WIRELESS NOTES.

Star (Christchurch), Volume XLIV, Issue 256, 28 October 1931, Page 4

WIRELESS NOTES.

How to Make a Permanent Crystal Detector. USEFUL IN VALVE SET. By “Aerial.” As those who have operated crystal sets know, the valve is inferior to the crystal where tonal qualities are concerned. Every now and then, one notices in technical journals accounts of experiments made in the direction of using a crystal detector in place of the leaky-grid condenser type of detection, but the lack of success attained is mainly due to the instability of the crystal when constructed on conventional lines. A device, which has been patented in U.S.A., overcomes this instability and as there is nothing to stop the experimenter from constructing one for his own use (he may not sell it) construction details are given below. Sow To Make It. The technical description is: “ The device comprises a pair of metallic plates having a thin dielectric sheet, such as tracing cloth, clamped between the plates, and having small grains of unilateral conductive material embedded in and extending through the sheet and in contact with and forming a restricted unilaterally conductive connection between the plates and through the non-conductive sheet.” Construction details: Obtain two plates (each one inch square) of any metal sufficiently thick (about 3-32nds of an inch), having parallel smooth faces. These are drilled in the centre to accommodate a No. 8-32 machine screw, one plate having the hole enlarged so as to allow of it being “bushed” with ebonite or fibre. This is essential as the plates must be insulated from each other (as would hot be the case were the screw not insulated from one plate by a bush and a fibre washer). The top plate should be that with the bushed hole. Between the smooth faces of the plates and permitting the machine screw to extend through it, is placed a piece of linen tracing, cloth slightly larger in area than the plates. (Tracing cloth Is used because it has been found to best answer the purpose.) Take the top plate off, removing the screw, but leaving the sheet of tracing cloth on the bottom plate. Sprinkle lightly over the tracing cloth sheet fine particles of galena, iron pyrites or other suitable powdered crystal. The size of these particles must be uniform and no larger than the thickness of the tracing cloth. -(The best way is to pass the particles through a 100-mesh screen. by which means powdered crystals of the correct size will be obtained). Now put on the top plate, insert the screw (be careful it is insulated from the top plate) and screw down lightly. By the way, solder a short length of wire to one corner of the bottom plate and another wire to one corner of the top plate, forming leads for connecting purposes. The detector may then be tested as described in these notes on Saturday, October 10. Tighten the nut until a deflection is obtained, reverse the current and note the deflection. Adjust the nut until a ratio of current in one direction to that in the other is at least 6 to 1, keeping the lowest side down to the minimum amount of current. The smaller the current flow of the low side the better will be the detector. Failing a test such as described above, the crystal may be mounted in a crystal set in the usual way and therj tuned by tightening the nut until the loudest signal is obtained. If properly constructed (as described above) a sensitive detector will be obtained, having many points of contact in place of the single, flimsy, contact of the catswhisker type. After the plates are adjusted to give best results, it is advisable to dip the whole device in paraffin candle grease or “ dope ” it with collodion cement, in order to maintain it in a more or less permanent state and to prevent the elements from oxidising. In hooking this device up in a valve circuit in place of the usual grid condenser and leak, connect it as you would an ordinary grid condenser, but with the low resistance side to the valve and the high resistance side to the coil. Note, however, that the grid return of the soil must be to the negative side of the filament. :: CORRESPONDENCE. To R.F.W.: Many thanks for the information- Have passed on your letter and enclosure to Mr Gardiner. — “Aerial.” j.j TO-NIGHT’S PROGRAMMES. 2YA, Wellington (416 m 7: News. 7.40: Lecturette, Mr F. M. Bateson, “Measuring the Universe.” 8: Ur Eugene Ormandy and his Salon Orchestra. Mirth Quakers. Weather report. International talk, Ur Guy H. Seholefield. 0.8. E. Stardust. Whispers. Clos'e. 3YA, Christchurch (306 m 7: News and reports. 7.30: Addington stock market report. 8: Paul Godwin’s Orchestra, “ Lustspiel.” 8.4: Tenor, Mr Jas. Shaw. “ Deeper and Ueeper Still,” “Waft Her. Angels.” 8.0: Piano, Miss Merle Miller, “ Ballad in A Flat.” 8.14: Soprano, Mrs W. B. Harris, “ Solveig’s Song.” 8.18: Studio Octet, “ Priests’ March,” “ Don Giovanni.” 8 27: Baritone, Rtev C. F. Cross, “By a Biers:de,” “Road to the Isles.” 8.31: Relay from the Radiant Hall (portion of Third Annual Children’s Festival of Music). Training College and Boys’ High School Orchestras, “ Northern Song,” “ Song of Sunshine,” “ Happy Song.” 8.43- Children’s Festival Choir, " Come, Let Us All This Day,” “May Dew.” Nymphs and Shepherds.” 8.53: From the Studio—Studio Octet, “Gavotte.” S 57: Mr Shaw, " The Sands o’ Dee.” “The Coming of a Dream.” 9.4: Weather forecast. 9.6: De Groot and his orchestra, “Autumn.” 9.10: Mrs Harris, “ Should He Upbraid,” “ Chanson Indoue.” 9.17: Miss Miller. “II Moto Continuo.” 9.21: Rev C. F. Cross, “ Fain Would I Change the Note,”" “Shall I Come, Sweet Love.” 9.24: Studio Octet, “ Cuban Dance,” “ Minuet.” 9.30: Programme of old-time dance music—Waltzes, “ Valse Memories,” “Destiny Waltz.” 9.36: Polka. “Polka Medley.” 9.39: Valeta, “Dreaming.” 9.42: Mazurka, “ Moonwinks.” 9.45: Lancers, “Community Lancers”; waltz, “Popular Songs of Yesterday.” 9.59: Boston two-step, “Barn Dance.” 10.2: Scotlische. “ The Birds and the Brook.” 10.5: Waltz, “Blue Danube Waltz.” 10.10: Maxina, “My Lady Dainty.” 10.13: Polka, “Grandfather’s Polka.” 10.16: Waltz. “The Missouri Waltz.” 10.20: Lancers, “ Fink-a-Lincke Lancers”; waltz, “Over the Waves.” 10.37: Maxina. "Maxina.” 10.40: Valeta, “Cuckoo.” 10.43: Schottische, “Dance of the Honeybees.” 10.46: Waltz, “The Costume Ball.” 10.55: “ Three O'clock in the Morning.” 11: Close.