electric field intensity or strength and not hattery voltage which was being described. In the experimental portion of this paper, Millikan describes the distance between the parallel plates as 1.600cm. A simple calculation shows that the battery voltage was much greater than 8000 V. (SI units are used.) Clearing-up Misconceptions about Millikan's Oil Drop Experiment To the Editor:
I wish to correct some misconceptions incorporated in the article entitled, "Some Misunderstandings about Millikan's Oil Drop Experiment," Castellion, M. E. and Bailar, Jr., J. C., J. CHEM.EDUC.,59,970 (1982). It is stated that the parallel plates in Millikan's experiment were charged 3000-8000 V hut usually close to 8000 V. Examination of the article, Millikan, R. A., Physical Review, 32, 349 (1911) shows that Millikan stated "an electrical field strength between 3000 V and 8000 V per cm is created between the plates by means of hattery, B." This shows that it was
If 12,800V potential difference is placed across 1.600 X m, we obtain the maximum electric field strength, 8000 Vlcm or 8 X lo5 V/m. Millikan used a voltage divider to bring his lowest electric field intensity to 3000 V/cm or 3 X 105 V/m. A similar calculation shows that a t the lowest electric field intensity, the voltage divider had reduced the output voltage to 4,800 V (Harris, S. P., J. CHEM.EDUC.,59,988 (1982)). A further quotation from the same sentence used previously follows: "if the droplet had received a frictional charge of the proper sign and strength as it was blown out from the atomizer, i t is pulled up by this field against gravity, toward the upper plate." Evidently, the charge was frictional in origin.
Volume 61
Number 1
January 1984
89
A charge cannot be produced from the electric field. There is a uniform electric field between the parallel plates except a t the edges. The electric force, F,, is directly proportional to the electric field intensity, E , and to the charge, q, on the oil drop. H
Everywhere hetween the charged and parallel plates, the electric field intensity is the same, a maximum of 8 X lO5V/m or 8 X lo5 nt/coulomh. Within this uniform field, the charged body moves with uniformly accelerated motion toward the plate having an opposite sign. The net force accelerating the charged oil drop is the difference between F,, the electric force and the eravitational force. on the oil dron. F.. ~ i l l i k states k that the frictionally chargedoil drops were blown out of the atomizer and fall into the viewine chamber. At thesame time hestates that the parallrl plates areshortcircuited bs meani 01 u switch. S.The. uil dnm is timed in its fall. ~ollo'ing this procedure, the electric field is switched on and the rate a t which the droulet moves uoward under the influence of the electric field s: timed for the same distance between two crms hairs. If the droplet meets an ion, the speed of travel between the two cross hairs is increased. Millikan was able to calculate from this kind of experiment "the sign and exact value of the charge carried by the captured ion." Sometimes, Millikan produced ions in the viewing chamber "by any of the usual ionizing agents like radium or x-rays." Millikan observed some drops for periods as long as an hour by utilizing a switch to reverse the polarity of the parallel plates. Millikan realized that his results were "limited in accuracy only by that attainable in the measurement of the coefficient of viscosity of air." The error in air viscosity was eventually shown to be the cause of the 2.50% error in Millikan's measurements. Sidney P. Harris
.
6 CH$OH
R R
Before incorporating the above adjunct method into a discussion of stereochemistry, it is best to present the traditional three dimensional representations for chiral molecules. T h e required restrictions for using the planar Fischer projection formula also should be stressed.
' Idoux, J. P., J. CHBM.EDUC.,59,553 (1982).
Natta. G.. Farina. M.. "Stereochernistrv." - . H a r ~ e rand Row. New York, 197'2, &.241-242.'
Justin W. Diehl St. Banaventvre University St. Bonaventure. NY 14778 To the Editor: In the July 1982 issue of the JOURNAL, John P. Idoux wrote an article. "A Simole Method for Soecifvine . . .. the R/S Confieuratiun about a Chiral Center," in which he uurlines o p n a e l dure fur idrn~itvin?:tht: R S tonfirur;~ti(,nI I ;I~ c h ~ r acenter via Fisher proje&n. He also suggests that this method has not been presented in undergraduate organic texts or chemical education journals. However, Carl R. Johnson has written and published a hook, "Organic Nomenclature: A Programmed Study Guide," Worth Publishers, Inc., 1976, devoting almost an entire chapter to finding the R/S configuration of a chiral center using Fisher projections. I have been using his manual in undergraduate organic chemistry for several years.
Queens COlleQe
Louis Miiakofsky Pennsylvania State University-Berks Campus Reading. PA 19608
Flushing. NY 11367 To the Editor Specification of RIS in a Multichiral Molecule To the Editor: The recent note' describing the specification of R or S to a chiral molecule by an even number of group exchanges hecomes extremely difficult for a multichiral molecule. Since an odd number of interchanges is equivalent to inversion,2 the following method might be useful regardless of the number . of invo1;ed chiral centers. 1) If the Fischer proiection formula has the lowest orioritv group, gener&;hydrogen, to the top or bottom, the; R or S is specified in the usual manner. For example,
H
2) If the Fischer projection formula has the hydrogen to the left or to the right, then the R or S is determined and then inverted. For example,
Two communications have appeared [J.CHEM. EDUC.,56, 451 (1979), 57,528 (1980); Biochem. Educ., 8,105 (1980)l in which simple ways to relate R, S stereochemical designations to Fischer projections were presented. Any system, simple or complex, cannot be used successfullv to relate the R. S designations to Fischer projections if an incorrect two-dimensional projection is made of the three-dimensional model of the structure [Appendix 2, IUPAC Tentative Rules for the Nomenclature of Organic Chemistry. Section E. Fundamental Stereochemistry, J. Org. Chem., 35,2849 (1970)l. These Rules (Section E-7.1) point out that one cannot rotate a Fischer projection 90° in the plane without changing the projection of the vertical bonds to in front of the plane. In many biochemistry textbooks the standard projection rules are not being applied to the amino acids, but are being used as if the rotated vertical bonds project forward. The amino acid in Dietzel's note [J.CHEM.EDUC.,56,451 (197911 is indeed (R), hut is f r e ~ u e n t l vassumed to he LB-see below. This assumption 1; only arreptable when it 1s cl(.arly >tared that it is I~pingused I / ~ i r . ~ . h v rKdttc., n. 2 , X cl.9:411.
COOH H-+CH, OH
,
,R
ongmal specification
s
inverted and now correct eonfigurational assignment
For a multichiral molecule, each chiral center can he quickly and correctly assigned without the possible confusion associated with the switching of groups. For example, 90
Journal of Chemical Education
John N. Aronson State University of New York at Albany Albany. NY 12222
