Page:Popular Science Monthly Volume 88.djvu/329

 Popular Science Monthly

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��The primary 3 is coupled to secon- daries 4 and 5, with their corresponding detectors 8, 14, and stopping condensers 12, 18, in the usual manner. The leads from the condensers 12, 18, which ordi- narily go direct to telephone receivers, are in this case carried to the two pri- mary windings of a telephone trans- former. These two coils, 10 and 16, op- pose each other's effects upon the sec- ondary 20, which has in series with it the telephone receivers 21. This trans- former is adjustable, so that either of its primaries may be caused to induce stronger signals in the secondary than will the other.

The operation of the device may be considered in connection with a condi- tion assuming simultaneous sending by two wireless stations, one nearby and the other distant. Suppose that with an or- dinary receiver the strong signals from the nearer station practically drown out those coming from a distance, and yet that it is desired to read messages on the weaker waves. With the apparatus of this patent the receiving operator would adjust one of the detectors, say 8, to a sensitive condition in which it would respond well to the weak signals. The other detector, 14, is then adjusted to receive only the strong signals. Thus there are set up telephone currents in primary 10 from both stations, that from the distant one being much weaker than that from the interfering set, and tele- phone currents in the opposite direction in coil 16, these latter being only from the interfering station. Since the sen- sitiveness of detector 14 has been re- duced, the interference currents in 16 will be weaker than those in 10. By loosening the coupling between 10 and 20, the signals from the interfering sta- tion will oppose in their magnetic effects on the secondary, and so produce no re- sponse ; if it has not been necessary to weaken the coupling too far, the signals from the distant station should still be heard.

Thus, if detector 14 is of the type which requires a certain fairly large cur- rent before it gives any response, and if the difference in signal intensities is not too great, some very advantageous interference reducing effects may be had.

���A Cheap Ground Clamp

A hose clamp can be purchased at any hardware store at two for five cents and a binding post taken fnjm the zinc side of an old dry battery. A hole is then drilled in the hose clamp and the post soldered fast.

Insulators can be made by taking a broomstick, sawed into 4 inch or 5 incli

��Strain insulator made from a broom stick

��lengths and having a screw eye put in each end. Baked and thereafter boiled in paraffine these make first-class strain insulators. To prevent splitting, a hole should be drilled in each end a trifle smaller than the screw eye and then filled with glue. The screw eye should have a coarse thread. The tough hard- v/ood holds the eyes so they will stand a heavy strain. Two coats of black as- phaltum make them resemble hard rubber.

Crystal Detector Hints

WHEN the best results are to be ob- tained, the crystal should be mounted in a fusible alloy. This can be easily made by melting equal parts of ordinary fuse wire and tinfoil and add- ing a little mercury.

In selecting pieces of galena the parts that are very shiny and have the most vein will be found to be the most sensi- tive. When breaking galena do not hit the mineral one hard blow : tap it three or four times lightly with a liammer, breaking it into square pieces, and there will be no waste.

With silicon or ferron as a mineral, use a gold wire having a sharp point. A fairly heavy pressure may be used, and results in a firmer adjustment. The re- sistance of a crystal is great, and there- fore as small a piece as possible should be used.

Different combinations of minerals will often work better than one mineral. Galena and graphite, silicon and graphite, zincite and bornite. or chalcopyrites (perikon), ferron and silicon, and galena and tellurium will all work better in combination than alone.

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