A Variometer Crystal Set.

Radio Record, Volume I, Issue 27, 20 January 1928, Page 12

A Variometer Crystal Set.

NOVEL CONSTRUCTION

‘iis is a variation from the solenoid method of tuning a crystal set, good volume and cotmpactness being its principal features. Variometer tuning is not particularly selective, but it ofters a satisfactory method of getting good volume from the local station with a smooth control equal to that of a tuning condenser at mmnch less cost, though it entails a little more work than the making of a plain solenoid, but those who care for construction will find this an interesting piece of work flint with a little care and patience will give good tesults. The panel 4} inches wide by 6 inches high, may be of ebonite or 3-ply, and the base of fin. rimu, is 4} by 4; inches. ‘tliis part of the construction is similar to that of the selective crystal set, only dimensijotis are all smaller. A cover can be made in the same way.

THE VARIOMETER. The essential part of this set is the variometer, which consists of two parts, the stator or fixed portion, end the rotor or moving portion, which is provided with a spindle fixed in such a way that the rotor may be turned round within certain limits, inside the stator. Turns of wire are wound upon both the rotor and stator, a space being left in the centre of the windings on each, to clear the spindle. he first construction is a former for the rotor, and this is made of wood to the dimensions shown, the corners being neatly rounded off. Now a strip of thin celluloid (about 20 mils) is required not less than 11 by 1} inches. ‘This is placed round the outside of the former, pulling tight, and overlapping an ihe centre of one of the long sides. ‘he overlap is cemented with celluloid cement made by dissolving small chips of celluloid in liquid acetone, which mav be obtained at the chemist’s. The celluloid dissolves in about two hours. .A very small bottle may be used, which shonld he kept corked. The cement is apniied with a thin stick. When the joint is made, the whole should be tightly bound with many durns of twine and left a few hours to: set. :

WINDING THE WIRE. Lhe Jap joimt being, set solid, the twine is removed and winding proceedtd with. The wire recommended is 2% eramelled, but 24’s may be used, a turn less being wound upon each of the four groups of winding. A small liole is made at the edge of the celluloid, which should be slid off the formcr fin. at one side, With this edge to the left, the end of the wire is passed through this hole and turned sharply back te hold during the winding. The turns are now put on in the direction shown in the diagram o 1otor, commencing at A and winding 1n the direction of the arrow. It should here be mentioned that the spindle to be used may be a straight wooden penholder about 34 inches Jong and a little over fin. thick. Fourteen turns of 92’s wire are wound on with the turns close together, then a space equal to the thickness of the spindle is left, the wire crossed over this diagonally, and the winding of 14 more turns proceeded with, the finishing end heing cut off with -five inches to spare, and temporarily secured with twine. The turns of wire are now to be secured with celInloid cement, especially the outer and

inner turns of cach group, as these will hold the others. When this work has set, a small aniount of cardboard is required. Sixteen-ounce strawboard, about 18 to the inch, will suit. A number of squares of this are cut, ljin. by ltin,, to make two lots each fin. thick. T'wo pieces are to be cut 3fin. by ljin. The tin. lots are placed one at each end of the rotor, resting on the windings, and a single piece is placed at each side over the wires. ‘The whole is now bound together with a couple of turns of twine in the same direction as the windings, but outside the cardboard, the twine to be exactly over the centre of the space between the two windings. Six pieces of celluloid are now to be eut about ljin. by Qin. One of these is slipped under the twine at each end ‘of the former, and the winding of the

stator proceeded with in exactly the same way as for the rotor, the beginning of the wire being temporarily se‘ured to a small nail driven into the wooden tormer. ILitteen turns are wound on each side, the space for spindle to be quite free, being left in the centre Now two of the remaining pieces of celluloid are slipped under the twine and windings, one above the position of spindle and one below on each side. The turns of wire are now cemented to each of the celluloid strips and left to set. Now the formers have to be removed, ‘and the easiest way to do this is to chip away one of the small ends of wooden former with a chisel. Then

this will come away and the cardboard packing’ may be removed. ‘The two parts ot the variometer are OW separate. ‘The ceutral position ‘of the opandie must mow be marked on each side of the rotor, and the holes made to be a tight fit ter the spindle. One end of the spindle’ should now be thinned to fit the kuob or dial to be used. FITTING THE VARIOMETER. Then the rotor is iuserted into the stator with the commencing end A near the end C on stator, and the spindle is pushed into place, through the spaces in the stator and through the celluloid of the rotor. At the same time a washer of cardboard or other suitable material not Mess than 1-16in. thick is to be put ever the spindle to separate rotor and stator at each side to keep them from touching. Two tin. holes for simali seews have to be made in the celluloid between the two windings at what is to be the bottom end of stator. Now the whole 1s stood on a piece of wood tin. thick on baseboard, and the best working position found and spindie height marked on back of panel for drilling. The back bearing for spindle is now made of wood as shown, the height 3 inches or so, 1 inch wide, serewed to a small cross-piece to screw ta baseboard. The pack end of spindle is drilled 1-16in. to take a small nail as 4 cotter on each side of the bearing. When this has all been got into smooth working order, the last part is to get the stator into place so that it allows the rotor to move freely. ‘Ihe stator is screwed to the stand-piece by two fin. brass screws through the celluloid. It inay be necessary to pack the stator up with strips of card, to get the necessary position, and when ail is O.K. the stator stand-piece can be screwed into »osition. : |

FINAL DETAILS, Panel arrangements are the same as for previous crystal sets, 28’s brass sheet being curled up for telephone clips if terminals are not used. The terminal on the left is the earth connection and that on the right for the aerial. Iiends A and C of variometer windings are connected together by five inches of thin flexible wire, and end B is connected inside to a short piece of the same flexible, which is brought through the rotor and connected to the back of the aerial terminal, which also connects to back or one end of crystal. Ind D of stator connects to earth terminal, and is continued to connect to *phoe. at that side. Be careful to thoroughly scrape off ail enamel from wires where a connection is to be made. Soldered joints are best, using resin-cored solder. . Any type of crystal can be used, but. if a cat’s-whisker type is preferred 1t could be placed on the baseboard. Tuning is accomplished by turning the knob until full volume is obtained. ‘The fewer the turns of wire put on, the lower the wavelength that can be received, but lowering the range in this way cuts off higher wavelengths. The construction allows of turns being easily taken off hoth sides of stator and rotor if found necessary. The following materials are required, approximate cost being given :-

Ss. d. Hb. 22’s enamelled wire 1 4 Crystal from Knob or dial : 2 terminals .......e...cce sees 0 8 1