Radio Broadcast (Nov 1923-Apr 1924)

306 Radio Broadcast eliminated, and the clarity and quality greatly improved." The neutrodyning condensers may be made in many simple ways. Perhaps the best and most easily varied for experimental purposes, is made by twisting together two wires, two or three inches long. A few inches of regular two-conductor lamp cord will do nicely, though its lack of rigidity makes it less desirable than some other conductors. A, B, C, and D are R. F. transformers, the primaries wound on 2 §" tubing and the secondaries on 3" tubing. The primaries consist of 17 turns of No. 20 double-silkcovered wire, and the secondaries of 68 turns of No. 22 D. S. C. The remaining values are indicated on the diagram, Fig. 5. The secondary of the first transformer may be replaced by a loop. It might, however, be advisable to retain the ground connection to the negative filament. SPIDER WEBS AND THE ONE-TUBE REFLEX THE photographs in Figs. 6 and 7 show a single-tube reflex set made after the complete instructions given in the November, 1923, issue of Radio Broadcast. The apparatus was constructed by Mr. C. H. Brown, and we are sure our readers will agree with this department that he made an excellent job of it. Mr. Brown, however, departed slightly from fig. 6 « Mr. Brown's neat reflex the plansworked out by Mr. Kenneth Harkness, substituting spiderweb coils for the conventional layer windings in the original apparatus. It is likely that the spider-web inductances are slightly more efficient. No. 28 enameled wire was used, all windings being made with the same number of turns as specified in the November article, which any one considering such a set is advised to read. The primaries are also wound over the secondaries. 3j-inch to 4inch winding forms made from pasteboard, thin wood, or fibre are about the correct size; the winding starts f-inch from the center. Any odd number of segments or slits in the spiderweb forms may be used. Between ten and thirty are the most common and convenient. Mr. Brown has also replaced the fixed resistance with a standard filament rheostat, a change of which this department approves. The rheostat is connected in the identical position of the fixed resistance. The crystal detector used in Mr. Harkness' set has not, at this writing, been placed on the American market, but the builder of this neat set found an efficient substitute. Any good crystal detector may be used in place of the French arrangement mentioned in the original write-up. A permanent crystal is now on the market, which needs no adjustment, and so reduces by one the controls that must be manipulated. The set illustrated in Figs. 6 and 7 is particularly interesting because of these deviations from the original described in the November issue. It is an excellent example of what we have been endeavoring to impress I upon our readers, that conW siderable variation is allowable (unless specifically warned against) in making up radio apparatus. Indeed, it is seldom desirable to folj low directions to the letter, for such a procedure is obviously antagonistic to progress. One tube will generally work as well in a parjjjlF I ticular circuit as will another, W , as long as the correct voltages, resistances, etc., are used. If you do not possess a certain make of audiofrequency transformer specified by one builder, use what