10. Electron theory

The foundations of the modern theory of electricity were laid in the sfficfy*of the electric discharge through gases, and in particular the so-called cathode rays. The nature of these cathode /ays w$s first described by Crookes Д879) when he cdnsTderecTthem as negatively electrified particles 'which were emitted from a metal under the influence of a strong electric fi>1H"

Further experiments made on these particles confirmed that they carried a negative charge jai}d the name "electron" was given to them. It is the rfttavem'ent of the electrons, wheth­er in a cohcluctor or a gas which gives rise to the phenomenon known as the electric current.

The student should^emember that an electron, being nega- tively charged, will move towards that end of the circuit or that part which is termedj'positive". The old conception of the electric current flowing from the positive pole or end of the circuit to the negative was accepted long before the ex- istence of the electron theory. It "is in direct oj^positioii to the real direction.of electron flow. This conception is, however, too firmly established and the current is_? still assumed to flow from positive to negative. ^щ

The number of the electrons comprising the uriit of cur­rent has been computed. At present, we know one microam­pere to be eqyal to the p'assjge of 6 milliard electrons per second. To keep a 100-watt lamp burning requires a flow of six milliard milliard electrons—not in a day, nor an hour, but every second. Six milliard milliard means the figure six with eighteen zeroes after it.

We know electrical energy to change into light energy in the electric lamp. Can light energy be changed back to electrical energy?

The emission of electrons under the impact of light energy is called photoelectric emissjpn.^ The more intepse the light, the more electrons are emYtfe(jfby the metal exposed to light. Although most metals will emit electrons when their surfaces are exposed to ultraviolet light, some metals, such as sodium, potassium, and certain others will emit electrons when ex-posed to ordinary visible light rays and infrared rays as well.

The first discovery came in 1887 when Heinrich Hertz, the prominent German scientist, found out that for a given electromotive force an electric spark will jump across a larger gap if this gap is illuminated by ultraviolet light than if the gap is left in the dark.

The second discovery came about a year later when it was found that ultraviolet light falling upon a negatively-charged metal plate caused it to lose its charge. As the plate was charged positively, there was no apparent change. The final discovery came about ten years later when Joseph Tomson, the famous English scientist, discovered that ultraviolet light falling upon a metalic surface caused it to emit electrons.

11. Thermocouple and photocell

There are two means of producing small electric currents for special purposes. One of these is the thermocouple or a thermopile. The other is the photocell, sometimes called the electric eye.-

The "iron-copper" thermocouple represents an iron wire and a copper wire, both being carefully cleaned at the end and making close contact with each other. At the point of contact between unlike metals, a current tends to flow from one metal to the other because the outer electrons in the atcms of one metal have more potential energy than those in the other metal. The measure of this potential energy difference is called potential difference. This potential difference de­pends both upon the nature of the metals and upon the tem­perature at the point of contact.

A number of thermocouples are sometimes connected in series. Such a combination called a thermopile is''more sensi­tive than a single thermocouple.

The photocell generates a small electric current in response to the action of light. In one type, the light ejects electrons from a photosensitive surface upon which it fall^JA photo­sensitive electrode ,usuaTty "consists of a thin layer of caesium or a caesium compound on a surface of silver. This is the pho­tocell cathode. The anode is a metal rod or a loop which, when the cell is in use, is connected to the positive terminal of a battery. It is the collector of electrons. The anode and,the.' cathode are connected to short, light metal rods which extend through the base of the tube to form the support,

Electrons moving from the cathode to the anode consti­tute a small'electric current whose magnitude is directly proportional to the amount of light falling upon the cathode.

Photocells perform a great number of very important ser­vices. Perhaps, the best known use is in connection with mo­tion pictures, where they are used in the reproduction of sound. They are also employed in television where they function in the signal transmission.

"Electric eyes" are also used in factories to give auto­matic control of illumination, by turning lamps on or off as required. Traffic signals, the devices for testing and recording the daily output of factories and many other types of safety devices are operated by photocurrents.