16
Chapter 1
(or grid) can be controlled by altering the voltage on the grid. Vacuum tubes are still used where large voltages and currents have to be controlled, such as for high-power radio transmitters, and also for cathode ray tubes (for older television receivers and for the measuring instruments called oscilloscopes), but their use for other purposes has died out. Even the oscilloscope requirement is now being replaced. n
Electromagnetic Waves AC would be important enough even if it only provided a way to generate and distribute electrical power, but it has even greater importance. A hint of this came in 1873 when James Clerk Maxwell published a book containing equations that showed that an alternating voltage could generate waves of voltage and magnetism in space, and that these waves would travel at the same speed as light. He called these waves electromagnetic waves, and from there it was a short step to show that light was just one of these waves. Why just one? These waves differ from each other in two ways. One is the number of waves that pass a fixed point per second, called the frequency of the waves. The other is the amplitude (the amount of rise and fall in each wave) (Figure 1.9). Amplitude determines the energy of the wave, so that a large amplitude of a light wave means a bright light. Frequency affects how easily a wave is launched into space and how we detect it. Definition Low-frequency electromagnetic waves are called radio waves, and we generate them and detect them nowadays using electronic methods. To put figures to these quantities, waves with frequencies below 100 kHz (one-hundred-thousand complete cycles per second) are classed as very low frequency (VLF) and are used mainly for time signals and for some long-distance communications. Waves of around 1 MHz (one-million hertz) frequency are called mediumwave, and a large number of entertainment radio transmitters use this range. Waves in the range 10 MHz to around 50 MHz are classed as short waves, used for communications, and the very high-frequency (VHF) range 50 MHz to 200 MHz is also used for similar purposes. As the frequency is increased, the range of useful communications along the Earth’s surface decreases, but the range in space (as from a satellite to Earth) is much greater.
n
Note The amplitude that is quoted for a wave is usually the peak amplitude. The figure of peak-to-peak amplitude is used when the wave is not symmetrical. n
The range from 300 MHz to 1000 MHz is ultra-high frequency (UHF), used for television transmissions, and once we get to using the unit of GHz (1 gigahertz is equal to 1000 MHz)