Center For Integrating Research and Learning

ArrowLodge's Experiment

This tutorial illustrates the early radio experiments performed in 1894 by English physicist Sir Oliver Lodge. Using equipment similiar to what is depicted here, Lodge demonstrated the key principles behind the invention of radio (then called wireless telegraphy). Other inventors soon expanded upon his findings.

The electric circuit created in the transmitter generates electromagnetic radio waves that are picked up by the receiver, causing the neon bulb to flash.

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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. The inductor and capacitor together form a parrallel LC 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.

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