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Page:Popular Science Monthly Volume 89.djvu/804

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790

��Popular Science Monthly

��to suit whatever material may be on hand. The base i may be made of hard rubber, fiber or hardwood, and should be about 4,^4 in. by 2 in. by 3^2 '"• thick. Four holes to take 8-32 machine screws

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��Fig. 2. Plan view showing the mounting of the spring brass that holds the needle

are drilled in the positions shown in Figs. 1 and 2 and directly Ijeneath the parts and are counterbored from the bottom to about \i-in. depth to take the nuts and washers. A disk 2, of copper or brass about 1/16 in. thick and i}.i in. in diameter, is soldered to the heatl of a machine screw 3 and forms a sort of table on which the crystal-cup may slide. The screw 3 is fastened in place by a washer and nut, as indicated, and is connected to binding post 14 through the ( hanncl shown in dash lines in Fig. 2.

A Hat brass cup as at 4, Fig. i, 5 s i"- '" di.mieter and H in. deep, may be made by cleaning out thoroughly the cap of a shotgun shell. In this is secured a piece of fused silicon, galena, or other sensitive crystal 6 (wliirh may be purchased from almost any wireless supply house) by melting and i^ouring in solder around it. The heat of molten solder will partially destroy the siMisitive- ness of some crystals, so it is better to use Woods' metd or a mercury amalgam if it can be obtained ; solder will general- ly do for silicon, however. A hard rubber or fiber ring 5, about }i in. thick, should be forced on over the outside of the completed cup, so that the crystal may be niovi-d around without luaking contact between the metal cup and the ojjcrator's fingers.

A needle ])oint 13 is to be su])ported <lirectly above the crystal, and this may best be <li)nc by the pillar arrangement shown. A long machine screw 7 is passed down through two bushings c) and 8, and is fastened below the base by

��a nut and washer. Between 8 and 9 is clamped a tapered strip of spring-brass 10, to one end of which is soldered a binding-post from the zinc tcrnu'nal of an okl dry cell. The shape of this strip may be seen in Fig. 2, where the upper part of the detector is omitted.

At the top of the pillar is fastened the adjustment arm 1 1, which should be made of brass about 38 '"• thick cut as shown in Fig. 3. The left hole is threaded to take the pressure-adjusting screw 12, Fig. I, and is slit as indicated at 15, Fig. 3. Thus the screw 12, Fig. i, may be hekl snugly by the screw-threads. A hard rubber or fiber hand-wheel should be affixed to the top of 12 by a washer and nut, as in Fig. i. Connection is made from the screw 7 to binding-post 14" by way of the shorter channel in- dicated by dash lines in Fig. 2.

The Telephones

Next to the detector, the most important thing in the receiving station is the telephone. Any ordinary tele- phone-receiver will give some sort of results, but to get the loudest signals for any particular set of conditions the best telephones should be used. There are on the market a number of heatl- receivers, designed for wireless tele- graph use. These are usually mounted in pairs, one for each ear, on a rtexil)le headband, and are woimd for resistances higher than ordinarily used in wire telejjhony. Reasonably good results can

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��Plan of the adjustment arm Detail of pattern of the tinfoil

��be secured from two ordinary 75-ohm watchcase receivers, if they are connec- ted in series and mounted upon an iiuprovised headband. Thus, there is no need for any one to be discouraged

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