Use of the Steele-Grant Microbalance. 1057 



instrument by introducing a bifilar suspension in place of the 

 knife-edge, and in this way he has succeeded in detecting 

 differences in weight of only 0'25 /* (l/x = 10~ 6 mg.) in a load 

 of 250 mg. This gives the almost incredible sensitiveness 

 of 1 part in 10 9 , though presumably the actual working 

 sensitiveness would not be so great. The Pettersson type of 

 balance was therefore eminently suitable, but it was not used 

 because the balance cases available were designed for the 

 original knife-edge type of Steele and Grant. The most 

 sensitive balance of the inventors was able to detect changes 

 in weight of about 4//,, but the load is not stated and 

 apparently it was not great. Their type-B balance with 

 doable knife-edges would measure differences of 100//, in a 

 load of 100 mg., giving thus a relative sensitiveness of 1 part 

 in 10 6 . This appeared to be sufficient for the proposed work, 

 and an instrument of this type was therefore constructed. 

 Its constants were as follows : — 



Period 33 seconds. Maximum load 43 mg. Minimum 

 indication 20/x. Accurate weighings could be made to 

 within 40/x, giving a relative working sensitiveness of 1 part 

 in 10 6 . 



Experiments were also performed with some other types 

 of balance, differing in structural details, and the general 

 conclusions reached through a comparative study are given 

 in the following paragraphs. 



The Beam. 



The correct design of the beam is of the first importance, 

 as it must combine rigidity with lightness and therefore with 

 economy of material. The best form is undoubtedly the 

 rhombus with vertical diagonal (fig. 1, A), as adopted finally 

 by Pettersson, but it is not suited for knife-edges. The 

 original design of Steele and Grant is exceedingly good, and 

 it is not easy to see how to improve it. Gray and Ramsay 

 (Proc. Roy. Soc. A. lxxxvi. p. 270, 1912) evidently considered 

 that it was not sufficiently strong, and in their classical work 

 on the atomic weight of radium they employed a new type 

 of beam (fig. 1, B). The radium balance which they con- 

 structed was a very trustworthy instrument, and the results 

 they obtained leave nothing to be desired ; the maximum 

 load was 24 mg., which could be weighed to the nearest 14 fi, 

 thus giving a relative sensitiveness of about 1 part in 2 x 10 6 . 

 However, from a theoretical point of view, it is not by any 

 means clear what advantages the radium balance beam 

 possesses over the simple double-triangle type of Steele and 



Phil. Mag. S. 6. Vol. 43. No. 258. JW>'l922 3 Y 



