Though they had been experimenting with electricity for many years, it was not until the beginning of the nineteenth century that scientists began to comprehend how it worked. This increased awareness was probably a result of the invention of the first battery--Alessandro Volta's voltaic pile--in 1800. By using batteries, scientists had a reliable supply of electricity to work with. They began to test the effect of electricity upon chemicals, metals, and other substances in an effort to better understand its nature and, eventually, harness its power. As worldwide trade increased so did the need for instant long-distance communication, and it was the science of electronics that addressed this need. By 1870 Alexander Graham Bell was experimenting with a means for sending a voice signal over a wire. While this was a viable idea, the signal was generally too weak to travel over long distances; a similar problem existed with Guglielmo Marconi's radio, which couldn't put out a signal powerful enough to clearly transmit a human voice. These problems were solved by the invention of the amplifying triode in 1911 and the high-frequency alternator in 1900. Stimulated by the two World Wars, electronic innovations came to dominate technology and industry in the twentieth century. Most electronic devices are based upon combinations of electric circuits. There are many possible components to such a circuit, including resistors (to hinder the current of electricity), capacitors (to store a charge), potentiometers (to control the amount of current), and switches (to prevent or allow a circuit being completed). A circuit can contain one or more of these components, in series or in parallel. As the understanding of electronics grew, the devices used became more and more sophisticated. By the late 1800s scientists were experimenting with Crookes tubes (named after the British scientist William Crookes). One such scientist, Sir John Ambrose Fleming, reconfigured the Crookes tube into a device which became known as the vacuum tube or thermionic valve. The vacuum tube consists of a hollow, oblong tube containing two electrodes--an anode and a cathode--at each end. In order for a circuit to be completed, most of the air must be pumped out of the tube, creating a vacuum. When a current is applied to the negatively-charged cathode it releases electrons; these electrons cross the gap to the positively-charged anode, completing the circuit. Because this particular vacuum tube contains just two electrodes it is called a diode. One useful feature of a diode is that it allows current to flow through it in one direction only: from the cathode to the anode. (Electrons are not released by the positive terminal, nor would they jump to the negative terminal.) Diodes are often used to convert alternating current (which travels in a back-and-forth motion) into direct current (which travels in only one direction). Such circuits are often called rectifiers . Sometimes a metal mesh is placed within the vacuum tube, between the cathode and the anode. This type of device, invented by Lee De Forest, is called a triode. In a triode, the current passing from the cathode to the anode (called the plate) varies in direct proportion to the voltage applied to the mesh (called the grid). Therefore, a strong current passed through the plate can be made to "mimic" a feeble voltage fed into the grid (for example, a weak radio signal), thereby providing amplification. For many years, triodes were an indispensable component of radios and amplifying devices.
Another type of vacuum tube is the cathode ray tube (CRT). In a CRT, a tiny aperture is placed in the anode, allowing some of the electron stream to escape from the tube. Using magnetic fields and deflection plates, the escaping stream is directed toward a fluorescent screen. Where they strike the screen, the electrons create a brief glow. The first invention to utilize this phenomenon was Karl Ferdinand Braun's oscilloscope, used to observe wave patterns in electrical signals. Today, televisions use similar electron beams to produce a visual image upon a screen. For the most part, vacuum tubes and thermionic valves have been replaced by semiconductors. A very primitive semiconductor called a "cat's whisker" diode (first discovered by K. F. Braun) was used in the early 1900s in crystal radio sets, but it was not until after World War II that the real importance of semiconductors began to be realized. A semiconductor is any material that is not quite a conductor and not quite an insulator--that is, it can act as either under the proper conditions. A good example of semiconducting material is germanium, which forms the base of many modern semiconductors. Generally, a small amount of impurity--such as antimony--is added, increasing the electrical effects. The key to using semiconductors is to place two or more different semiconducting surfaces together, for the semiconducting effects manifest themselves most dramatically at the place where two materials meet. For example, if two pieces of germanium--one coated with antimony, the other with indium--are placed next to each other, and area called a p-n junction is formed; at this junction, electrons pass back and forth easily. If a current is applied in one direction the electrons will flow along, completing the circuit; however, if the current is applied in the other direction the electrons will build up, impeding the flow and preventing the circuit from being completed. In this way, semiconductors serve the same function as vacuum tube diodes. The most important kind of semiconductor is the transistor--a device that performs all of the functions of a triode. The first transistor was built in 1948 by a team at Bell Telephone Laboratories in New Jersey headed by William Shockley. Shockley realized immediately that transistors could almost completely replace the vacuum tubes used in most electronic devices; they were much smaller, allowing for the designing of hand-held and portable appliances, and did not give off heat as tubes did. The first transistor radios were mass produced in 1955; Shockley's team was awarded the Nobel Prize for Physics the next year. Today, the transistor is itself becoming obsolete, as electronics manufacturers perfect tiny components called integrated circuits (IC). Made from semiconducting materials, an IC less than 10 millimeters square can hold thousands of different electronic components. Because they are so small, ICs have virtually eliminated the problem of overheating while dramatically increasing the speed by which functions can be performed. In 1971 a microprocessor chip--containing a complete central processing unit for a computer--was introduced by the Intel Corporation. The generation of electronic components that will replace the integrated circuit can be seen on the horizon. Some scientists believe that metal-oxide semiconductors (MOS); just a few millimeters in diameter, will replace the IC. These devices perform the same functions as an IC, only faster--and require less energy to do so.
Another important development in electronics has been that of the electronic countermeasure, which emits signals that block reception of radar and radio signals. Such a device is commonly used in military functions to steer radar-guided missiles from their targets.
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