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Tuesday, 11 December, 2001, 16:38 GMT
Q&A: Wireless communication, the science

Little was known about electromagnetic waves when Marconi started his experiments on wireless communication. And the reason why radio waves are able to bridge the Atlantic was not discovered until later. Some even doubt that Marconi heard what he claimed to hear - the dot, dot, dot of the Morse letter "s". BBC News Online's Helen Briggs looks at some of the facts.

What are electromagnetic waves?

An electromagnetic wave is energy that consists of related magnetism and electricity.

An electromagnetic wave can be thought of as an oscillating electric force travelling through space accompanied by an oscillating magnetic force in a plane at right angles to it.

There are many types of electromagnetic waves, including radio waves, microwaves, and X-rays.

They all have different wavelengths (distance from a point on one wave to the same point on the next), as shown below.

Electromagnetic waves can pass through air, liquids such as water, and space. Most of them are invisible; our eyes are sensitive only to a narrow band of wavelengths - visible light.

How are electromagnetic waves formed?

Electromagnetic waves are formed when an electric field couples with a magnetic field.

The scientist James Clerk Maxwell was the first to predict that an oscillating electric current should be capable of radiating energy in the form of electromagnetic waves.

This was later shown by the German scientist, Heinrich Hertz, who developed equipment to send and detect electromagnetic waves in the 1880s.

The experiments laid the foundations for the work of Marconi and the use of electromagnetic waves in radio communication.

What are radio waves?

If electricity oscillates at thousands or millions of times each second, it produces radio waves.

Radio waves have wavelengths of between about 1 millimetre and 10 kilometres.

Most radio signals have frequencies (the number of complete waves in one second) of 30,000 Hz to 300,000 million Hertz.

They are used extensively in wireless communication, including radio systems and mobile phones.

What is the relationship between frequency and wavelength?

Frequency is related to wavelength by the equation c = f x w where c is the speed of light (metres per second), f is the frequency (Hertz), and w is the wavelength (metres).

In other words, the product of frequency and wavelength is a constant.

How do radio waves cross the globe?

Electromagnetic waves travel in straight lines. At the time that Marconi was carrying out his experiments, many scientists believed that they could not be sent across the Atlantic.

It was thought that they would either be bounced into space or absorbed by the curvature of the Earth. However, Marconi was convinced he was right after conducting experiments that involved sending radio signals over ever increasing distances.

We now know that radio waves can beat the curve of the Earth because of the ionosphere, a reflective layer around the planet where some matter exists as ions.

Radio waves are bounced off the ionosphere and are directed back to the Earth's surface. In this way, they are able to travel around the globe, allowing international radio broadcasts.

What sort of radio signals were used during Marconi's transatlantic experiment?

Marconi's equipment was probably transmitting at medium wavelengths (wavelengths between about 100 metres and 1,000 metres).

Some have claimed that Marconi could never have got a medium wave signal across the Atlantic during daylight, when signals cannot travel so far.

However, Marconi's honesty has never been challenged and there may well be a technical explanation. Marconi's equipment might have been sending out signals at other wavelengths that could travel much longer distances.

Doubts in the British press and among scientists were largely settled in February 1902 when Marconi sailed for the United States on board the SS Philadelphia, which had been fitted with aerials.

The ship's captain confirmed that readable signals were received from Poldhu up to 2,496 kilometres from shore.

Furthermore, attempts by the Eastern Telegraph Company to spy on the Poldhu transmitter from its base just down the coast at Porthcurno appeared to verify that the equipment worked, albeit unreliably.

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