Item ST 38448 Display - Spark Wireless, Experiment, Lodge, Post 1898

Summary

Early display demonstrating spark wireless, reproducing the experiment by Sir Oliver Lodge, circa 1898.

Display demonstrates both the production and detection of an electromagnetic field and also the effect of tuning, which Lodge referred to as syntony. This display was created by the Museum of Applied Science Victoria to commemorate the centenary of the birth of Sir Oliver Lodge (1851-1940).

A spark coil would have been attached to the transmitter circuit. When activated, the spark would jump across the gap at the top of the condenser causing a damped oscillation in the inductor (transmitting) loop (the rectangular section of wires on the transmitting side). The electromagnetic field produced would then be picked up by the tuned lecher line (receiving loop). The presence of a signal would be demonstrated by a spark between the brass spheres on the side of the receiving condenser. If a coherer were connected, this would also detect the signal.

The tuning is achieved by moving the shorting bar (a thin wire connecting the top and bottom horizontal wires) to achieve the maximum spark at the receiver.

It is worthy of note that the use of the term 'lecher line' in this display is not correct; the receiving loop and shorting bar form a variable inductor, which with the receiving capacitor form an adjustable tuned circuit. A lecher line is used for measuring wavelengths and looks similar to the arrangement in the display; it consists of a pair of wires mounted a short distance apart with a movable short circuit and a means of detecting high frequency current circulating in it, such as a globe or other type of detector. The lecher line must be longer than the wavelength to be measured.

Physical Description

Green base board with labelled holes and supports to secuely locate items. 2 Leyden jars. Thick insulating rod with a thin metal rod attached to either end. The two thin rods are parallel to each other; one thin rod has a spherical connector at one end. Another thin rod (shorting bar) is connected across the two thin rods, parallel to the thick metal rod. One spherical brass sphere with insulated connecting wire. U shaped thin rod with connector at one end.