The materials required for the construction of a selenium cell are as follows: Twelve feet of spring brass 1/2 inch wide, 1/16 inch thick, two small machine screws, two 3-inch bolts and nuts, a piece of thin mica 6 by 12 inches, 1/4 ounce of selenium, a small piece of thin board, some wood screws, and a piece of glass about 3 by 3 inches.

From the brass cut 40 pieces 3 inches long, and drill a hole to take the bolts 1/8 inch from one end. Also make up 39 washers by cutting pieces 1/2 inch long and drilling holes in the center. From the mica cut 39 pieces 2 1/2 inches long by 5/8 inch wide. Take half the brass strips, place a washer between each, pass a bolt through the holes in the ends, and screw up the nut. Do the same with the rest of the strips, and you have two sections of the cell. Now slide one section into the other, tighten up the nuts, and place in a vise. File down and polish the edges of the strips so as to form a perfectly smooth surface on one side.

Next separate the two sections again, place a piece of the mica between each strip, so as to insulate one section from the other, then assemble as before, being careful to get the top surface perfectly level and smooth.

Take another piece of the brass, 4 1/2 inches long; 1/2 inch from each end drill and tap a hole to take the machine screws, and bend up 3/4 inch of each end. Use this piece to clamp the two sections together in the center,.being careful to insulate it from them. After making sure that the sections are properly insulated from each other, the selenium may be applied as follows: Hold the brass over a flame until the selenium melts freely, then rub the stick of selenium over the polished surface. If the brass is hot enough the selenium will adhere readily, but if too hot it will burn off. After applying the selenium, and while it is still soft, pass a knife blade lightly over the surface. This removes the surplus selenium, and leaves a thin smooth coating.

Now bake the cell in an oven for one hour, having the tem-perature just below the melting point of the selenium. Then take out and allow to cool in the open air.

Make a box with a glass cover, and wedge the cell in this with small pieces of wood. Fasten two binding posts in one end, and connect each binding post to one of the sections. The cell is now complete.

The advantages of this type of cell are that it is easy to get the top surface of the brass strips perfectly smooth, and, as the insulation is of mica, there is no danger of burning it and thus spoiling the cell. It is not necessary to tin the edges of the brass strips, and it is better not to do so, as the solder used in tinning is apt to melt and run between the strips, short-circuiting the cell.