WIRELESS NEWS.

Press, Volume LXI, Issue 18569, 19 December 1925, Page 5

WIRELESS NEWS.

♦ VARIABLE CONDENSERS. THEIR MECHANICAL EVOLUTION (specially whtttes Jroa "the Tixss.") (By 'Electra.'') The variable condensex, being the most important control instrument of a radio set, has naturally received much attention from radio engineers and designers, and it has gradually evolved from a large, crude device, innocent of accuracy in either manufacture or operation, to the compact, neat little affair of to-day, which is assembled to factory requirements of infinitesimal thousandths of an inch, and which operates with uncompromising precision. Just how much it has been improved, and refined, can be seen from an examination of the instruments available on the market at the present time.

It is necessary to classify the various types, for the total number of instruments is great to the point of confusion. They all display the same fundamental idea in construction; that is, the employment of two adjacent metal surfaces closely spaced in air or other insulating material, but not actually touching, and with some means of regulating the total active area of facing plates; however, the effect is achieved in four diiierent, mechanical fashions, so each must he considered mdividuThe first system involves the use of a closed saucer of moulded insulation, divided into two sections by means of a sheet of mica. Small wells in the sides of the dish hold mercury in such a fashion that when the knob of the device is turned, the two portions meat each other, but remain unmixed, because of the intervening mica. The more mercurv in the facing wells, the greater the capacity, and vice versa. This particular type of condenser is mentioned only as a matter ot interest. Being highly sensitive to jars and other vibratory disturbances, it never was a success, and enjoyed a short-lived vogue about two years ago. There are too many types of condensers and too much to be said about them to be able to get it all in_one article, so I will continue this interesting subject next week*

ELECTRICAL TERMS. WHY THEY WERE NAMED. Last week I explained how such name*, as "v01t, ,: "ampere," "ohm" and "mho" came to he used in radio, and tht, various others will be dealt with iu this article. The "farad" and its hybrid the "microfarad" or "mid" is a shortening of the name of Michael Faraday, probably the most brilliant scientific experimenter of all time. The son of a humble London blacksmith, Faraday started his career as an assistant in the great laboratory of the world-renowned iioyal Society, and displayed such geuius that he finally rose to tht exalted position ot director. His achievements in tlieheld of chemistry alone were prodigious, but he is best known for his discovery that a magnet moving inside a coil of wire produces a current of electricity in that coil, and for his measurement ot the effects of electricity on non-conduet-incr mediums; in other words the measurement of the capacity of condensers. The period of his work was .about l&U. Faraday also is responsible for the electric arc, and for a substance which is the hase of aniline dyes. _ The most interesting story is that ot Joseph' Henry, for whom the "henry, the unit of coil size, is named. Henry, an unDretentious native of Albany, ranks with Faraday as a founder ot this acre- of electricity, yet he is comparatively unknown. He tortainly did more to develo'o the science than any other American", Franklin and Edison no. excluded. -..->'

The Electro-M^agnet. It was Henry who developed the ejectro-magnet, improving greatly on tho fundamental idea or an. earlier Englishman. He wound bars or iron with copper wire insulated with strips of silk his wife tore from her wedding gown, and he obtained magnets stronger than any dreamed of. In loot lie sent a current of electricity through a, mile of wire, and caused an electromagnet to actuate a signal bell, thereby creating the first telegraph. Morse is generallv regarded as the inventor ot this great boon to humanity, but it was Henrv who really was responsible, for it. Morse had an idea, but he made no headway on it until he read of the inventions of Henry and applied the latter's electro-magnets. Ef 1 furthermore, was tho inventor of the telegraph relay, without which longdistance telegraphy would not have been possible. . Henry's greatest contribution was Ins discovery of a system of producing electricity with the aid of magnetism, in this labour antedating the famous Faraday by several years. All tho generators arid transformers employed in electrical and radio practice operate on the principles he set forth. Henry, also, is authoritatively credited with the first electric- motor, in which his brave wife's nuptial finery again figured. . The biggest surnriso revealed by investigation of this great American's life is the fact that it was he, in 1842, who discovered that the electrical discharge of a Leyden jar (a large fixed condenser) was oscillatory, and also that it could induce similar discharges in circuits a considerable distance away; this, in a word, is tho basic, underlying principle of wireless telegraphy. Henry did not seem to realise the tremendous importance of his disclosure, but in any event he anticipated a similar discovery on the part of tho better-published Hertz by at least thirty years. "With his other work on electrical induction and this on oscillatory discharge he naturally laid tho rock-bottom foundation for radio as wo have it to-day.

NOTES. T was listening-in this week down in the dry town of Ashburton. The air there "is comparatively free from local induction, and man-made static ind howling valves are not as bad as I expected from reports I have heard. Stations VLON, IYA, and 2AQ came in good and strong, but 3AC, I am sorrv to state, is a perfect dud, ail carrier wave and no modulation, except for about two items at about 9.20 ,) m last Tuesday. The announcements cannot be heard. Apparently 3\C's range is 30 miles or thereabouts —some rnuge for a 10-watt outfit. The \ussre stations are OK in the above"town; in fact, the Christchurch station is the worst of the lot by far. Ashburton can hardly be called an experimenter's paradise, as there is not a shop down there stocking radio accessories for the home builder. Mr B. C W. Spackman, of Napier and Z2BM fame, is" going in for broadcasting again. He will use 100 watts power on a wave-length of 190 metres and will use the call sign 2YL. Mr Spackman's transmissions ■will be appreciated by all. It is also announced that 2\K Wellington is starting up again. Let's hope so: all the more the merrier. |