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 



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 



Y' 



4. 1 



E"- 



FIG.J 



-zi 



FiR. 3. 

 Fig. 4. 



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 



