Page:Popular Science Monthly Volume 91.djvu/816

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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.

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