WIRELESS NEWS.

Press, Volume LXI, Issue 18365, 24 April 1925, Page 12

WIRELESS NEWS.

GRID-BIAS BATTERY.

REMARKABLE SAVING IN EXPENSE. (SFECIALLT WRITTEX FOE "TUE P3ISS.") Bv "Electra." Because of the remarkable, saving ill "B" battery expense resulting from the use of the "C" or bias battery, this effective irnjt is being incorporated almost universally in multitubo sets. No one who uses a "~C" battery doubts its value, but comparatively few understand how and why it accomplishes such wonders. The usual answer is that by keeping tke grid, negative, reduces the plate current. This much is true, but it is far from a complete explanation. If we reduce plate current-, why do we not reduce signal strength? The amount; of plate current has no direct bearing on signal strength. As soon as you light the tnbos both the "A" and "B" batteries begin to expend some of their energy. When' a station is tuned in there is no change in the milliamperes drawn from the plate battery. The only change which the incoming signal produces upon the plate current passing through the loud speaker unit are variations or fluctuations at audiofrequencies, as determined by the speech or music being broadcast. The volume of th-j signal is determined by the amplitude of the fluctuations, and not by the amount of plate current. In the same way the height and frequency of waves of a turbulent sea are quite independent of its depth. As long as thero is sufficient depth of water available to permit the development of the highest possible wave by a given set ot weather conditions, there is no gain, in tho size of tho waves by an increaso in the depth of a sea by a hundred or a thousand feet. Variations in Plate Current. Likewise as long as the plate current is sufficient to accommodate the variations caused by changes in the grid potential as impressed upon it by the incoming, signal, you obtain the coming signal cause the plate current ■variations. Often «n clfieient set drawing but seven milliamperos from the plate battery may give a louder signal than an inefficient one drawing 25 nmlianiperes. A very strong loud speaker signal is produced by variations in plate current amounting to four milliamperes. "With the set drawing an average of seven milliamperes, therefore, the variations caused by the incoming signal cause the plat© current to rise and fall between limits of fire and nine milliamperes. With the wasteful 2o milliampere set the variations still remain at tho .iverago drain, rising to a maximum of 27 and falling to minimum of 23 milliamperes, Tho only difference in actual result between Wie two sets is that the "B" battery drain in the efficient set is about 30 per cent, of that of tho wasteful one. This is reflected directly in upkeep ciosts.

Now that it is explained that! there is no loss in signal strength through the addition of the "C" battery the inquisitive listener still seeks to knowwhy the battery serves to reduce the plate current so effectively.

Why Current is Reduced. The heated filament of a vacuum tube is a source of electrons, whicr? are liberated as soon as the- filament is lighted. A strong tendency to drift to the plate is created through the connexion of the high voltage "IV' battery between the plate and the filament, which causes a continuous flow of plate current. En route from the filament to the plate, tho electrons which constitute this current flow, passi ■through the meshes of the grid. Tho charge on the grid varies in accordance with the signals impressed upon it by tho antennae system. "When the grid is positive, it supplements the plate potential in attracting electrons to it causing increased plate current. When the grid is negative, tho charge opposes the flow of electrons toward the plate. Consequently, the plate current varies in accordarco with the voltage impressed upon the grid. Indeed, this element of the tub© may bo considered as a sieve, the meshes of which, vary in accordance, with an incoming signal. Tho addition of the "C" battery simply causes these meshes to reduce their size by a definite proportion, resulting in smaller plate current and ' f ß" battery economy. Incoming signals servo to reduce and'increase their size to tho same degree aa before, exwept that tho fluctuations in plate current thus produced are impressed upon a smaller plate current. Thus We havo no sacrifice in tho amplitude of fluctuations, which produce the, all essential signal, hut wo do secure them at less expense of plate current.

A RECORD LOG. I have to hand a list of calls heard on one Sunday evening by Mr M. R. Jago,; of Essex street, Master ton, and they constitute something like a leading record, there being a total of .51 different stations heard during 2hrs 53min listening. The details of the set used arc as follows: —Flat spider web coils, primary S turns, secondary 10 turns, tickler 30 turns, cleteotor and two stages of A. I* 1 , amplification. The aerial is a twin wire 40ft high and 60ft long "L" type. The timo Mr Jago started tn listen in was C.l p.m., and he continued till 8.53 p.m. The list is rather lengthy to give in detail, as space is valuable 111 this column, but it is comprised mostly of Now Zealand amateur transmitting and broadcasting stations, though there is also_ a pretty fair list of American and Australian stations, both amateur and commercial. Mr Jago is to bo complimented on this big "bag," and it is to be hoped thatsome time later I may have the plea-sure of announcing a still larger one.

THE AMERICAN TESTS. These da not seem to have heen a great success, a very poor perccntago indoor] having reported anything ]iko worth-while results. Mr 0. ltussell, of this city, reports having received W.L.W.", the big station owned and operated l>v the Crosley Radio Corjioration, KFI and WOK. (Newark). KFI was received on an indoor loop aerial using a 7-valvo super-het. Using a low loss tuner WLW and KFI were picked up. this, of course, on an outdoor aei'ial. Other reports have come in from Ashhurton and Wellington concerning the reception of the above stations. Mr W. Edwards, of Domain terrace, Spreydon, reports that on Saturday, April 18th, at. 10.10 p.m.. ho tuned in IvDKA. This station was asking .for reports from their vario;:-. Australian and New Zealand friends, and announced that they would bo testing out again at an early tiatc. Only the three last items of their programme were heard, and Mr Edwards states that the strength of the station was surprising, being as strong as some of the >iew Zealand stations. It- is strange that tha tests on Thursday, April 16th, did not come in stronger than th*y did, for T. have heard l rejx>rts irom various amateurs who were listening to the long wave stations that signals were very QSA indeed. A number of European stations came in that are very seldom heard here.

CORRESPONDENCE. <- S." rapanui—Yours is rather a selfish wav'of looking at it, tor until the 30s license has been paid, it will bo' imnossiblo to erect the high-power broadcasting stations. 1 advise you to read u» the regulations and seo what the penalty is Tfor owning an unlicensed radio set. Information has been received'by tip Canterburv Progress League m reply to representations niado by it that the fees- for wireless broadcasting will bo increased from os to 30s and that the efficiencx of the broadcasting service will be' increased. The increased Juvenile will he solely expended in in 9 improvement of tho service.