C. J. Hochanadel Oak Ridge Notional Laboratory oak Ridge, Tennessee
A
A Simple Device for Determining Slopes Using the Glass Rod Method
clever, simple, and rapid method fol dctrrmining the slope at, any point on a curve was described recently by Hoarere.' The method employs a round glass rod placed across t,he curve a t the point. If the rod is perpendicolar to the curve, an unbroken image of t,he cnrve is observed throngh the rod. Two glass rods held together give hetter accuracy. After proper alignment, a line is drawn along one rod, and a perpendicular drawn to this is tangent to the curve at,-th; point. In nractice., it is diffienlt to hold the rod in dace while - - ~ drawing the line. To facilitate the operation and t,o allow t,he tangent t,o br drawn directly in one manipulation, t,he device shown in the figure is useful. Two picces of glass (or plastic) rod were glued into a slot. cut prrpendicular to one side of a clear plastic straightedge. The rods were glned in place flush with one side of the plexiglas. When the rods are aligned on the curve, t,he t,angentcan be drawn directly along t,he st,raight edge. A
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small magnifier fixed in place ahove the rods aids accurate alignment. The point of tangency can he placed precisely by drawing a line between the points made using the small hole and the bottom v between the ends of the rods.
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5/16" P L E X I G L 4 S
6 m m G L b S S ROD, PLEXIGLAS
6 ' ' long ,l".id*
A blackboard model ran be constructed simply by mounting two u-ater-filled vials on a straightedge.
Volume
39, Number 6 , June 1962
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299
