Dumont cathode ray oscilloscope, c 1945.

Dumont cathode ray oscilloscope Type 208, serial number 3905. Seeing the waves made by oscillators was difficult until German scientist Ferdinand Braun (1850-1918) invented a form of cathode ray oscilloscope in 1897. An internal oscillator repeatedly sweeps a beam of electrons acros the screen at constant speed. The input voltage moves the beam up and down, drawing a picture of the wave. Modern oscilloscopes often display the picture digitally.