a beat (heterodyne) signal at the difference between the two frequencies, which was amplified until it was audible as a tone in the earphones. The frequency of the multivibrator was adjusted until the beat decreased to zero. This meant the two frequencies were the same.

Then the output of the multivibrator was applied to the wavemeter (center). The multivibrator output contained harmonics at exact multiples of 1 kHz, up to several hundred kilohertz, the radio frequency range used by early transmitters. The wavemeter was tuned to resonance with one of the harmonics of the oscillator, and the wavemeter's scale was set to show the correct frequency.

Caption: "To the left is shown the multivibrator, and close to it is the tuning fork mentioned by the author. To the right is the amplifier-detector, while in the center is a special wavemeter"

Alterations to image: Partially removed aliasing artifacts (crosshatched lines) introduced during scanning of original halftone image, using Gimp FFT filter, and lightened and increased contrast to bring out more detail.