BATTERY ELIMINATORS

Radio Record, Volume I, Issue 31, 17 February 1928, Page 3

BATTERY ELIMINATORS

SUCCESSFUL DEVELOPMENTS. One of the chief technical developments which has been made in the construction of wireless receivers in the last year has been the steady improvement in equipment to enable the set to be worked from the electric supply mains. For some years an eliminator for the high-tension battery has been on the market, but early types of hightension battery eliminators lacked the flexibility of a high-tension battery, and as they were also rather expensive they were very little used. Many improvements in detail have now made a battery eliminator a far more satisfactory source of high-tension supply for a large receiver than a high-tension battery, and the chief fault with the early models-a tendency for some of the hum or ripple of the electric supply service to make its way into the receiver when a heavy current was drawn from the eliminator-has now been entirely overcome. Prices of completed eliminators are still high-higher, in fact, than they should be in proportion to the cost of other wireless equipment, but so wide a range of moderately-priced eliminator parts is available that a person cat assemble one of these units at home at a reasonable outlay.

"A" BATTERY ELIMINATED. From a technical point of view the progress made in the elimination of the low-tension, or A, battery has been more interesting than the development of the high-tension eliminator. | Comparatively little difficulty is experienced in rectifying and smoothing out into a direct current the alternating current having the necessary characteristic for hightension supply, because the intensity of the current uecessary is very small. The opposite, however, applies to a currert for filament lighting. Because the necessary pressure is low it is difficult to produce an efficient rectifier for converting the current for filament sup-, ply from an alternating to a direct current, and because the necessary current intensity is comparatively high an efficient rectifier would need to be bulky and expensive. These difficultics have been so grat that except in one or two special eases the attempt to rectify and smooth an alternating current for filament supply has been abandoned, and attention has been concentrated on the application of an alternating current supply direct to the valve without introducing into the receiver the hum of the alternating current supply. Many different kinds of valves have been produced in the last year which will work effectively from alternating current. These valves differ materially from ordinary valves, because the filament pro-

per is surrounded closely by a sheath of very light metal which is not in electrical connection with the filament. The grid and the plate of the valve are placed outside this metal sheath. The filament, which is heated from a small transformer working from the electric supply mains, plays the part of a radiator, which heats the. sheath surrounding it to a very‘ dull red heat. This sheath is treated with the energising material used in dull emitter valve filaments, and when. heated in this manner it takes the place of an ordinary filament. By the use of this arrange--ment the alternating current supply is_ used to do the necessary heating work in > the valve without being actually connected into the receiving circuit proper, and it cannot interfere with the ordinary working of the set.