800 



will make a fair-sized cell, but the more 

 strips and washers, the lower the resistance 

 of the completed cell. From the mica cut 

 50 strips 2-9/16* in. long by Yi in. wide. 

 One of these strips should be very lightly 

 shellacked to each brass strip, as shown in 

 Fig. 3, so that the end of the mica covers 

 the end of the brass strip farthest away 

 from the 3/16-in. hole. 



Twenty-five strips and 25 washers are 

 then mounted alternately on each bolt, 

 with the mica always on top, as shown in 

 Fig. 4. The two sections are slipped 

 together and the nuts tightened down, as 

 shown in Fig. 5, so as to hold them securely. 

 A clamp, as shown in Fig. 6, may be made 

 to fit over the center if desired, but it must 

 be insulated from both sections by mica, or 

 fiber, etc. 



The top edge surface of the strips should 

 be well smoothed down, with a file or other- 

 wise, so as to be perfectly smooth. The 

 two sections should then be tested with a 

 voltage 5 to 10 times as high as that to be 

 used in connection with the cell, so as to be 

 sure that both sections are insulated from 

 each other. If the insulation is perfect, the 

 smoothed surface should be heated until the 

 selenium melts freely when rubbed over it. 

 Be careful not to get it too hot so as to 

 oxidize the selenium; yet it should be hot 

 enough to cause the selenium to adhere well. 

 The parts of the cell where the brass strips 

 overlap should be given a thin, even coating 

 of selenium, and while hot, the excess may 

 be removed with a spatula, so as to leave a 

 smooth, even surface. 



The heat treatment is then given. The 

 cell is placed in an oven and heated to about 

 260 or 270 deg. C. The temperature is 

 then lowered to about 180 deg. C. at which 

 point it is kept for about 2 hours. The cell 

 is then removed and cooled in the air, the 

 "A" selenium resulting. 



The extra nut on each bolt may be used 

 for connections. It is best to mount the 

 cell in a small box with a glass lid, so as to 

 protect it from injury, dust, etc. 



The higher resistance cell is made by 

 winding two wires in parallel, or side by 

 side, on a narrow strip of heavy mica, 

 asbestos board, slate, or other suitable 

 insulating material, as shown in Fig. 7. 

 The edges of this insulating material maybe 

 notched before winding, so as to hold the 

 coils in place. Holes should be bored in each 

 end, through which the ends of the wires 

 may pass, so as to secure them. A medium- 

 sized cell is about I by 3 in., with the top 



Popular Science Monthly 



side only coated with selenium. It should 

 be wound with copper, brass, silver or 

 platinum wire of about No. 35 gage. The 

 resistance of the cell depends upon its size 

 and the spacing of the wires. It is best to 

 have the wire as warm as possible when 

 winding it on the form, so that as it 

 contracts on cooling, it tightens, thus 

 holding it more rigidly in place. A 1/16-in. 

 spacing or less is very often used. The 

 closer the wires, the lower the resistance. 



After winding, the selenium is applied in 

 the same way, and the same heat treat- 

 ment given, as in the construction of the 

 low resistance cell. 



Ballast Weights for Antenna to Pre- 

 vent Aerial from Overturning 



TO prevent an aerial from turning over, 

 even though the lower end is not 

 fastened to a mast or tree, the following plan 

 was found effective. Take a piece of 2-in. 

 pipe about 8 in. long and place inside it a 

 piece of wire bent as illustrated. Then fas- 



The weight hung to the ends of the spreader 

 to keep the aerial from overturning 



ten this with a piece of fine wire so it will 

 hang about 10 ft. below the aerial. Each 

 end of the wire is fastened to the two 

 ends of the spreader. If the pipe alone does 

 not. weigh enough fill in with some melted 

 lead or babbit metal, fastening the wire 

 securely in place. 



