A DEMONSTRATION POLARIMETER HOWARD BURKETT DePauw University, Greencastle, Indiana
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I x DISCUSSING the optical activity of organic compounds students learn more quickly if a few demonstrations are given. Hence, in addition to using models to show the asymmetry of molecules, the author has devised a simple demonstration polarimeter. This saves time in describing plane polarized light, the fact that polarized light is not transmitted by a "crossed" polaroid, and the effect of optically active substances on plane polarized light. What is more important than the time saved is that the students accept the demonstrated phenomenon without the skepticism which was TOP VIEW COMPLETE ASSEMBLY, EXCEPT POINTERS common before the demonstration was used. In essence, the demonstration polarimeter is a crude, large-bore polarimeter. The details of construction are not critical and may be varied to fit the materials available and to suit the whims of the builder. The apparatus as described has been ent,irely satisfactory. The light source is a 50-watt light bulb mounted in a small recepticle a t the rear. The height of the holes in t,hepanels is made to correspond with the light source and will depend upon the type of lamp base and lamp POINTER HOLDERS '/ which are used The light passes through the hole in 'p(6fwi&PaNEiithe rear panel, through a polaroid lens recessed in one PANEL of the pointers, through the t,ube for the solution, through the front polaroid, mounted in the same way P~W.. 2 as the rear one, and onto a piece of translucent paper, fastened in the hole in the front panel. The polaroids are mounted so that they are "crossed" when the A metal tube for holding the solutions was first tried. pointers are in the same plane. The author employed Not only was there the problem of sealing the glass Polaroid Laboratory J-filters, No. 310. windows into the ends so that they would not leak, but also it was found that the plane polarized light was so scattered by the shiny wall of the tube that no difference in the intensity of the transmitted light could be noted upon rotating the front polaroid. It was thought to paint the inside wall black. However, a black auto radiator hose solved both difficulties. Two glass disks were inserted, one at each end, about three-sixteenths of an inch into the radiator hose. If these fit snugly, the seal will he water-tight without using cement.. A pair of nearly flat watch glasses give satisfactory results. The whole apparatus, except the pointers, was painted black in order for the pointeh and the illuminated paper to show well by contrast. The details of construction are shown in the photograph and diagrams. figure 1
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