Text 2. Electron Emission

There is little doubt that wireless, radio, and television are among the greatest miracles of modern science. Travelling with the speed of light, code signals, the human voice and music can be heard around the world within the very second they are produced in the broadcastingstudio. Through television, world events can be observed in full colour at the same moment they occur hundreds of miles away.

The more we learn of the fundamental principles of radio and its operation the more amazing does their reality become.

The heart of a tube is the source of electrons.

Every vacuum tube depends for its action upon a stream of elec­trons that acts as a carrier of current. As necessary as the stream of electrons is the electrode that emits them. Whatever the nature of the tube and the arrangement of electrodes, an emitting electrode cannot be dispensed with. Even in cold-cathode tubes, one of the electrodes is treated with a low-work-function material to facilitate the production of some electrons that will initiate the action.

In general, the excellence of performance of a given tube de­pends upon the efficiency with which free electrons are produced. When the emission fails, the tube is useless. We infer correctly then, that the subject of electron emission is worthy of consider­able study.

The types of electronic emission may be listed as follows:

1. Thermionic, or primary, emission.

2. Secondary emission.

3. Photoelectric emission.

4. Field emission.

Thermionic Emission. The velocity of electrons and atoms, as they move about within the confines of the material they comprise, is dependent on the temperature. At a temperature of absolute zero all molecular activity is supposed to cease. As the temperature is increased, the activity of electrons and atoms increases until a point is reached where the electrons have sufficient velocity to enable them to break through the potential barrier of the material. This evaporation of electrons from the body of a solid at high tempera­ture is known as thermionic emission. The emission or evaporation of electrons takes place at lower temperature than does that of at­oms. The mass of electrons being smaller, it reaches the higher velocities necessary for evaporation at low temperatures than does the heavier atom. The temperature becoming high enough for the atoms to evaporate, the material or solid that they compose rapidly disintegrates.

Secondary Emission. One knows to a high degree of certainty that being accelerated to a sufficiently high velocity an electron may have enough kinetic energy imparted to it to knock one or more electrons out of any material it comes in contact with, either a metal conductor or an insulator. A positively charged electrode situated near the source of these “secondary” electrons will collect them. Inactual tubes the secondary electrons may be attracted back to the electrode they come from, as from the plate, or they may be collected by another electrode which is positively charged. In many tubes these secondary electrons give rise to undesirable effects, design steps be­ing taken to reduce their number and to control their movements. In a few tubes, such as electron multipliers, the desired operation is based on the principle of secondary emission.

Photoelectric Emission. When light of proper wavelength is al­lowed to fall upon certain metals, electrons are released from the surface of the metal as a result of the energy imparted by the light. Here, then, is another electron source. Such sources are used in pho­totubes and in certain types of television camera tubes.

Field emission occurs at cold surfaces under the influence of ex­tremely strong fields.

All types of emission are most effective in vacuum. If the emis­sion did occur in air, the emitted electrons would not get very far through the relatively dense surrounding atmosphere. Most metals would burn up in air at the temperatures to which they must be raised to emit satisfactorily.

Notes

emission [l'mi/n]	випромінювання
doubt ['daut]	сумнів
miracle ['mirskl]	диво
code ['koud]	код, шифр
tube [tju:b]	трубка; електронна лампа
stream [stri:m]	потік
carrier ['kæris]	носій
to dispense [dis'pens]	розподіляти
efficiency [i'fi/nsi]	ефективність
to infer [in'fa:]	робити висновок, виводити
velocity [vi'lositi]	швидкість
to confine [kan'fain]	обмежувати
to comprise [ksm'praiz]	охоплювати, містити в собі
to cease [si:s]	припиняти
evaporation [j^vaepa'reijn]	— випаровування
One knows to a high degree of certainty...	—Достеменно відомо...
to accelerate [ask'sebreit]	—прискорювати
insulator ['xnsjuleita]	— ізолятор
to reduce [ri'djurs]	—скорочувати