Page:Popular Science Monthly Volume 92.djvu/490

 474

An Improved Design for a Ground- ing Switch

THIS article describes a short-throw lightning switch having the base of marble, and the contact supporting blocks of bakelite. Bakelite is a perfect insu- lator, marble is not. Marble is cheap, bakelite is not. The combination gives maximum insulation at minimum cost. Raising the contacts from the marble eliminates surface leakage to a large ex- tent. By having the contacts mounted

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Base board with the lever attachments for the single throw switch

��with their flat sides in contact with the supporting blocks, alinement becomes automatic, because bakelite sheet is extremely accurate in thickness. By placing the contacts at the vertices of an equilateral triangle, minimum throw with maximum space between contacts is se- cured.

A summary of the material required is as follows:

Marhic, 1 Pc. 1 in. xW]/^ in. x 14 in.

Hakclile Slieet 5i in. Thick, Vi IVs. 2 in. x 2 in. x % in.

lius Har ("opper, 1 IV. ]/« in. x 1 in. x '25 in.

Copper Sheet Y^ in. Ihick, I I*c. 2 in. x .'5 in.

6/1(5 in.-lSx l^in. IM. Hd. Iron Cap Scrcw.s, f) Ut-q.

K i"- X lYi in. 11(1. 11(1. Iron Wood Screw.s, 3 Kc<).

No. 10-32 x 13/10 Fil. lid. Hru.ss Math. Screws, 9

Kcf). No. 10-32 ]{ras.s .Xforn Nuts. 1> ]{cq. No. 10-32 x Yh in. Fil. lid. Itrass Macli. Sc, 3 Rcq. No. U-20 x Yz «"• Hd. 11<1. Hra.s.s Ma<h. Sc, 3 Req. No. U-20 X % in. lid. Brass Much, Sc., 1 Keq.

��DEI 5

��DET7

���No. 14-20 Hex. Brass Nuts, 8 Req.

5/16 in. Iron Washers, 9 Req.

}/i in. Brass Washers, 3 Req.

No. 10 Small Pattern Brass W^ashers, 9 Req.

Felt Washers with 5/16 in. Center Hole, 9 Req.

Felt Washers with Y, in. Center Hole, 3 Req.

3 100 Amp. Terminals.

1 Handle.

Wooden Blocks ^in. xlj^in. xlj/^ in., 3 Req.

The switch can be made by the average

amateur with ordinary tools. Get a piece of unpolished white marble for the base, from some marble setter or stone cutter, and chip it to size with an ordinary cold chisel; taking small "bites," so as not to crack it. Smooth up the edges with an or- dinary coarse rasp, and finish them off with coarse emery cloth. Clean up the top surface with coarse emery, also, and file a bevel all around. As your next step drill the base as per detail 1. This takes time, patience and labor. Drill the holes with an ordinary twist drill, using plenty of water as a lubricant. As the drawing shows, there are nine 5/16 in. holes for fastening the bakelite blocks to the base, and three ^-in. holes to fasten the base to the wall or support. Be sure to drill all holes from one side, as the drill always chips out a small piece of marble around the hole when it breaks through.

There are required three insulating blocks, 2 in. square, made out of ^4-in. sheet bakelite. You should be able to get the bakelite from any up-to-date dealer in wireless supplies. Bakelite is the best insulation for this purpose, as it weathers well, does not warp, and does not decom- pose with age. Cut the bakelite with a hack saw, file the edges smooth and pol- ish it with a fine emery cloth and oil. Drill and tap it as shown in detail 2.

Four jaw clips are needed, two for the bottom and two for the top. Use bus bar copper 1 in. wide by 3^ in. thick. Cut this to length with a hack saw, file the edges smooth, and file a bevel on one end, so that the blade of the switch will enter the jaw clips smoothly. After filing the bevel, drill the clips as per detai) 5.

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