144 



Popular Science Monthly 



in. deep, to receive the coil flush with 

 its top. The leads of the coil are run 

 through the disk. The surface of the 

 coil is then plastered evenly with retort 

 cement. The legs are fastened to a 

 second piece of insulating material 

 with round-headed brass machine-screws, 

 )4 in. long, with nuts. See Fig. 6. 



Cartridge shells make neat electrical 

 contacts for a rheostat 



Using Cartridge Shells for 

 Electrical Contacts 



A NOVEL use for cartridge shells of 

 the old center-fire kind, certain 

 to interest the electrical experimenter, 

 is in making rheostats, small switch- 

 boards and important contacts on wire- 

 less apparatus, where efficiency is 

 considered. A hole a trifle smaller than 

 the diameter of the shell is made in the 

 base and the cartridge shell forced into 

 the hole made, as shown in the diagram. 

 The proper wires are then soldered to 

 the metal on the inside, or the wire ma>' 

 be placed inside of the shell and held 

 securely in position by driving a wooden 

 plug into the empty shell, as depicted. 

 A complete rheostat may be so made. 

 The heads of the shells ofi^er efficient 

 contacts. 



The Best Crystal Detectors 



IN spite of the fact that crystal detect- 

 ors play so important a part in the 

 experimentation of electrical amateurs, 

 their use is not understood as well as it 

 should be. There arc various combina- 

 tions in use. It will be found, in general, 

 that the more .sensitive a crystal is, the 

 more readily will it lose its adjustment or 

 "knock out" from loud signals (jr static. 

 The a\'erage amateur will get more satis- 

 faction from using a single crystal than 

 from a combin.ition. For exam|)le, al- 

 though perikon has many desirable char- 

 acteristics, there is apt to be trouble from 

 particles of one of the crystals rubbing 



ofif, and adhering to the surface of the 

 other. This is constantly occurring. 



Chalcopyrite and zincite, arsenic and 

 silicon, and antimony and silicon are 

 all used in combination, and are remark- 

 ably sensitive. An occasional wash 

 with carbon disulphide helps to remove 

 grease and dirt from the surfaces, and 

 often restores them to sensitiveness. 



Carborundum is proof against all 

 manner of knockouts, but is unfortu- 

 nately not very sensitive. A stifif wire or 

 needle makes the best contact with 

 this substance, and should be pressed 

 down into it with considerable force. A 

 battery must be used with this detector. 



Silicon is more sensitive than carbo- 

 rundum, and is correspondingly more 

 easily knocked out by static. A light 

 contact is required, and the efficiency is 

 often improved by applying an exceed- 

 ingly small potential. 



Galena is probably the most sensitive 

 of the crj'stal detectors, but is hard to 

 keep in adjustment. It is especially 

 sensitive to static. The wire used 

 should be as fine as possible. Certain 

 violin and mandolin strings are wound 

 with very fine silver wire; this wrapping 

 is most suitable for use with galena. 



A minute fraction may be cut off the 

 end of the wire from time to time, thus 

 always insuring a clean point. Ceruslte 

 requires about the same treatment as 

 galena. Iron pyrite works best with a 

 firm contact, and is almost as sensitive as 

 silicon. 



Testing Electric Lamps Quickly 



WHERE many 

 incandes- 

 cent lamps are 

 tested, much time 

 is lost in screwing 

 them in and out of 

 the testing-socket. 

 This can be reme- 

 died by means of 

 an ordinary porce- 

 lain lamp-socket. 

 Remo\e the inner shell and hammer its 

 threads down on a ^g-'m. [lipe. The 

 tliamotor should then be great enough to 

 admit the llnvailed base of the lamp. 

 After re[)laciiig the shell, lamps can be 

 readily tested by merely sliding them 

 into this socket. 



A test-socket 



