To the Editor: In regard to the accurate value for the charm on the electron, Millikan should have found 4.919 X 10-in esu, but hecause his measurement of the electric field intensitv was done to only three significant digits, he is only entitled & use three significant digits. Thus,the proper value for the charee on the electron should have been reported as 4.92 X 1 0 - ' 0 ~ 1 . In SI units, the electric field intensity is expressed in newtons/coulomb but is measured in the units voltslmeter because these are equal quantities. Today, we report the charge on the electron to three significant t i m e s in ces units as 4.80 X 10-10 esu. It is invalid to compare a three-significant-figure value to anything else but a three-significant-firmre value. so the error is 2.50%. I t is well !mown, hut also immaterial, t i t these charges are known to seven or eight significant digits. Millikan, [Phys. Rru., 32,349 09Ll1l states that the parallel plates in his experiment "have surfaces which are ground so nearly plane that the error is nowhere more than 0.02 mm. These planes are held exactly 16 mm apart by means of three small ebonite posts, held firmly in plare by ebonite screws." The fractional error here is 0.04 mm116 mm = 0.0025. Kvidently the plates were between 15.96 mmand 16.04 mm apart and not really exactly 16 mm apart. This valuealone gives an uncertainty in the charge on the electron which he obtained of (4.919 0.012) X 10-1° esu. The value of the charge on the electron should have been renorted as 4.92 X 10-'~&u -Further, Millikan describes his field strengths as "between 3000 and 8000 volts per cm created" by applying a large battery across the plates. These field strengths are in SI terms, 3 X lo5 nt/coulomh to 8 X 105 ntlcoulomb or equally in voltslmeter as noted previously. These electric field strengths show that Millikan used batterv voltages between 4.800 and 12,800 volts. Such large voltages wereiead on analog meters readable to only three significant figures. Strong states that he wishes to make a small correction. He purports t o show that the error for the Millikan experiment is 2.4% and not 2.50%. The value 2.4% was derived from a four-significant-figure comparison (4.919 - 4.803 = 0.116 and 4.917 - 4.803 = 0.114). Thus, the charge on the electron should esu, and this value as stated have been reported as 4.92 X in the article is 2.50% away from the value correct t o three significant digits, 4.80 X 10-'0 esu. The "so-called" accurate value is stated to be so because accepted values are constantly changing. In the 41st edition of the "Handbook of Chemistrv and Phvsics" (1960) the charge on the electron is given as 4.8029 10-lo ksu &d as 1.5921 X 10-l9 coulomb. In the 47th edition (1967) of the same handbook, the charge on the electron is given as (1.60210 f 0.00007) X 10-l9 coulombs and as (4.80298 f 0.MX)20) X 10-10 esu. The uncertainty values are hased upon three standard deviations applied to the last significant digits. In the 52nd edition (l972)of the same handt,ook, the rhsrge on theelectron is given as (1.602191': r O.lllUW701 X 10-'%coulombs and as (4.803250 f 0.00020) X 10-lo esu. Here the uncertainties are standard deviations computed on the basis of internal consistency and are taken from Rev. Modern Physics [41,375 (1969)l. Thus, there is really no accurate value for any physical constant. We may expect t o see further changes in these values. Strong expresses concern about the use of one CES svstem even thoughmost chemists use it to this day. ow every all of my students have taken a modern physics course where they use the rationalized MKSA system, which is almost identical to the SI,
Another Econornlcal Reflux/Distillatlon Apparatus To the Editor: Marahella' describes an "economical and efficient" reflddistillation apparatus which uses three ground joints and one condenser more than strictly necessruy. A single condenser which can be swivelled around an angled-joint is the more economical alternative.
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' Marabella, C. P., J. CHEM. EDUC., 57, 221 (1980). A. A. WooU University of Sath Ciavenon Down Balh BA2 7AY England
To the Editor:
Strictly speaking, the apparatus Woolf sketches is more economical. However, several drawbacks are apparent when compared to the apparatus which I suggested. The most obvious pedagogical objection is the lack of a thermometer well. This, along with the introduction of a bulky moving part (increasing the chances of breakage), leads me to conclude that, in the long run, Woolfs apparatus is not as economical as it might a t first seem.
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C. P. Marabella The University of Arizona Tucson. AZ 85721
The VolumeNolurne Problem To the Editor; I t was good to see the volume/volume problem discussed in print by Beaulieu and Woodin [J.CHEM. EDUC.,58,740
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11 ,.9 x 1 11 , A
In my biochemistry laboratory courses I have dealt with it without compromising the analytical chemist's svmholilim hut by introducing as a less ambiguous symbol a short arrow that is read as "made up to." In this way the analytical chemist's 1:3 becomes 1 4. There is no excuse for the usage where 1:3 is taken to mean 1made up to 3. As a biochemist, I am appalled that this is regarded as the biochemical nomenclature.
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Sidney P. Harrls
Trevor Robinson
Chairman and Asst. Principal John Bowne High School Flushing. NY 11367
Departmen! of Bechem stry
Journal of Chemical Education
Unwerson, ol Marsachusens Amhersf. MA 01003
