262 OUR PHYSICAL WORLD 



slider guide. A piece of metal, brass or copper, is tacked or 

 screwed on the under side of the block to hold it in place on the 

 guide rod. This piece of metal should be cut with a narrow 

 strip which may be bent down and then back under the slider 

 so that it makes spring contact with the turns of wire on the 

 cardboard tube. In order that contact may be made successively 

 with each turn of wire on the tube, the insulation must be scraped 

 off in a narrow strip extending the full length of the tube, directly 

 under each slider rod. The pressure of this sliding contact on 

 the wire must be strong enough to insure positive connection 

 between slider and each separate turn, but not so strong as to 

 wear the wire rapidly or to require much force to move it along 

 the slider. Contact with the slider rod is made by the metal 

 covering over the groove on the slider. The end of this metal 

 piece may be cut slightly so that it can be pressed tightly against 

 the guide rod. 



Two of these sliders with guide rods are to be provided. One 

 is to be mounted directly over the tube with the ends of the rod 

 secured to the end pieces. Square notches may be cut in the end 

 pieces and the rod fitted snugly into these and secured by a screw 

 at one end through a hole in the rod, and by a binding-post at the 

 other to which the connection may be made. The slider rod is to 

 be mounted at the side of the tube and directly over the center. 



The winding on the tube is to be of No. 22 insulated wire. 

 Enameled wire may be used. First shellac the tube. Punch 

 two holes in the tube about a quarter of an inch apart and one 

 half-inch from one end. Pass about ten inches of wire through 

 one hole from the outside and then secure it by bringing it up 

 through the other hole and then again through the first hole and 

 back out through the second hole. Now wind the wire closely 

 and smoothly over the tube to within about half an inch of the 

 other end. Secure the wire on the same side of the tube and in 

 the same way as before, allowing about ten inches for connection. 

 To hold the wire in place, a second coat of shellac may now be 

 applied. 



