Use of the Steele-Grant Microbalance. 1063 



The slight variation in the percentage alterations as given in 

 the fourth column is doubtless clue to an unfortunate accident 

 during the two sets of weighings. The stand carrying the 

 silica weights was overthrown, and the weights had to be 

 cleaned from dust and fluff which adhered to them. Such 

 cleaning operations, even if very carefully carried out, are 

 always liable to leave slight residual changes in weight. 



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It is very likely that the variations shown in the tables 

 could be materially diminished by the employment of thinner 

 and longer fibres. The long and free fibre suspension used 

 by Gray and Ramsay is probably much superior in these 

 respects, but no tests have been made with it. Attention is 

 merely directed to possible sources of error in the use of the 

 balance. It should also be borne in mind that the risk of 

 breaking the fibre during the ordinary manipulations con- 

 nected with weighing is considerably increased with very 

 thin fibres, and the delay consequent on the insertion of a 

 new fibre and the readjustment of the whole balance may 

 be very annoying. In this connexion it may be worth 

 mentioning that a very neat device, due again to Mr. G. A. 

 Ampt, renders such manipulations comparatively safe. All 

 weights, counterpoises, and objects to be weighed are pro- 

 vided with two hooks, one above the other, as shown in fig. 2 

 They then hang on the balance or on their stands from the 

 upper hook, and are carried from place to place on a slender 

 silica rod from the lower hook. When it is desired to hang 

 an object on the balance, the upper hook is allowed to engage 

 with one of the arms of the rack, and the silica rod is slipped 

 out of the lower hook without straining the fibre in the least. 

 The operations are reversed when an object is removed from 

 the balance. The adoption of this simple device will save 

 much worry and trouble. 



General Arrangement of Apparatus. 



It is almost needless to remark that the precautions against 

 dust, mechanical shocks, and temperature changes, empha- 

 sised by various workers, must be carefully followed if 

 accurate results are required. In the present work the 

 balances were mounted on rubber pads in a cellar with con- 

 crete floor which was almost free from vibration and 

 maintained a fairly steady temperature. The cases were 

 surrounded by metal boxes as advised by Gray and Ramsay, 

 and the indicating light was only allowed to impinge on the 

 mirror momentarily and after passage through at least 25 cm. 

 of distilled water. The manometer was connected to the 



