File:Lecher line experiment 1907.jpg

English: Experiment demonstrating standing waves with a Lecher line in 1907 using equipment very similar to Ernst Lecher's 1888 experiments. The two parallel wires (5) in the foreground constitute a Lecher line, a length of parallel transmission line terminated by a short circuit. The spark gap transmitter (1,2) in the background generates UHF radio waves, which are coupled into the wires through the two metal plate capacitors (3,4) in the background. The glowing tube (6) suspended from the wires in the foreground is a Geissler tube, similar to a neon lamp. The high radio frequency voltage on the line causes the gas in the tube to ionize and glow. The Geissler tube is slid up and down the wires until the light goes out. This marks a node in the standing waves, where the voltage goes to zero. The distance between two nodes is equal to a half wavelength of the radio wave. This technique was used to measure the frequency of radio waves until World War 2.