166 The Model Engineer and Practical Electrician. February 13, 1930.
QUERIES and REPLIES
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Selections from Queries recently replied to.
3534. Electrical and Scientific Instrument Makers. -W. W. (Cape Town, South Africa). Q. As a reader of THE MODEL, ENGINEER since igor, I shall be glad if you will let me have the addresses of the following firms: (1) The Universal Electric Supply Co. (at one time Manchester); (2) Messrs. W. Watson & Sons (scientific instrument- makers); (3) Cambridge Scientific Instrument Makers; (4) Messrs. Ambrecht, Nelson & Co. (sup- pliers of radio-active substances, at one time of 71, 73, Duke Street, Grosvenor Square, London, W. A. We append the required information :-(1) Now, Voltalite, Ltd., 4, Frederick Road, Pendleton, Manchester; (2) 313, High Holborn, London, W.C.1; (3) Cambridge Instrument Co., Ltd., 45, Grosvenor Place, London, S.W.1; (4) now, A. Nelson & Co., 73, Duke Street, London, W. 3489. Electric Heating Apparatus.-W. McL. (Nossend). Q.-Could you tell me where I could obtain resist- ance wire for electric heaters, also price and quantity required for same working on 240 to 250 volts 25 cycles at about 2-3 amperes, AA kind of wire much used for electric heaters is nickel chrome. It can be obtained from the London Electric Wire Co. and Smiths, Ltd., Anchor Works, Playhouse Yard, Golden Lane, London, E.C.; possibly the Economic Electric Co. could supply this or a similar kind. You will have to experiment. We suggest about 14 yards of this wire, No. 28 gauge, for a trial to take a current of about 2 amperes. The construction of electric heating appliances is a section of electrical work requiring experiment and experience. 3488. Fitting Engine and Boiler in Metal Hull.- R. W. (Gillingham). Q. What is the usual way of fixing engines and boilers to a metal hull boat? I have recently had a model tramp steamer given to me, and wish to finish it off in the most practical way. The boat is roughly 5 ft. in length and I ft. beam, and is built-up of sheet zinc. A. We suggest that you solder brass blocks or cross bearers to the inside of the hull and drill holes in these first which can be tapped to receive holding screws or left plain to receive bolts, according to the arrangement adopted. If the engine is not pro- vided with holding-down lugs, these can be attached to the base. The boiler might be held by thin brass straps passing around it and provided with means of attachment to the holding-down pieces; it might rest in hardwood blocks. Thoroughly wash the joints between brass and zinc and paint over them to guard against electrolytic action. Sometimes the entire plant of engine and boiler is mounted upon a base of sheet metal and this arranged to attach to holding blocks or bearers by screws or bolts so that the whole plant can be lifted out. Experiment first with soldering blocks on to a piece of sheet zinc. Our book, Machinery for Model Steamers," price 6d., post free, may interest you. 3486. Wire for Medical Coil.-G. B. (Norwich). QI am making a 4-volt medical coil. Will you tell me the S.W.G. for the primary winding, also necessary quantity of enclosed sample for the secondary winding? A A. The wire you send is approximately No. 37 gauge double silk covered copper wire. The quantity will depend upon the dimensions of the coil. description of a medical shocking coil is given in Chapter 3 of our book ** Induction Coils for Amateurs," price rod., post free. This coil takes about 16 ozs. of wire in the secondary winding. The primary winding consists of about 4 to 5 ozs. of No. 20 gauge n.c.c. copper wire. A condenser is not required and should not be used, it would cause the shock given to be too violent. Your No. 37 gauge wire will serve quite well for a secondary winding. There is no particular quantity, within limits of reasonable proportion, the greater the number of turns of wire in the secondary the more powerful will be the shock effect. 3535. Fayette.-L.. E. V. (Tooting). Q. I cannot quite understand the dimensions given for the clearance radius for bogie wheels in the main frames. Please explain how the ins. given on the drawing is arrived at. A. In reply to your query re clearance radius for bogie wheels in the main frame, and your statement that the ins, figure given is wrong; we think that if you set out the dimensions correctly the distance question will come out to 1 ins. as near as possible, and the radius 11/16th ins. also must be adhered to, to give a safe working clearance. You have evidently come to a wrong conclusion by assuming that the centre of the radius is level with the bottom of the frame, but this is not so. This centre is actually 9/32nd in. below the bottom of the frame, the top of the I 1/16th-in. radius being 1 7/32nd in. from the top of the frame. This is found from the position of the bolt holes for the cylinder, the centre line through which is also the centre line through the driving wheels, and which dimension it will be seen works out at 1 3/16th ins. from the top of the frame. Owing to the wheels being sprung this fixed wheel centre does not exist. = 3536. Fayette.-L. H. W. (Birmingham). Q-I am building Fayette, and, knowing very little about it, cannot understand the following: Inside length of valve chests 1 5/16th ins. Overall length of valve buckle in. two-in. bosses 1ins., leaving maximum available space for travel of in., whereas the valve, being in., requires to travel at least in. to open ports at all at either end of stroke. Also, Baker gear only gives in travel, so ports do not open at all. Would it be satisfactory to reduce lap from in. to 1/32nd in., making valve 11/16th in.? It would the open ports with 4-in. travel. If there are any other readers in this district (South Birmingham) who are building Fayette (or any other type), I should be very grateful if you could put me in touch with them. A. In reply to your query in connection with the valve travel of Fayette cylinder valves, we think that if you adhere to the drawings, the valve travel in full gear will come out just under in. We would not advise you to reduce the lap to 1/32nd in., as this would give a very late cut-off, and the engine would use an enormous amount of steam even when linked up. With regard to the question of inside length of the valve. chest, this is correct, but the valve spindle guides should only screw in about half the thickness of the valve chest, that is, the screwed part of the guide should only be about in, long whereas the drawing shows them cross hatched the full thickness of the chest. This, of course, allows the bosses of the valve buckles to partly enter the tapped holes in the chest at each end of the stroke. We would advise you to look up the articles on Baker valve gear in THE MODEL ENGINEER dated July 5 and 19, also August 2 and 9, 1928.
