Simpler Weighing
w
GORHAM W. HARRIS
Simmons College, Boston, Massachusetts
ISHING about the systems . to .get mformation ' of welghmg m use for elementary quantitative analysis, the writer sent a shoe questionnaire to about fifty colleges and universities of larger size throughout the country. The forty-one replies received yielded the following statistics: Mahod of weighing Multiple swing. 3. 5. etc. Double swing Double and single swing Magnetic damping Single minx only Totals
24
7 5 4 1
41
2890* 1725 855 355 50
5875.
*These totals are probably 200-300 larger in reality. as t h r r institutions did not report the number of students.
the single deflection method when they adopt it a little later. We have applied the term "swing point" to the spot a t which the pointer stops a t the end of its swing to the right, usually three or four spaces from the center. The student determines, when necessary, the zero swing point in starting a weighing and then, after two or three quick exploratory positions of the rider, computes the correct final position of the rider from the sensitivity of the balance. It might be objected that when the balance has been set off-center for single deflection, it would be necessary to readjust i t whenever one wishes to count multiple swings, as when calibrating weights to 0.01 mg. Nevertheless, adjustment of the nut a t the end of the beam is not necessary in order to change from one system of weighing to the other, although that is the usual device for altering the center of swing. With its zero point a t center, the balance can be made ready for one-swing weighing by placing on the left pan a one-milligram weight or a small piece of aluminum wire, as from a broken rider. This special adjusting weight may be stored in the weight box. %here& the single deflectionprocedure was hardly mentioned in textbooks of a decade ago, it gets adequate attention in several standard texts of recent years. First described briefly by Turner1 in 1916 and more fully by Brinton2 in 1919, i t has been gradually gaining favor. Besides its speed, this technic has the advantage that there is but one value to remember or note--the swing p o i n t a n d no computation is required as in finding the center of swing. Another decided asset arises from the off-center condition of the balance. When the l5e.m and pans are freed, the pointer moves without further assistance to the swing point. Fanning with the hand, often suggested in texts, causes delay until the air currents have subsided. Lifting the rider momentarily gives a satisfactory impulse, but is slow and clumsy in comparison with the quick automatic start with the offcenter adjustment. Since all one-swing readings are made a t about the same point on the scale, one is less troubled by parallax than when reading both ends of a long swing, and a small lens gives easy magnification. S i c e a change in the rider position affects only one end of the pointer swing, the sensitivity of the balance measured by the swing point amounts to twice as much as when computed from the center of swings. The two-swing routine shows the same increased sensitivity when the sum of the swings is used.
In summary of the table: 24 institutions of the 41 reporting use the method of several swings for weighing, while 17 teach one or other of the time-saving methods listed. In the majority of colleges the multiple swing method is taught, but the number of students involved is only a little larger than that of those who are given. one of the quick& methods. Only two of thenine institutions replying from west of the Mississippi were using the multiple swing method exclusively. One reply, while stating that both double and single swing methods were taught, marked the multiple swing as the only one in actual use "for pedagogical reasons." The writer believes that the demands of logic and pedagogy can be well served if the multiple swing routine is taught and used a little, then replaced by a simpler technic. Another response read: "Because of the large number of students taking our beginning (ourse, and the fact that each one cannot have a balance for hisexclusive use, i t has not been found practicable to use the single deflection method." Each of our balances a t Simmons College is used by five or more students, yet we have found it entirely feasible to use the one-swing procedure. Our experience may be of value to others in this very common situation. It is carefully explained to the students that the pan arrests must have equal tension, as shown by the pointer remainimg steadily a t the center of the scale when the beam is released, and that the pan arrest button must be pushed in with a quick touch so that the pans will be released simultaneously and neither pan receive a greater impulse. The system has been in use a t the college now for several years, and little or no more attention is required to keep the balances in order than was given in the past. More important, the results of ' TURNER,"A rapid method of weighing," Chemist-Analyst, analyses are quite as satisfactory as before, when mul(1916). tiple swings were used. The students serve an appren- 16, 21-2 B R ~ N T O N , "The single ddection method of weighing," I. ticeship with multiple swings, and learn to appreciate Am. Chen. Soc., 41, 1151-5 (1919).
Obviously, this greater sensitivity does not indicate enhanced accuracy or precision. The writer timed and recorded zero points by each of the four methods reported in the questionnaire--~ne-, two-, and threeswing and magnetic damping. The readings by one and by two swings were about fifty per cent faster than the others (the single-swing the fastest), and more than twice as fast as five swings, but the precision of the quicker procedures was only about one-half that of the threeswing or damping. In no case, however, was the average deviation from the mean greater than a tenth of a space. Recent tests reported by Lin8 and by a LIN, "An evaluation of the methods of using analytical balance." J. CREM.E~uc..16,34C-3 (1939).
Booth and Damere114 agree in finding the single deflection method entirely satisfactory for ordinary quantitative weighing. This method, although apparently rather unconventional, deserves open-minded consideration by instructors of quantitative analysis, as it is somewhat quicker even than the two-swing system, offers the simplest device for putting the beam in vibration and, like magnetic damping, avoids all computations of center of swing. Not only is time and effort saved, but errors can be reduced in weighings of substances affected by contact with air.
4
BOOTH AND DAMERELL, "Quantitative analysis," McGrawHill Book Co., Inc., New York City, 1940, pp. 22-3.
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