The first astable multivibrator circuit, invented by French engineers Henri Abraham and Eugene Bloch, from their 1919 paper. The Abraham-Bloch multivibrator, built with triode vacuum tubes, was the first relaxation oscillator circuit, and the predecessor of the Eccles-Jordan flip-flop widely used in modern digital logic. Abraham called the cross coupled-oscillator a "multivibrateur" because its square-wave output was rich in harmonic frequencies, unlike the existing sine wave oscillators.