RADIO Early in the 19th century, Michael Faraday, an English physicist, demonstrated that an electric current can
produce a local magnetic field and that the energy of this field will return to the current when the current is
stopped or changed. James Clerk Maxwell, professor of experimental physics at Cambridge, in 1864 proved
mathematically that any electrical disturbance could produce an effect at a considerable distance from the
point at which it occurred and predicted that electromagnetic energy could travel outward from a source as
waves moving at the speed of light.
Radio aroused worldwide excitement in December 1901, when Guglielmo Marconi, the Italian physicist,
received the first transatlantic radio signals in St. John's, Newfoundland, sent from a transmitter in England.
Radio messages and signals travel across space by way of electromagnetic waves. Light is another type
of electromagnetic wave, as are X rays, gamma rays, and cosmic rays. Since it is difficult for humans to perceive
the action of these unobservable waves, electromagnetic wave action is often compared to that of water
waves. Like water waves, radio waves also radiate away from a center. They can travel through the air and
through a vacuum. Like light waves radio waves travel in straight lines at a velocity of about 300,000
kilometres (186,000miles) per second and have amplitudes that vary cyclically with time; that is they oscillate
from a zero amplitude to a maximum and back again. The number of times the cycle is repeated in one second
is called the frequency in cycles per second, and the time taken to complete onecycle is sometimes called the
period. To commemorate Heinrich Herts a frequency of one cycle per second is called one hertz. The distance
from one wave crest to the next is known as the wavelength. Wavelength and frequencyare related. Dividing
the speed of the electromagnetic wave by the wavelengthgives the frequency.
Exercise 1. Say whether the following statements are true or false.
1. Michael Faraday was the first to notice the existence of a local magnetic field produced by an electric
current.
bet alatan
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