Retro Science Strikes Back: Detecting Lightning with a Coherer

Polish version is here

Back to the Roots of Radio

When it comes to the development of wireless communication, alongside Tesla and Marconi, we also owe much to Alexander Popov.

Source: http://upload.wikimedia.org/wikipedia/commons/a/a8/Alexander_Stepanovich_Popov.jpg, accessed: 12/18/2011

Popov was a Russian scientist who graduated in physics from St. Petersburg University. At the end of the 19th century and the start of the 20th century, he was one of the first to explore the idea of long-distance wireless communication. In 1894, he built a lightning detector that was essentially one of the first radio receivers. Between 1895 and 1897, he independently developed a wireless telegraph system, similar to Guglielmo Marconi's, and in 1897, he successfully transmitted radio signals over a distance of 5 kilometers (3.1 miles).

In his experiments, Popov frequently used a device known as a coherer, which we will now examine in more detail.

The Coherer

The coherer is a remarkably simple yet surprisingly effective detector of electromagnetic waves. Its low selectivity was not a drawback during the early days of radio development.

Its construction is as follows:

It consists of a glass tube with electrodes placed inside. Between the electrodes are loosely packed metal filings, typically made of steel or brass. You can adjust the insertion depth of the electrodes to fine-tune performance.

My coherer:

Kliknij, aby powiększyć

Of course, obtaining an original coherer today is quite challenging. However, you can build one yourself using metal filings obtained from filing steel parts. Instead of a glass tube, a plastic one can be used, and the electrodes can be replaced with nail heads.

The coherer works on the principle that the loosely packed metal filings provide high electrical resistance between the electrodes. However, a sufficiently strong pulse of electromagnetic radiation causes the filings to rearrange and sometimes even form microscopic welds due to micro-discharges. As a result, the resistance of the coherer drops significantly. It can be restored to its original state by shaking the device. In more automated applications, the activated coherer would trigger a bell, whose hammer would shake the coherer, resetting it to its standby state.

With the coherer ready, you can attempt to build Popov’s first lightning detector. The schematic is shown below:

The antenna must be relatively long: at least 10 meters (32.8 feet) of wire suspended at a height of no less than 6 meters (19.7 feet). The grounding must also be of good quality. This device can detect thunderstorms with lightning strikes from a distance of over 25 kilometers (15.5 miles). A lightning bolt, being a powerful electric spark, generates an electromagnetic pulse that is received by the antenna and transmitted to the coherer, causing its resistance to drop. This closes the circuit, and the output component (a bulb or bell) indicates the occurrence of a lightning strike. To detect subsequent strikes, the coherer must be shaken to reset it.

Warning: In the event of nearby lightning strikes, it is essential to remove or ground the antenna, as leaving it connected during a storm can be extremely dangerous.

You can also test the lightning detector on a smaller scale: instead of an antenna, use a wire segment a few centimeters (about an inch or two) long and create electric discharges nearby using a Ruhmkorff coil. A spark discharge near the device will trigger the detector.

Enjoy experimenting with this fascinating and educational device! :)

Weird Science