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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. 



