284 Kansas Academy of Science. 



THE ACCURACY OF A MODERN BALANCE AND SET 

 OF WEIGHTS. 



By H. I. Woods and Alice K. McFarland, Washburn College, Topeka. 



TT would seem to be a matter of some interest to know just what 

 -*- may be expected of a good precision balance of modern con- 

 struction, bought in open market, without any special requirements 

 having been imposed on the maker or dealer. The construction, as 

 regards material and workmanship, perhaps may be judged to some 

 extent by general appearances to a critical observer, and by facility 

 of adjustment and smoothness of operation. The excellence of the 

 instrument in the precise determination of weights, depending 

 largely on the equality of lengths of arms and smoothness of the 

 sensibility curve, can be determined only by careful tests. 



The balance chosen for the tests here reported is by Paul Bunge, 

 Hamburg, and is of the type described in his catalogue as number 

 "0." It is also described in Watson's Physics ( pp. 109, 110). The 

 length of the beam is nominally 13 cm., and is of the short-arm, 

 high-stayed triangle form. The rider scale, also 13 cm. long, ex- 

 tends along the entire front of the beam, and is numbered from 

 to 10, each tenth of the length being subdivided into 10 parts, or 

 100 in all. The rider weighs nominally 5 mg., and is to be at the 

 "0," beam horizontal and no load. The pointer is long, 25.5 cm. 

 from the central knife edge. 



As ordinarily used the balance has the usual sensitiveness of ^\ 

 mg. but a delicacy of yJo mg. may be readily secured by placing 

 a small cylindrical weight on the top of an arm extending vertically 

 upward back of the center of the beam. The swings are then read 

 by a microscope extending through the front of the case at an angle 

 of forty-five degrees and focused on a small ivory scale attached to 

 the pointer about 4 cm. from its lower end. This scale is 10 mm. 

 long and is divided into 100 parts, microscopic figures being placed 

 at each tenth division. This fine scale, in addition to its use in 

 very precise weighings with the fine adjustment, enables one to 

 make more accurate determinations with the ordinary -^^ mg. sen- 

 sitiveness than is possible by reading the swings in the ordinary 

 way on the coarse scale. The vibration time being, of course, the 

 same, readings may be made with almost as great facility through 

 the microscope as on the coarse Scale. 



