ANOMALOUS BEHAVIOUR OF DELICATE BALANCES, ETC. 399 



At the conclusion of these observations the beam was fatigued for some considerable 

 time ; a second series of observations was then taken, and the successive values for the 

 E.P., shown in curve ft (Fig. III.), deduced. The second curve shows that the beam 

 had been reduced to an almost perfectly normal condition for the load (10 gr.) used. 

 With this particular load, and with this alone, similar results could be obtained at 

 any time, provided a sufficient period had been allowed for the beam to recover from 

 the strains set up during any preceding experiment. 



The slight upward trend of the curve ft is due to a small change which took place 

 in the temperature (vide infra) during the observations. 



The load which is just sufficient to produce the changes which we have here 

 discussed, may be regarded as a critical had. We think it is highly probable that 

 for many balances of certain types there are similar critical loads. In refined 

 weighing this is a point of some importance, and calls for careful attention on the part 

 of the observer. 



During the investigation it was observed that some three or four different modes of 

 securing the knife-edges to the beam have been adopted by various makers. Three 

 of these are outlined in Fig. IV. Balances having their terminal knife-edges secured 



Fig Z7 



as in a appeared to be the more reliable. In this case the knife-edge is formed along 

 the upper extremity of an agate plate, which is sunk and cemented into a brass block. 

 This block is fastened to the beam by screws which enter from beneath ; the tips also 

 of the horizontal screws bite into and so firmly grip the block. 



The curves shown in Fig. III. were drawn from data obtained when experimenting 

 with a balance the terminal knife-edges of which were secured to the beam after the 

 manner indicated in b. 



This method is, in our opinion, faulty and objectionable. The solid agate prism is 

 simply held in position by adjusting screws which can be brought to bear upon but 

 cannot possibly grip so hard a substance as agate. As no screws enter the knife-edge 

 prism, the prism cannot seriously be regarded as forming a definite and incorporated 

 portion of the beam itself. 



We observe, further, that the direction of the upper adjusting screws is incorrect. 

 When the beam assumes an inclined position during weighing, there will be a 

 tendency on the part of the loaded pans to thrust the two terminal knife-edges in one 

 direction along the inclined plane. The slender and horizontally placed adjusting 

 screws will be unable to completely resist the wedge-like action of the agate prisms ; 

 the screws will be bent upward and, consequently, one arm will be shortened and the 



