THE USE OF CONDENSERS

Evening Post, Volume CX, Issue 104, 29 October 1925, Page 14

THE USE OF CONDENSERS

It is sometimes stated that condensers should be used in wireless receiving apparatus as little as possible, because they 'causa losses;;: and a" result of .'this' die-! turn, which, like many others, is a halftruth, is that condensers are sometimes left out of places where tney would be of bsnefit. It is not the capacity of the. condenser which causes losses, but the bad quality of the condenser,'if it is

bad. A condenser, as used in a highfrequency circuit, is an electrical spring; it ia ready to store up energy tv a certain extent, and lo give il. out again ; and a perfect condenser will render up again all the energy which is pushed info it. Thus it will not cause any losses unless it is connected in such a way as to form a path for high-frequency currents in a. direction in which they should not go. A condenser of poor quality will, however, absorb some energy. The most important direction of loss is in the dielectric of a fixed condenser. If this is an imperfect insulator, the cause, of the loss is self-evident; current actually flows . through the material, and so is not stored np in the condenser ready to be restored to the circuit. But a substance may be an excellent insulator, so far as preventing the passage of current . is. concerned, and yet be defective as a condenser dielectric. In such a substance there appears to be a change in molecular or atomic arrangements under electrical strain, such that a chai-ge in one direction will have sn effect from which the structure of the material does not recover on the removal oi the charge. A reverse charge results in this change being reversed, and the double process involves the expenditure of energy which means loss. The effect is similar to tho phenomenon in magnetism known as hysteresis; in fact, it is electrostatic hysteresis. Tho word itself simply means "a. lagging behind." The energy used up in the, material itself is converted into heat, which in extreme cases can easily be detected. But the effect on the elec^ trical conditions in the circuit—assuming that it is handling oscillating current, as is usually the case in a wireless circuit— is not confined to a' simple loss of en,ergy by conversion into heat. As the reversal of the structural changes in the dielectric takes time, the oscillations in the condenser do not keep step with those which are impressed upon 1 it, but lag behind; they are "out of phase," and in this condition some of the power represented by the restored energy in the condenser becomes unavailable for use. This loss is the "power factor" of the dielectric. As phase relations are graphically indicated by a diagram showing lines more or less at right angles, and the hysteresis effect causes one of the lines to be displaced from the perpendicular, the hystctesis loss is expressed as the; angle by which the lino is displaced. This is the explanation of the frequentlyseen statement' about condensers that the: "phase angle," is so and so, the figures' being usually minutes of arc. The smaller l.he angle, the less the loss.

In a fixed condenser, the choice of a, bad dielectric, though a good insulator, may cause quite serious losses. • The standard material for fixed condenser dielectric is mica, in which the .losses aro as a rule much too small to be worth worrying about. Tho moral of all this is that a pro-' perly-placed condenser will not cause, appreciable losses unless it is a poor con-; denssr. And one should naver use poor' components in a wireless set. ;

Let \is say it again : use a "O. Bat-,! tery" in your amplifiers. If you have no spare battery, buy one; it is the cheap-! est thiag in the set. At a rough esti-/ mate, two shillings spent on a bias battery- will save, during its life-feime, two; pounds worth of B. battery, regardless of any improvement in tone quality that may be expected.

tip PS