158 MECHANICS 



part of the instrument. The knife edge (1) may be made 

 of a pocket knife blade or of a piece of hard brass filed 

 to a straight sharp edge. The knife edges for the tray's 

 bearings (n) are made by filing the under side of the tray 

 wires where they cross the beam. The tray wires are 

 made of No. 12 wire. The trays (d) are 3"x3" and #" 

 thick. Two holes near the opposite edges receive the 

 wires which should be bent in the opposite directions be- 

 neath the trays, thereby holding them firm and level. 

 When the instrument is finished it may be made to bal- 

 ance, that is, the needle may be caused to move to, and 

 remain at, the zero point, by moving the nuts on the ends 

 of the beam toward the lighter side. The whole instru- 

 ment may be made level and steady by means of the four 

 screws at the corners as shown in the illustration, 

 although this is not absolutely necessary. A paper box 

 (j) may be used for small objects. The other tray must 

 have an equal weight placed upon it and the instrument 

 must be balanced before weighing any object. 



b. Place the object which is to be weighed, upon the 

 lefthand tray, and upon the righthand tray place a weight 

 which is a little more than necessary. Then remove this 

 weight and put on the next smaller weight. If this is 

 too little, add the next smaller weight. Continue this 

 until the instrument balances. The pointer must always 

 swing free of the screw eye. If, instead of using the 

 large weights first, the small ones were used there would 

 be no small ones left to make the final balance. Always 

 begin with the large weights. 



