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should then be filed or cut so that the cork will just keep the bolt on the surface of a glass of water. Both the bolt and cork should then be dipped into hot melted paraffin wax so as to leave a thin coating on the surfaces. This keeps the

��Popular Science Monthly

���Current tester, consisting of test tube of water, cork and bolt, all surrounded by coil

cork from becoming saturated with water and also prevents the bolt from rusting.

Now take the test tube and tie it in an upright position to any convenient object. The round end should face toward the bottom. Fill the tube two-thirds full of water and put the cork with its bolt in it. Get a cork of sufficient size to fit the open end of the tube. Dip the cork in shellac, varnish it, then drive it into the tube, so that the side of the tube projects about Ys in. above the top of the cork. Let the shellacked cork dry, then fill up the space between the top of the cork and the edges of the tube with plaster of paris or cement — the latter preferred — making it level with the edges of the tube. When it has hardened, take a piece of emery paper and roughen the glass on the sides of the tube for about H in. from the end. This is to enable the cement to grip the tube tighter when it is fastened to the base board. Place the base of the instrument on a level surface and put the tube, corked end facing downwards, in the center of the small hole, the end of tube to be flush with the bottom of the base. Then fill the cavity between the sides of the base and the tube with cement, keeping the

��tube in an upright position until the cement hardens.

The flange holding coil is made from cigar-box wood or other thin material. Nothing is to be gained by making the coil more than three times the diameter of the test tube; that is, if the outside diameter of the tube is 1 in., the total diameter of the coil including the tube will be 3 in. In other words, there will be 1 in. on each side of the tube. Measure the outside diameter of the tube, then mark on the board a circle of the required size and also mark with the dividers, a circle for the outside diameter. If a 1-in. tube is used, the diameter of the flange should be 3^4 in. wide, the l-i in. oversize being left to extend over the wire coil, as will be seen in the sectional view of the battery tester. Put the wooden flange on the test tube and slide it down to the center (it should fit the tube as tightly as possible), then glue it in place and let it dry. Get H lb. of No, 20 double cotton covered magnet wire and wind a sufficient amount of it on the spool to nearly fill it up, but allow the top flange of the spool to project }/s iw. on each side of the wire.

In winding the wire, start and finish at the bottom, leaving 6 in. of wire on each side of the spool to connect the magnet with the binding posts, there being one of the latter on each side of the coil. Scrape the insulation off of the connecting wires and insert it under each binding post. The wood work should be enam- eled so as to make an attractive instru- ment. The enamel should also be put on the bottom and over the plaster of paris or cement seal and the joints where the tube is flush with the base. Four brass tacks spaced at equal distances should be placed on the bottom of the base. The test tube is then ready for use.

Connect a wire from the zinc of the battery to one binding post and the car- bon wire to the other. The cork and bolt will then dip and rise as the circuit is closed and opened. The stronger the battery is, the quicker and deeper the bolt will sink. If the battery is nearly exhausted, the bolt will merely move about on the surface. A very artistic tester can be made by enameling the base of the tester red: coloring the spool flange black, the wire coil green, and the cork and bolt black.

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