THE WIRELESS WORLD.

Otago Daily Times, Issue 20222, 7 October 1927, Page 5

THE WIRELESS WORLD.

NEWS AND NOTES. By Magna Vox. Items of local interest are invited by Magna Vox” for publication in this jolumn. It is necessary that such matter should reach this office by Tuesday of each week for insertion on the following t viday. ANSWERS TO CORRESPONDENTS. “Valve,” Mandcville.—Yes; if your valve has become useless as an amplifier it can often be used as an excellent detector. Arthur F.. Palmerston. —Certainly not. On an average a. loop aerial is about 5 per cent, as efficient as an outdoor one. J. M‘K., Port Chalmers,—(l) Your acid is far too strong. (2) Do not he misled regarding tho voltage at the rectifier terminals. Your voltmeter requires a largo current to operate it, and thereby causes a big voltage drop. K. A. A., Dunedin.—Tho faintness in reproduction is probably duo to demagnetisation of tho earphones or tho load speaker. A good click should be hoard as they are connected in tho circuit. In tho case of tho ’phonos, when in doubt, unscrew ono ear-cap and it should bo found that tho diaphragm adheres strongly to the magnets. . \ M B„ Mosgiel.—Yes: laying tho batteries on their side would cause the deterioration you mention. If you wish to obtain tho longest possible life from them, you should keep mom upright. “Accumulator,” Alexandra.—lf, as you say you emptied tho wrong mixture out’ of the accumulator immediately on finding your mistake, 1 do not think you have anything to worry about. As an additional precaution, however, wash tho plates well in a strong solution of baking soda and water, and afterwards rinse them again with distilled 'water. This will have tho effect of neutralising any of tho old mixture which may remain on the plates. Thanks for your appreciation. VALVES AS AMPLIFIERS. Often when a signal has been detected it is 100 weak and has to bo amplified by utilising coupling' transformers suitable to the audio-frequency of the detector. Signals or music may then bo built up many hundreds of times. More than on s‘a"e of iron-cored transformer coupling is quite practical, but there is a limit to their number. If. carried beyond three stages the parasitic noises due to the inter-actions between the transformers will bo so loud as to drown out the music, or if they do not drown it put will -on ously interfere with tho reception. In practice, the limit is set at two stages. When a large number of stages of audiofrequency amplification arc required, it is desirable 7 to use the resistance capacity method. As many as six stages “ used without fear of distortion. Owing to the drop in voltage m + he resistance, it is necessary to use a higher battery voltage than when using a transformer About 130-150 volts are required. The amplification from the fcsistancoeanacitv coupled amplifier is not as efKc«v P 0 from tSo point of view of Of amplification as in tho case of trans formers, and is in the proportion of three (statres of resistance to two . stages it transformer coupled amphficat ’.on ihtis thrift valves would be required to give tne same volume as two in the transformer coupled The ultimate distortionless re ception," however, is much greater. prentice it is found that resistance coupling is nA satisfactory for radio-frequency unnlifiers below wave-lengths of 2000 metres ’ owing to heavy losses m the dreuit For audio-frequency, Inver t h excellent except that higher B volt 3? I? A*# * tine other. «» being connected in parallel. TRICKLE CHARGING THE A BATTERY. The radio set owners who have direct current laid on to their homes are probably not aware (states the New Zealand Motor Journal) that it is a very simple and economical matter to keep the A batterv up to concert pitch, io those wfth even a very slender know edge of electrical matters it is known that tlm ordinary electric light bub passes a rather small current, depending upon the size or candle power of the light Thus a 50 • c.p. lamp passes, roughly, i ° ampere (the current passed is calculated by dividing the watts by the volts). Two Lamps of this size will pass just about half an amp., which is just about for slow or trickle charging. As the principal room of the house is usually equipped with a double lamp fixture, thip, then will be the room to do your charging from. The procedure is: Unscrew the cap from the lighting switch and bind a wire around each terminal of the switch. These arc led down to the accumulator on the floor, but do not connect. (Note: Keep the switch closed until you are ready to connect the wires to the accumulator and also whilst binding the wires on the switch.) The next procedure is to determine the polarity of the wires. Ihe .simplest way to do this is to place the ends of both wires in a tumbler of salt and water or vinegar and water. t“ en open the switch, and the wire from which most bubbles arise is the negative, io charge the battery, connect the positive wire on to the positive terminal of the accumulator and the negative wire on to the negative terminal of the accumulator, opening the switch just before connecting the latter ware. Do not close the switch whilst the accumulators arc connected up or they will discharge across it. Charging the battery thus from the mains consumes no more current than is being used for the lights and does not impair their brilliancy, so that charging can be carried on during the evening while the lights arc being used. Should the charging rate be too low for a badly run down battery, more powerful lamps could bo placed in the sockets, but for ordinary cases half an ampere is quite sufficient. DISTORTION AND ITS CAUSES. Do not bring in the music too loudly, or you will overload your valves. There are physical limits to the capacity of radio valves just as there are to our appetites or the speed our car will develop without also developing trouble. Very few machines of any kind are efficient when overloaded. If we eat beyond our normal requirements we suffer all manner of disagreeable conditions—indigestion, fatigue, sleeplessness, etc.; if we drive our motor beyond its normal capacity we lose traction, arc bounced about, and are as likely as not to burn out a bearing or two. If, in our radio receiver, we are using small valves and we turn out enough volume to fill a dance ball wc cannot expect music, because we are sure to get noise instead. Overloading our valves —and such operation is surely doing just that—results in a heavy drain on our batteries and produces distortion which almost any ear will detect. Where small valves are employed we may well be satisfied if our load supplies volume enough for a medium sized room. Where semi-power valves are properly used we may expect enough volume from our speaker to fill a large room without sacrificing tone quality, but where music loud enough for dancing in a fair-sized hall room is required ordinary valves will not do. Wo must use a power amplifier, equipped with power valves. The idea that an ordinary receiver may be used in a club or hotel dining room without such a power amplifier is doing much more to hurt radio than to make it popular. In almost every case of this nature the receiver is cranked up to the last notch in order to have it heard above the rattle of dishes and buzz of conversation. The result is bedlam. Radio receivers, when properly designed and properly operated, are capable of tremendous volume, accompanied by beautiful tone colour. They may bo called upon to serve where every other means for providing cntc’-tainmcnt fail, and they will produce music in a most satisfactory manner if we employ them with some regard for their limitations and do not look for Rolls-Royce performances from a Ford. SHORT WAVE RECEIVING. In operating a short waves set (says “Megohm” in tho Radio Record) there are a number of points to be considered in addition to those common to both broadcast and tho short wave. Tho following hints are given as tho result of practical

experience, and may bo of assistance to readers who are starting out on shortwave reception. Keep detector filament voltage as low as possible. This makes for quiet working and smooth oscillation. The plato voltage must also bo kept clown, and probably from 20 to 30 volts HT will give satisfactory results. Very often bettor results are obtained by discarding the earth connection. In most circuits it is then necessary to connect the A battery side of the secondary tuning coil to the earth terminal. If the set shows any tendency to squeal when off oscillation point, try lowering filament voltage with the rheostat and also try a larger grid condenser. The radio choke is not at all critical. “Megohm” has used half a dozen, some elaborately wound on test-tubes, with 100, 150, and '2OO turns, but 100 turns of 30's roughly wound on an empty cotton spool with a small centre answers as well as any. Whore plug-in coils arc used, secondary condenser should be small in capacity, and may bo made so by removing plates from a variable condenser until only five are left. This spreads out tho stations on the dial and makes tuning less critical. There is no need to shorten a IOOU aerial set for short-wave reception, and a greater length than this is said to bo quite suitable. Loose aerial coupling and right reaction coupling are usuallv best. For tho guidance of those contemplating tho construction of a set using movable plug-in coils, the following are approximately the three sets required to cover the range from 20 to 100 metres with a small secondary condenser as mentionou above: — No. of Turns. Pri. Sec. Rcao. Metres 3 15 6 50-100 3 8 6 50-55 3 4 4-6 20-35 In houses with an _ alternating current lighting supply there is sometimes trouble in getting rid of the a.c. hum, and working with an earth connection is sometimes impossible in such a case. The hum has been removed in a particular case by placing a metal plate four or five inches square a few inches above the detector valve in a horizontal position and connecting the plate to tho ground by means of a thin wire. Other cures must be sought to overcome individual troubles of this kind. Switching the a.c. supply off is sometimes feasible in the daytime. WIRELESS PROPAGANDA. During the past year signs have not been wanting of competition between the larger countries in the wireless business. Both in the propaganda and in actual business, as reflected in export trade, has the evidence been striking of intense development. High-power stations, for the purpose of sending out propaganda in the form of press messages, have been operated for some years in Europe and America, but Great Britain was slow in getting a really strident voice on the air. A moderately high power arc station was started at Leafield near Oxford in 1921, but it was not useful. The Rugby super station was opened last year so that Britain could shout as loud as the other countries. America had several moderately high-power stations—really transatlantic stations —in 1918, but the opening of the station or group of stations at Long Island in 1921 was heralded throughout the world in true American fashion. But when the use of short waves became popular the competition became greater. With great economies in power short wave stations were started in America and Europe, and while their transmissions were primarily of an experimental nature —in order to test out thoroughly the new system—the opportunity was not lost to impress on the listening world the importance of the country in which the transmitting station was situated. Now we find these short wave stations being utilised for telephony as well as telegraphy. (The whole of the foregoing refers to telegraph signalling.) Particularly in the United States do we find many stations arranging, at no inconsiderable expense, programmes for distant countries. Early last year the American stations broadcast special programmes for English listeners—to be rebroadcasted by the British Broadcasting Company s stations. Later on similar services were sent out for German stations to pick uxi and rebroadcast. Then Australia was specially favoured with programmes from the principal American short wave stations, and some of the stations transmitted special programmes on their normal wave lengths. We have recently had some excellent transmission from the now famous station of the Phillips Valve Company, POJJ. And strangely enough that station, as is the company’s factory, is in Holland. There is yet no British station capable of broadcasting on a short wave length for reception in distant dominions. At all events no such station has done any successful work in that direction. The matter is in need of urgent attention, and it is refreshing to note that the press in England is taking the question up seriously. Station 3LO Melbourne, which has broadcast Dutch and American stations, is naturally keenly anxious to rebroadcast a British station, but while it does not exist the stations in other countries with a “ world voice ’ must be utilised. Ask to hear the SFERAVOX, the reproducer supreme, and note the difference in volume and purity over the usual horn type loud speaker. The SFERAVOX is non-directional, and is heard with equal volume from any angle. New price £4 4s, of all leading radio dealers. —New Zealand Representatives: Brown and Dureau, Ltd., Wellington. —Advt.