The Cine Technician (1938-1939)

112 THE CINE TECHNICIAN Nov. -Dec, 1988 RECENT ADVANCES IN ELECTRONICS and THEIR APPLICATION to KINEMATOGRAPHY By A. G. D. WEST, M.A., B.Sc. The British Kinematograph Society opened its winter session on October 20th. We are pleased to print a summary of the President's address on a subject of importance lo alt film technicians. Until the electron tube made possible the introduction of sound accompaniment to the moving picture, there had been no new principle introduced, since its foundation, in the practice of kinematography. The position of sound talking picture has now been consolidated and we are faced with the next fundamental change due to the incidence of the electronic art on the picture aspect of kinematography. It is imperative that technicians become familiar with the trend of modern technique in this direction. PRINCIPLES OF ELECTRONICS Application of the science of electronics (that branch of science which relates to the conduction of electricity through gases or in vacuo) specified the use of electron tubes in which electrons were controlled to do different jobs. There are four different types of such tubes: 1. Thermionic vacuum tubes. 2. Photo-sensitive tubes. 3. (las-filled conduction tubes. 4. Gas-filled lamps. In every case the process consists of three distinct stages : — 1. The release of electrons from conductors into the tube. 2. The control of these free electrons as they move about inside the tube. 3. The making use of their movement and the energy which they represent in a specific way. ELECTRON EMISSION Electrons are produced or released from conductors only by giving them enough energy to break through the surfaces of the conductor. This can be done in five ways : — 1. Photoelectric emission due to action of light. 2. Secondary emission, caused by the impact on a conductor of electrons travelling at high velocity. With certain substances an electron is capable of releasing from one to ten electrons when it strikes their surface. 3. Thermionic emission due to heat. 4. Cold emission of electrons which takes place when the surface of a metal is subjected to the presence of a very intense electric field of force. 5. By ionisation of gases whereby an electron impacting a molecule of gas causes it to rerelease a further free election. Their movement is controlled in three ways: — 1. By electrostatic field of force. 2. By electromagnetic field of force. 3. Due to presence and motion of nearby electrons. Their energy is utilised in three ways: — 1. Bj making them enter a conductor and charge it up, resulting in a flow of current from the tube. 2. By causing them to bombard something in the tube to heat it up. 3. By causing them to impact on a screen consisting of fluorescent material giving rise to light. CONSIDERATION OF ELECTRON PROCESSES Certain processes are very familiar and the following are two further processes which have attained very consisiderable importance in electronic work and illustrate how they can he used in furthering the progress. SECONDARY EMISSION OF ELECTRONS It has been found that if a stream of electrons is allowed to bombard a metallic surface having certain definite characteristics, each electron can be made to displace a definite number of electrons, depending on the type of surface which is used. The cold emission of electrons so far has not been made use of with advantage in developing electron tidies, but the use of electron tubes where the ionisation of gases is employed lias made great strides in recent years. ELECTRON OPTICS The movement of an electron under the influence of electro-magnetic or electro-static fields is defined in accordance with certain well specified and well known laws. The analogy between electron optics and geometrical optics is seen when we realise that corresponding to the laws of the rectilinear propagation of light and the laws of refraction and reflection are similar laws holding for the movement of electrons. It is the study of electron optics which has enabled television engineers to make great progress in the cameras and picture reproducers which they now use. FLUORESCENCE The use of fluorescent materials at the present moment constitutes by far the most important of the methods employed for the reproduction of television pictures. PRACTICAL ELECTRON TUBES The photo-electric cell is the fundamental device for use in the transmission of vision from one point to another by electronic methods. The maximum sensitivity available at the moment for the type of cell which can be used for this purpose, namely the caesium cell, is about 50 microampures per lumen. If electrons from the photosensitive surface, before being led out of the tube, can be made to strike a surface which has secondary emitting qualities, then the electron current can be magnified many times, in cases up to 10 times. If this process can be repeated, then it is possible to obtain much greater effective magnification.