VOL. XXIII, No. 14

VOL. XXIII, No. 14

No. 475. WEDNESDAY, OCTOBER 3RD, I928. VOL. XXIII, No. 14 fidence in regard to the future if we first assume that we Editor: HUGH S. POCOCK. shall continue to operate broadcasting on the present Assistant Editor: F. H. HAYNES. lines, and that there are no radical changes either tech- Editorial Offices: 116 -117, FLEET STREET, LONDON, E.C.4 nical or otherwise ahead of us. Captain Eckersley is is `( Editorial Telephone: City 9472 (5 lines). undoubtedly right in emphasising that there so we to Advertising and Publishing Offices: much more to be done," but may not expect find that the progress of the future will be just as full of the DORSET HOUSE, TUDOR STREET, LONDON, E.C.4. iephune: City 2847 (13 lines). Telegrams: "Ethasrnrtd, Fleet, Loudon." necessity for compromise expedients and hit -and -miss COVENTRY : Hertford Street. methods as the past. Short-wave broadcasting has not I ete,mo. ('Tull Nt, L'oceutry" Telephone O210 Coventry. received any marked attention in this country during BIRMINGHAM : Guildhall Buildings, Navigation Street. past months, but we may anticipate that the coming 7 i rm, AUtopress, Birmingham." Telephone: 0970 anal 2971 ht t lles t. winter will revive its importance and that with improved MANCHESTER : 260, Deansgate. etepro.nrc " 1110e, Manchester." Telephone '970 City ;A linesI. improvisations (for we may still legitimately so term efforts made to obtain stable long- distance short-wave Subscription Rates : Home, 17s. 4d.; Canada, 1.75. 4d.; other countries abroad, tos. 6d. per annum. reception) the link -up between this country and the As ratant. 01 f/ic . trnlits and apparatus described In these pages are rarrrcd by remoter continents by broadcasting may be looked for patenta, readers are advised, brfoal Peaking use of them, to satisfy themselves that they ;emdd no; te Defriuging patents. with some degree of confidence. New Avenues of Development. CONTENTS OF THIS. ISSUE. PAGE Picture transmission is a new sphere of broadcasting lnrrtlRlAt .. .. .. .. .. .. 4413 CHOOSING AN A.C. Iìl.rnnNATOR. 14Y A. L. M. SI/WEBBY .. 45n activities of which we are promised a foretaste almost' EXPERIMENTS ON 5 METRES . .. .. .. .. 453 the sceptic may see nothing BUILDING H.F. TRANSFORMERS. BY H. F. SMIT'n .. 4-,r, immediately, and although FURTHER NOTES ON THE "MEGAVOx TIHiEE" (COX(I.Ullell). BY likely to maintain the interest of the public in the repro- N. \V. 11CLACTILtN .. .. .. .. .. .. 458 Till TliI:ND Or PROGRESS .. .. .. .. .. 4111 duction in the home of still -life pictures of some topical PROGRAMMES FROM AIfROAD .. .. .. .. .. 41i!1 to that no other CURRENT TOPICS .. .. .. .. .. .. 47:1 event, yet it would be rash predict FURTHER GLEANINGS FROM 'OLYMPIA 474 applications will be forthcoming to provide variety and WHAT DO THEY 1inone or RADlo? By 1)'URSAY BELL 477 BROADCAST BREVITIES .. .. .. 481. enhance the value of the service. Again, television, 11SEFi'1. DATA CHARTS NIL '11) .. .. .. .. 483 incomplete we may regard its present state of REVISION OF T'RANsMuTTING PERMITS .. .. .. .. 4ßli however LETTERS TO THE EDITOR .. .. .. .. .. .. 487 development, may, almost at any time, perhaps through R(:.v,r.P,;' PROBLEMS .. 4013 some system hitherto unheard of, be brought to a state of practical development justifying its application as an WHAT OF THE FUTURE ? adjunct of broadcasting. Again, with the development in this country of the THE doors of Olympia have just closed upon what national scheme for general electrification, is it not more has undoubtedly been the biggest and most suc- than likely that some form of wired wireless will be cessful Radio Show ever held in this country, and applied as a means (perhaps alternative or auxiliary to the occasion suggests itself as one for reflection on the the present system of broadcasting through the ether) of trend of progress and a consideration of the possibilities distributing information or entertainment throughout the which lie ahead of us as broadcasting and its associated country? influences extend and develop. There are so many directions in which development may come that it is impossible to regard the present So Much More to be Done. service provided by broadcasting as in any way final. In a contribution to our issue of last week Captain Yet we have referred above only to technical con- Eckersley related reminiscences of the early days of siderations and have not touched upon the very wide broadcasting in humorous vein, but concluded (in all possibilities which still remain open to development in seriousness, we believe) with the remark, ( ( There is so the nature of the material to be broadcast. Is it to be much more to be done, but sadly enough in some ways, supposed that the programmes of to -day and the pur- never in the hit -and -miss improvisations of former poses to which broadcasting is being put will remain times." Broadcasting as we understand it to -day has, we stationary? Rather we would be prepared to believe will agree, reached a standard where results can be fore- that in the not far distant future on looking back on the casted and the hit -and -miss methods of bygone days are present service which broadcasting provides we shall be eliminated,; but we are only able to express such con- astonished that we were so slow to realise its potentialities. B 9 www.americanradiohistory.com .,,T?,1,.,,ls+i: W?46,PFINWPRI4tIri ' ?ws. Wflpohoo OCTOBER 3rd, 1928. WopIld1 How to Avoid Overload and Instability. By A. L. M. SOWERBY, M.Sc. HIì writer does not pretend to know how many here, that the voltage at the output terminals of an elim- commercial receivers there are on the market, inator varies with the current that is being taken from nor would he like to hazard a guess as to the it, the voltage dropping fairly rapidly as the cilrrent is number of different designs that have been published increased. One commercial model, for example, pro- from which the home-constructor can build his own vides 14 milliamps. at a voltage of zoo, but when 40 set. Inspection of the catalogue of a well -known whole- milliamps. are taken from it the voltage drops to iio sale house reveals the existence of some forty different volts only- barely more than half the value given at eliminators, designed to supply anode current from al- the smaller load. ternating- current mains, and there are probably at least Apart from one or two instruments, which have fitted another forty models available. It will be seen, then, to them a device for keeping the output voltage practi- that the chance of acquiring a suitable eliminator for cally constant whatever the load, this interdependence of any particular set by going into a shop and blindly current and voltage must always be expected. It will purchasing the first offered is tolerably remote. This be less marked in an eliminator in which the smoothing fact is the justification for the present article, in which chokes are large and robustly built than in one where will be made some suggestions which, it is hoped, will the chokes are small ; heavily built chokes are thus a be of assistance to those who are faced with the task recommendation. Even the heaviest chokes, however, of selecting from among the many eliminators offered cannot do away with that part of the voltage drop that the model which will most completely fulfil the require- is due to the internal impedance of the rectifying ments of their own particular receiver. valves. In choosing an eliminator, it must be remembered This variation of voltage with load throws on the that the output stage of manufacturer of the elim- the receiver is that which inator the duty of provid- requires a high MAXIMUM VOLTAGE MAXIMUM VOLTAGE ing his prospective cus- most OF ELIMINATOR voltage, and that except OF ELIMINATOR tomer with the necessary in a few unusual cases information to enable him O 0 the last valve draws about z z to know what voltage to of the total expect with every differ- three- quarters TAP F- I- TAP current required by the 0o5 ent load ; this information î¢ is most conveniently ex- set. So far as the total rn V output of the eliminator is 0 o pressed in the form of a concerned, it will therefore I- I- curve, though a short table be sufficient to select a o of figures, which is more model which will maintain (a) easily printed, may be the voltage desired while equally useful. Not all essen- passing a current some Fig. 1. -In (a) the voltage -drop in the series resistance R is used makers provide this to reduce the voltage for the " Tap " terminal ; in (b) a potential tial information ; unless the thirty to fifty per cent. divider P is employed. In the former case the presence of R with the condenser tends to check instability in the receiver by purchaser is content to ac- greater than that required Tap " terminal. by the last valve. isolating the valve fed from the " quire an eliminator with- For several different reasons, some of which depend out a proper knowledge of its capabilities, his choice upon the presencq of the rectifying valves and smoothing is, therefore, unnecessarily limited. circuits, the selection of an eliminator is not quite so Quite apart from any possibility of undue voltage - straightforward a matter as the simplicity of the condi- drop, there are other reasons making it desirable, if tion just laid down might suggest. It is a well -known there is no great objection to the extra outlay, to pur- fact, upon the reasons for which we need hardly dwell chase an eliminator of such dimensions that the output 11 TO www.americanradiohistory.com OCTOBER 3rd, 19 28.

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