394 ME. J. J. MANLEY: OBSEEVATIONS ON THE 



Possible Causes of the Observed Irregularities. 



From a study of the data available after experimenting for a considerable time with 

 one long and five short beam balances, we were led to conclude that the changes in 

 the K.P. were, although small, very real ; and that they were probably due to one or 

 more of the following causes : 



(1) A slipping to and fro of the central knife-edge upon its plane whilst the beam 

 is executing vibrations having a maximum amplitude. 



(2) A slight but continued differential flexure in the two arms lasting for some 

 minutes after the release of the beam. 



(3) A very small lateral displacement of one or more of the knife-edges, due to the 

 yielding of the screws or blocks which hold them in position. 



(4) Unequal lengthening of the two arms by contact with the warm air currents 

 ascending from the experimenter's hand during the process of loading the pans. 



We proceed to consider each of these causes in detail. 



(l) Side-slip. 



That the changes observed in the R.P. were at least partly due to the central 

 knife-edge slipping to and fro upon its supporting plane appeared to be rendered still 

 more probable by a discovery made during an examination of the Oertling balance. 

 This particular balance has been in use for nearly forty years ; it is still very sensitive 

 and highly satisfactory. A careful search revealed the existence of a fine rectilinear 

 groove cut along the middle of the central plane where it comes into contact with 

 the knife-edge. Oft repeated side-slip would easily account for the formation of such 

 a groove. 



Admitting the possibility of side-slip, we may determine what effects, if any, this 

 would have upon the value obtained for the II. P. 



In the case of strictly uniform side-slip, purely theoretical considerations show that 

 its effect would merely be to increase the amplitude of the vibrations of the pointer 

 without in any way changing the value for the R.P. 



With regard to the possible existence of non-uniform side-slip, it is, we think, only 

 necessary to here state that certain experiments based upon an optical method were 

 conducted with a Bunge beam; and that the results obtained conclusively proved 

 that if any side-slip whatever took place its magnitude was something less than 

 0-0001 mm. So small a value is for our purposes quite insignificant, and need not be 

 further considered. 



We are, therefore, justified in concluding that the beam, even when executing 

 vibrations of exceptionally large amplitude, is practically free from side-slip, whether 

 regular or otherwise. It likewise follows that the relatively large undulatory 

 movement of the R.P. must be due to some other cause. 



