schlink: variance of measuring instruments 



401 



In gear trains, moreover, there is especial likelihood of the oc- 

 currence of the rhomboidal type of hysteresis loop since actual 

 discontinuities in the transference of motion from one part of 

 the train to another will follow directly from any clearance at the 

 pitch line. 



Evaluation of irregular variance. In some instruments, especi- 

 ally those characterized by poor workmanship or ill repair, 

 successive hysteresis loops may be far from concordant in their 

 shape or. magnitude on account of actual variations in the fric- 

 tion and journal displacements at any particular indication. In 

 cases of this kind, the variance may be expressed or defined by 



Fig. 3. Illustrating operation of instrumental backlash 



plotting frequency curves of the readings or error's obtained for 

 a given value (or for a series of definite values) of the same meas- 

 ured quantity repeatedly applied — one series of points being 

 plotted for increasing readings terminating in the value under 

 investigation, and another set for decreasing values terminating 

 at the same point. Such a curve may exhibit, for example, 

 probability of occurrence of any amount of variation from the 

 mean instrumental reading. A succession of such frequency 

 curves taken over various parts of the reading range will deter- 

 mine a surface enabling one to ascertain the probability of a 

 given error at any point of the reading scale, but its principal 

 utihty will be in affording a criterion as to the comparative 



