This tutorial illustrates experiments performed by English physicist Sir Oliver Lodge in 1894 demonstrating the first tunable radio receiver, which in the early days was called a coherer. With equipment similar to what is illustrated here, Lodge demonstrated key principles behind the invention of "wireless telegraphy" (radio), which other inventors soon exploited.
The electric circuit created in the transmitter generates electromagnetic radio waves that are picked up by the receiver, causing the neon bulb to flash.
In the transmitter, a Van de Graaff generator produces enough voltage to jump the spark gap and charge the Leyden jar. The electrodes of the Leyden jar are connected to an adjustable metal loop. The Leyden jar is a capacitor (which can store electricity) and the adjustable metal loop is an inductor, or coil of wire that generates a magnetic field when current is passed through it. Together they form a parallel LC (inductor-capacitor) tank circuit.
The receiver consists of another Leyden jar capacitor connected to a metal antenna loop, but the circuit also contains a neon bulb. When current jumps the spark gap, the adjustable metal loop generates electromagnetic radio waves that cause an electric current on the antenna loop (inducer). The antenna loop then charges the Leyden Jar until enough charge is built up to flash the neon bulb.
By manipulating the loop position slider, you can tune the transmitter to find the resonant position that will produce frequencies most detectable to the receiver and produce the brightest flash from the bulb. Tuning the transmitter is easier when the spark interval slider is moved to the “short” position.
Lodge's Experiment