Glossary of Wireless Terms

Radio Record, Volume II, Issue 1, 20 July 1928, Page 15

Glossary of Wireless Terms

UNDER this heading we will give regularly sections of the glossary . of wireless terms which is a prominent feature of the N.Z. Radio Listeners’ Guide. In that book, although set in the smallest type, it occupies some 18 pages, and is definitely in our opinion the most comprehensve and complete glossary on modern lines which has been made available in the Dominion. For the benefit of our readers the glossary will be reprinted in our columns..

DEMAGNETISATION. — Telephone { and lJoudspeaker magnets, having a definite polarity, may become demagnetised and consequently weakened, if the instrument is wrongly connected to the set, so that the current opposes instead of assists the permanent magnetism of the core... The positive terminal of phones or loudspeaker should be connected to that terminal of the set (sometimes also marked "plus"’), which goes directly to H.T. plus (B battery plus). DETECTOR.-A device, usually either a valve or crystal, for converting into andible sounds the radio impulses received from the aerial circuit. A valve which carries out the function of detection by means of a grid leak and grid eondenser (known as grid rectificaton) or by variation of the grid potential by, means of a potentiometer (known as anode rectification). A detector valve should never have as high a battery voltage applied as in the case ef an amplifier valve. To obtain the best results various B battery voltages should be applied to a detector valve before the correct voltage is decided upon. ee ag Tectification® and "Condenser, rid. DIELECTRIC.-The substance, or element, occupying the space between the plates of a condenser. An air dielectric is now generally used in variable condensers, and a mica dielectric in fixed condensers. AIR DIELECTRIC.-The most usual form of variable condenser makes use of air as a dielectric medium, but as the dielectric constant of air is low (unity, in fact), a greater area of overlapping plates is required for a given capacity than if mica, ebonite, oil or glass were used. For power purposes where high voltages are used, air has the advantage of being self-healing in the event of a sudden surge of pressure breaking down its insulating power and causing a spark; but as the same applies to oil, which has a higher dielectric constant and consequently necessitates a smaller bulk, this is more commonly used for power purposes.

DIRECTIONAL RECEPTION.-For all practical purposes the usual outdoor aerial may be regarded as non-directional -that is, it is capab,: of collecting energy from any direetion. Theoretically, however, the free end of the aerial should point away from the transmitting station. Generally speaking, the lower an aerial the more directional is its effect. A loop aerial is distinctly directional. BISCHARGE.-The operation of exhausting the accumulation of electrical energy in either an accumulator or a econdenseer, A lightning flash is a visible and audible discharge of the aceumulation of energy in the atmosphere to earth or any point-such as a cloud of lower potential. DISTORTION.-In the endeavour to obtain increased range or volume without extending the set, the latter may be overworked, with the result that the desired increase is obtained at the expense of quality. If reaction is pushed too far, distortion reaches painful samits. Distorted loudspeaker reception is usually due to a badly-adjusted set rather than a fault in the loudspeaker, to lack of a CG battery, poorly designed. audio transformers, or transformers of too high a-step-up ratio. With respect to the lastmentioned the second audio-frequency transformer should, as a rule, not be of a higher ratio than three to one. A higher ratio transformer may be used immediately after the detector yalve-that is, in the first stage. DISTRIBUTED CAPACITY.-It is desirable that a tuning coil should possess a natural frequency in which the induct--anee component is high and the capacity is low as possible. To this end, various systems of winding designed to reduce the ‘inherent distributed capacity of the coil to a minimum are adopted. Bank, honey- | comb, or basket windings may be cited. DS.C.. WIRE-Double © silk-covered wire. As silk is a better insulator than cotton there is less likelihood of breakdown in coils wound with silk, It is more expensive than cotton, naturally. DUAL AMPLIFICATION.-A. form. of cremtt in which a single valve is made to do