VOL. 1, NO. 9
DIFFERENCE BETWEEN THE HAL# SUM AND THE SQUARE ROOT 211
THE DIFFERENCE BETWEEN THE HALF SUM AND THE SQUARE ROOT OF THE PRODUCT WHEN WEIGHING BY THE METHOD OF GAUSS* S. F. HOWARD AND T.A. MARTIN, NORTHFIELD UNIVERSITY, NORWICH, VT. For the calibration of weights the method of Gauss is usually recommended. As the calculations a t best are long, any device for shortening the computation should be welcomed by those who calibrate their weights from time to time or who wish to weigh by the exact method of Gauss. If we use the decimal place next beyond that of the sensitivity of the balance we often have to deal with numbers having five or six digits. To multiply these and to extract the square root involve time and care. The use of logarithms simplifies matters somewhat but there is almost always an interpolation to cover three digits, at least, and the third digit cannot be very accurate with the common tables. If the arms of the balance are the same length the two weights will be the same and the square root of the product will equal one half of the sum. If the arms are unequal the half sum will be greater than the square root of the product. The greater the diiereuces in the lengths of the arms the more will these values diier. It is the magnitude of this difference which we are considering. Morse in his "Exercises in Quantitative Chemistry," says that the half sum may be *Read in the Division of Chemical Education of the American Chemical Society, at the Washington Meeting, April 24, 1924.
used when this difference "is too small t o be detected by the balance." It is our purpose to derive a formula which may be used to tell when this difference is so small that the half sum may be used and correct weights obtained within the limitations of the balance. Consider a semicircle with the diameter equal to the sum of the weights. If ac is the diameter, then ab and ac are the weights W, and W2. When these weights are equal the radius, R, represents the true weight. When the weights are unequal, x, the perpendicular a t b, cutting the circumference, represents the true weight. R - x is the difference between the half sum and the (L b o square root of the product. The formula for this diierence or error, is derived as follows:
a x =
R=-
d m
(1)
2
(2)
w*+ w2
ws-w, R-w,= wt+w9-w, 2 2
(7)
As the diierence between WI and Wx grows smaller x approaches R as a limit. Since the formula is to be used when the two weights diier very z may be written as 2R. little, R
+
Hence,
The table has been worked out by the formula for weights differing from one-fourth to five per cent and shows the difference or error for each per cent indicated. The smaller weight is taken as 1.00. weight. differ
%
Errm by formula
Per cent error
0.25 .5 1.00 2.00 3.00 4.00 5.00
0.00000078 ,000003 ,00001244 ,0000495 .000111 .000196 ,000305
0.00008 ,0003 ,00125 W5 .01 .02 .03
.
Vor.. 1, No. 9
THETEACHER'S VOW
213
From the table i t is evident that the half sum may be used if the arms, or weights, differ by one-fourth of a per cent with an error of only eight one hundred thousandths of one per cent. If they vary by one per cent the error is l/saa of a per cent, while a difference in the arms of five per cent involves an error of '/&of a per ceat. If the half sum is used when the error is well below the limitations of the balance,it will be found that much time is saved with noloss in accuracy.
