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 



Popular Science Monthly 



OET. V 



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. 



