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Aviation History
1931
1931 - 1322.PDF
FLIGHT, DECEMBER 18, 1931 said that a lens such as he useo. (a Zeiss 70-cm. F.5 camera lens) was not an absolute essential, and that a lens with a 3-in. aperture would give quite good results in smaller halls. The smoke tunnel redesigned by Mr. Farren from Mr. Simmons's original model is shown in section in one of our illustrations, and an external view in a photograph. Mr. Farren's explanation of the apparatus was as follows: — " The base of the apparatus is a box approximately 3 ft. long, 12 in. deep and 12 in. wide. One end of this contains a honeycoml. (A) 12 in. X 12 in., through which the air passes vertically lt is contracted at (J5) to 6 in. x 6 in. and turned through 90 deg. by vanes at (C). At (D) it is contracted to 3 in. x 3 in., and passes to the working section (E) round another 90-deg. bend at (F) into the other end of the base. contains the device for removing the acid. This consists of a per- forated box (G) surrounded by a gauze curtain (H), which consists of one layer of butter-muslin be- tween two of stiff gauze. The gauze dips into a wooden tray (A') containing a solution of ammonia. After passing through the gauze the air passes through a throttle (L), in the central part of the base, and from this to the exhaust fan by a 2-in. rubber pipe at (M). " Various types of fan are used, according to require- ments, but a very simple one seems to be quite effective for all ordinary purposes. The rotor is driven at speeds up to about 1,500 r.p.m. by a motor whose speed is con- trolled by a rheostat arranged as a potentiometer. " The whole of the tunnel and fan, with the exception of the vanes (C) and (F) and the shaft and bearings for the rotor, is ot woodwork. The optical arrangements are simple ; a 250-watt metal filament lantern lamp is used. The light passes first through a 6-in. condenser of 6^-in. focal length, and then to a 10-in. condenser of 13-in. focal length. This is directly in contact with one side of the working section, and converges the light through the tunnel to the projecting lens." Method of Producing Smoke Mr. Farren said that the technique of using titanium tetra-chloride was one that had to be learnt by somewhat bitter experience. He himself appeared to have mastered the technique very well, and showed very considerable dexterity in dealing with it. For introducing the liquid into the air stream or for depositing it on the model, he used pointed glass rods. It was important that the holes '/ V V SOME OF THE MODELS USED : Top, mounted on glass, interchangeable withsides of tunnel, are an aeroplane wing, a sphere, R. 101, and rotating cylinder. Centre, mounted on wood, interchangeable with lid of tunnel, are pipe of varyingshape but constant cross section, sharp-edged orifice, and contracting pipe (three- dimensional). Bottom, models with moving parts. Venturi tube, poppet valve, and barrel throttle. through which the rods were inserted should be closed at once when the rod was withdrawn, as otherwise the flow in the tunnel might be considerably disturbed. If the rod was to be left in place, the opening around it should be plugged with cotton wool. The nature of the " smoke " varied considerably accord- ing to the state of the titanium tetra-chloride. The best effects were produced when the liquid was fresh. If kept free from contact with the air, it would remain quite clean, but if air was present in any considerable quantity, solids began to form, which might be picked up by the glass rod. It was important to keep the end of the glass rod quite clean by wiping it every time it was withdrawn from the tunnel. A great deal of trouble could be saved by the judicious use of paraffin wax on all woodwork with which the liquid came in contact. Care should be taken to keep everything as dry as possible, since the reaction of titanium-tetra-chloride with water was extremely rapid and produced solid deposits which were very troublesome. The Demonstrations It is not, unfortunately, possible for us to convey in any adequate manner the fascination of actually watching the demonstrations which Mr. Farren gave. Phenomena with a 3 FIGURES FROM MR. FARREN'S DEMONSTRATION : For explanation of these and following illustrations see text. •"••' •:-•••• •. ••—.-:.• I.::...:*:•;•:••••• ...... . 1244 ••••••• •••.••..•..•
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