Page:Popular Science Monthly Volume 90.djvu/809

 Popular Science Monthly

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��600, 1,000, 1,500, etc., meters. Make final adjustments, using very loose coup- ling. If a series primary-condenser is in the circuit, leave it at the same setting for all stations. Make a note of the number of primary turns used for the best adjust- ment for each of the stations.

On a sheet of cross-section paper, scale off the bottom in wavelengths the full range of the loose coupler, beginning at the lower left-hand corner. Scale off the pri- mary turns at the left-hand side and from the lower left-hand corner. Plot the points at the intersection of the turns and the wavelength, and draw through them a fair curve, such as shown in the diagram.

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��ZDO 400 600800 K)OOI20OMX)l60O lt00ai0022OO3(OO260O2S0O WAVt UN6TH Plot the points at the intersection of the tiims and the wavelength, and draw the curve

If the f)oints do not line up, see if the plotting has been done correctly; if so, the assumed wavelength is in error.

Do not try to include the loading coil on the same curve, but make a separate one.

Only the primary is mentioned, because the majority of amateurs use "untuned" secondary systems. But if this is not the case the secondary may be calibrated the same way, if a fixed loose coupling is used. The secondary shunt-condenser value must remain constant. A set so calibrated may be instantly adjusted for any station, even though it has not been heard before, pro- vided, of course, that its wave is known.

The curve shown was drawn from results obtained according to the foregoing explan- ation, with a large loose coupler. Note that the line is straight at the shorter wave- length increases. If the loose coupler had been much larger, the curve would have reached a place where it would be nearly a

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��Making Small Slots in the Frame of Pancake Helices

IN the construction of the pancake type of helices, the amateur is often puzzled over the method of sawing the slots in the frame of the helix in such a manner as to insure a snug fit of the brass or copper rib- bon used. In the construction of the primary of such helices, where the ribbon is usually about 11/16 in. thick, probably the best method of sawing the slots for the ribbon is to use an ordinary hack saw. The blade generally runs pretty close to the size of the ribbon, and this gives a snug fit, without the necessity of "shims" — or small pieces of copper, brass, or wood fitted in the slots, which often result in a "messy" job.

In the secondary, however, the ribbon sometimes runs as small as gage 64. Saw blades of this size are very difficult to obtain, so the best way is to make your own blade. This can be easily done by securing a piece of an old clock-spring, which as a rule is very thin. Cut off a piece as long as your hack-saw blade, putting the two ends in a flame for a few seconds to draw the temper. A hole may then be bored in each end to fit the hack-saw frame. Draw the spring taut, taking care to hold it away from the face, as it will sometimes snap. Then press the "blade" against an emery- wheel till a series of "teeth" are notched in it. This does not make a very artistic looking job, but it will be found that it cuts into the frame — whether it be hard rubber or any of the finer woods — rapidly and with a clean stroke. Care should be taken to keep the blade cutting straight, as it will have a tendency to curve. If the right kind of clock-spring is used, a snug fit for the ribbon is assured.

The pancake type of helix is probably the easiest for the amateur to make up, pro- vided he can secure the necessary ribbon. Although fiber or hard rubber is best for the frame, mahogany is very good, and is easily handled. On the primary, it is customary to space the ribbon ^ in. be- tween turns, and on the secondary, where finer ribbon is used, J^ in. — Paul Oard.

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