A N IMPROVISED ULTRAMICROSCOPE C. C. KInINamt Lycominq College, Williamaport, Pennsylvania
TRE ultramicroscope is an effective device for demonstrnting the probability of molecular motion by its revclation of the l3rownian movement. The instrument herewith descrilml and illustrnted is easy to asseml~lr and adjust, and the lens system is efficient and relntively inexpensive. This arrangement hn.5 the ndvnntage of ennhling one to determine the chnracter of t l ~ r rhnrge on the rolloidal pnrticlrs under obwrvntion. The materials required are as follows: one onlinary 100-watt Westinehor~.~ strniaht-filament clear elass .. .. lamp; a t\w-foot. lrnpth cut from n squnrotuhe strrl garnge door trnck (the Intter m n k o an rsrrllent optirnl bench when properly snpportecl); six 1'/. inch \vmhcrs; six 2-inch bolt7 with thumb nuts and wa&rn: t , h m small right-angle irons; cardl~oard,brns~,or copper tubing; some tin or aluminum strips for the tube holders;lamp sockets;acouple of tin cans for the lamp shield; lamp cord and plug: a few \vood screws; two achromatic lenses, 32 mm. diameter and 171 mm. focus, nnd one achromatic lens 18 mm. diameter and 35 mm. focus. Such lenses can hepurehaaedfrom \varsnlvngefirrnsfor ahout thrce clollnrn. A pierr of \vood is cut of mtch size that it will f i l closely in the lamp end of the trnck m t i o n and extend out a few centimetern. The lamp ~ w k e is t rnountrd 011 this extension.
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The tubes are supported with metal strips, angle irons, and bolts as ~ h o w nin the photogmph. Slots cut in the angle irons permit a vertical ndjustment of the light beam. The mic.rowopc show^ here is a rebuilt Ilannch and Lomb instn~mrntof modmnte cmt. T h r tr:~cksection is slipprd I w t w m the pronG of the micronwpr?bnsc and tlvn \vood \\dge:exw r w to hold the track nnrl mirrosc~opcin finn nlipnnirnt. No slit is used in this instrument. Instend, the reduced imnge of the strnipht linr lnmp filament is proj e c t ~ into l n thin film of iwlloidnl mlut ion hrhl by capillary nttrnction i n n cell of s l ~ r i : dtlesipn. This rcll and mounting rnnsists of the following eleu t wide is bent, mmts. A strip of shrct metnl n l ~ ~ 4~ cm. n?i shown in tlir dinpmm, to form ir shelf-like superntnge. h piwe of blnrk gl:~es,hnkelite, or ordinary glmq plnte, 4 cm. nqrlnre, is rrmcntnl to the top. If onlinnry g l m is u ~ l it, should Iw pnintd hlnrk on the Imttom and (Y~~cR.
Two parnllel wires are nttachml to t,he glass plate by cementing them to the glasa a t the rear end of the superstage nnd of sufficient lrn@h to extend out for battery ~ w m e r t i ~ ~ n\Ye s . use $28 chrome1 wire, but platinum win would be lwtter. 'I7ie.w wires mu& lie flat on LIglnsq. A cover g l n s lnid nrmsa them forms a thin rect:mwlnr cell open a t the front to the t ~ n mof light. 'I'ho r d ~ of e the cover ~ln.%q must come even with the front rd* of the blnck glnss plnte. The e d w of the former should also IE hlnrkenrd. A few drops of the colloid solution n~ffireto fill this cell. The\virrspermit theuse of a battery todetermine tho sign of the particle rharge. For example, n colloidal snlution of arlfur \vas prepnred by dissolving 2.48 grams of s d i u m thiawlfate in 10 ml. of water, then adding
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JANUARY, 1631
Bome HCI gas by blowing acroas the top of an unstop pered bottle containing concentrnted HCI solution. Aa soon as the d u t i o n became cloudy it was diluted to 100 ml. yielding a tenth-molar solution. When this solution wna placed in the cell and prop erly illuminatecl three ll/rvolt cells connected in aeries with a key and low resistor were connected to the cell. On clminp; the circuit through a resistance of one ohm the sulfur particles moved rapidly toward the positive pole, thus mvenling their negntive charge. A piece of polaroirl film laid over the eyepiece of the microscope often will give a clenrer field by cutting off the light p o l a r i d hy wflrction. Thr tots11 cost of this equipment, the microscope ex-
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cepted, should be less than ten dollars. In conclusion, it should be pointed out that them is nothing critical about the lens system used for illumination. Any arrnngement that will project a bright and shnrp image of the lamp filament into the cell will prove entirely sstisfact,ory. c.u CeNblroUon
