Use of the Steele-Grant Microbalance. 1059 



corresponds to a relative sensitiveness of 1 part in 4x10' ; 

 in use, however, the resting-point was found to vary slightly 

 from day to day, so that trustworthy weighings could not be 

 obtained beyond the nearest 4 p. This gave a relative 

 working sensitiveness of 1 part in 10 7 . One may therefore 

 venture to state that the simple Steele-Grant type of beam 

 leaves little to be desired as to trustworthiness and constancy 

 of behaviour, and there is no reason to discard it in favour 

 of other types. For comparatively heavy loads, the thick- 

 ness of the members may be proportionately increased and 

 the requisite sensitiveness attained by suitably raising the 

 centre of gravity. 



ilie Knife-edges. 



Steele and Grant, in their original memoir, described the 

 process by which the knife-edges had been made by them. 

 They were ground in sets of three by means of a small rect- 

 angular metal holder into which silica rods had been 

 cemented. Bv resting the ends of the rods and the lono- 

 edge of the holder on a ground glass plate supplied with a 

 little fine abrasive, alternate grinding on both sides resulted 

 in chisel edges. The thickness of the holder was then 

 increased by securing steel slips on either side, and the 

 operation was finished on a very finely-ground glass plate 

 with a little glycerine and water but without abrasive. This 

 method requires only comparatively simple apparatus and is 

 capable of giving very satisfactory results ; it has been 

 employed in the present work. The greatest difficulty is 

 found in the finishing touches. The glass plate employed 

 must be heavy and very steady, truly flat, and with so finely 

 ground a surface that it is almost transparent when dry. It 

 has been found advantageous to use a very light forward 

 stroke with abundance of lubricant for the final polishing, 

 and the edges must slide over the glass without the slightest 

 harshness or hesitation. The progress of the work is 

 watched with a microscope, the edges being carefully dried 

 before examination ; they may be taken to be good enough 

 when the irregularities are barely visible with a good 5-mm. 

 objective. 



Pettersson criticizes rather adversely the knife-edge type 

 of balance on the ground that dust collecting on the quartz 

 bed -plate will interfere with the free movement of the edge 

 and that it will gradually become blunted through use. 

 These disadvantages do not attend the bifilar suspension 

 which he adopts, and he makes out a strong case for the 



3 Y2 



