Use of the Steele-Grant Microbalance. 1061 



very sharply at one point so that it approximates to the ideal 

 point and plane suspension. The bending, however sharp, 

 must necessarily take place through a small arc, with the 

 result that as the end of the beam rises it lengthens slightly, 

 and as it falls it shortens slightly. This departure from the 

 ideal suspension may show itself in three ways': — 



(i.) The sensitiveness of the balance will depend to 

 some extent on the positions at which weights, 

 etc., are hung on the rack carried by the fibre, 

 (ii.) The actual resting-point of the balance will depend 



to some extent on these positions. 

 (iii.) The values obtained by calibrating the weights 

 against the ;ur-bulb will depend to some extent on 

 the relative positions of weights and bulb on the 

 rack. 



All these points could be observed with the double knife- 

 edge balance before described, which was tested for the 

 purpose. The suspended system here consisted of a short 

 fibre about 2 cm. in length which carried the air-bulb of 

 volume 332*8 cubic mm. Hanging from the lower end of 

 this was a fine rod about 10 cm. long, fashioned below into 

 a rack and terminated by a small hook. The whole system 

 was about 14 cm. long and is shown in fig. 2. Table I. 

 shows the alteration in resting-point and in sensitiveness 

 consequent on hanging a standard weight of 43 mg. in 

 positions A, B, or C. 



Table I. 



Position of Sensitiveness in Equilibrium case Change in 

 weight. divisions per 100 /x. pressure. weight. 



A.... 5-0 542-1 mm. 



B.... 3-8 541-9 „ 



C... 3-5 541-8 „ 



100// 

 50^ 



It will be observed that the sensitiveness increases with 

 the height of the load. The resting-point has also altered, 

 for a vertical change in position of the load of 2 cm., by 

 150yu. in a load of 43 mg., i.e. by 1 part in 3 x 10 5 . Though 

 small, the alteration is appreciable, but it becomes negligible 

 if the load is always hung at the same level. 



Table II. shows the alteration in apparent value of three 

 selected weights as calibrated against the air-bulb when two 

 different fibres were employed. As these fibres differed 



