Early 1902 Lecher line identical to Ernst Lecher's original 1888 apparatus. Waves generated by the Hertzian spark-gap oscillator at right are coupled into the wires by the two metal plate capacitors (circles) and travel down the parallel wires. The wires are short-circuited together at the left end, reflecting the waves back up the wires toward the oscillator, creating a standing wave of voltage along the line. The voltage goes to zero at nodes located at multiples of a half-wavelength from the end. The nodes were found by sliding a Geissler tube, a small glow discharge tube like a neon light, up and down the line (two are shown on the line). The high voltage on the line makes the tube glow. When the tube reaches a node, the voltage goes to zero and the tube goes out. The measured distance between two successive nodes is equal to half the wavelengthλ/2 of the radio waves. The line is shown truncated in the drawing; the length of the line was actually 6 meters (18 feet). The waves produced by the oscillator were in the VHF range, with a wavelength of several meters. The inset shows types of Geissler tube used with Lecher lines.
Lecher-line educational kit sold by Central Scientific Company in the 1930s for teaching radio theory in college. It contains everything necessary, including an absorption wavemeter for independently measuring frequency.