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Education and outreach

Education and outreach

Amazing science demo three: Talking Sparks

15 Apr 2015
Taken from the April 2015 issue of Physics World

This is the third in a series of “five amazing physics demonstrations” presented by science-demo guru Neil Downie and his adept assistant Matthew Isbell.

In a special feature in the April issue of Physics World, Downie describes his five best demos of all time, all of which use everyday equipment to illustrate fundamental physics concepts. In the article, Downie describes how his fondness for the five experiments comes from the fact that, with a bit of creativity, each one can be easily adapted to explore physical concepts further. In the digital edition of the April issue, each demonstration is accompanied by a video in which Downie walks you through how you would present each demonstration to an audience. Full details of how to access the digital edition are available at the bottom of this article.

In this third demo from the series, Isbell demonstrates one of the earliest forms of radio communication: the Marconi spark-gap transmitter. Unlike other demos in the series, there are no bazookas, canons or bobsleighs, but the beauty of this demo lies in its simplicity. It is easy to see how the Victorians would have been amazed at how a few metal poles and a spark could transmit messages through the air as if by magic.

Talking Sparks

So what’s this all about? Electromagnetic waves – and radio waves in particular – are so integral to modern life, from mobile phones to satellite communications, that it is easy to forget how frustratingly hard it was for the early pioneers to detect radio waves. Sparks were used to create electrical oscillations down a transmitting rod, creating radio waves that could be sent to a receiver of similar size. But the received radio waves were almost imperceptible and the medium only became useful with the invention of the “coherer” – an incredibly simple but formidable amplifying device. This experiment shows why.

What bits and pieces do I need? To make your radio transmitter (shown above), you need two, metre-long metal wires or tubes as well as a small, flat piece of wood. Attach one end of each tube to the wood, leaving a gap of a couple of centimetres between them. You’ll also need a piezoelectric lighter to create a spark between the two ends. The receiver consists of two metal wires or tubes of the same shape and size as the transmitter, also attached to a piece of wood. To make the coherer, take a 2 cm-long plastic tube with a 4 mm inner diameter and half-fill it with metal filings between two metal end-plugs. Place your coherer between the two tubes of the receiver and then connect a multimeter – set to the “beep” position – across it. If you don’t have a multimeter, you can connect a 3 V battery and light-emitting diode (LED) with a ~100 Ω resistor and the coherer in series, which will light up when a radio signal is detected.

How do I get going? Position the transmitter and receiver facing each other a metre apart, with both arms lying horizontally. Place your lighter in the gap between the two rods on the transmitter and click it to make a spark. As soon as you click the lighter, you should find that the receiver will beep and stay on. Tap the receiver with your finger and the beep will go off. Try moving the transmitter and receiver further and further apart; with luck, your simple radio will have a range of at least 10 m.

And what physics will I learn? The igniter’s piezoelectric crystal generates an electric charge when squeezed. When this is discharged, the resulting spark sets up a (briefly) oscillating current in the two halves of the transmitter, which act as a dipole antenna. This current turns into a radio wave that is picked up by the receiver, in which similar (but much weaker) oscillations of current are produced. The coherer is actually an insulator because the metal filings inside it mostly do not touch; it only conducts when the receiver picks up radio waves, creating a tiny spark across the filings, which then melt and fuse together.

  • If you’re a member of the Institute of Physics (IOP), you can now enjoy immediate access to the April issue of Physics World with the digital edition of the magazine on your desktop via MyIOP.org or on any iOS or Android smartphone or tablet via the Physics World app, available from the App Store and Google Play. If you’re not yet in the IOP, you can join as an IOPimember for just £15, €20 or $25 a year to get full access to Physics World both online and through the apps
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