File:Lecher wires and oscillator 1932.png

English: A Lecher line demonstration kit used to measure the frequency of a vacuum tube UHF oscillator, from a 1932 radio magazine. The Lecher line, invented by Ernst Lecher in 1888, was widely used to measure the frequency of short wave UHF transmitters during the 1930s. It consists of a pair of parallel wires that serve as a length of balanced transmission line which is short-circuit terminated by the metal plate (right). Radio frequency currents applied by the triode oscillator (left) form standing waves along the line. At multiples of a half-wavelength (λ/2) the voltage goes to zero, a point called a node. This is indicated by a light bulb on the oscillator across the input to the line. To measure the wavelength λ of the radio waves, the shorting plate is slid up the line until the light goes out. Then the distance between the shorting plate and the oscillator feeders is λ/2. Once the wavelength is determined, the frequency can be calculated by f = c / λ where c is the speed of light.

Caption: Lecher wire demonstration apparatus with lamp for indicating the point of resonance