schlink: variance of measuring instruments 403 



hence the number of bearings at which backlash effects can 

 occur. 



The action of vibration in reducing instrumental variance is 

 found to accord well with the principles previously set down. 

 Owing to the minimization in the static friction, occasioned by 

 the vibration, on account of the momentary disengagement or 

 separation of coacting bearings and journals, a considerable 

 reduction of instrumental variance arising in mechanistic causes 

 will normally take place,— thus the energy required to bring the 

 parts to their theoretical equilibrium point is in a sense supplied 

 from without the sj^stem. 



Variance as a limiting factor in design and adjustment. In 

 designing and constructing instruments, consideration should 

 be given to the effect of variance errors in practically limiting the 

 sensitivity obtainable by adjustment, as well as in determining 

 the optimum interval between graduations or the smallness of 

 the units of graduation. It is suggested that the mean interval 

 of graduation of laboratory instruments should not be less than 

 five times the mean variance, while for commercial or plant 

 instruments the ratio of mean scale interva|l to mean variance 

 may be of the order of two to one. 



Inconsistencies between the values of sensitivity, variancy, 

 and smallness of the units of graduation of instruments are com- 

 mon; tachometers, for example, are often graduated to a single 

 mile per hour, while there may be a variation of reading at a 

 given rotational speed of five miles per hour or more. Similarly 

 weighing scales and balances often show variations of a full 

 graduation or more. Likewise the sensitivity may easily be 

 disadvantageously high, inducing erroneous estimates of the 

 precision of results and requiring special care in the calibration 

 and use of the instrument. 



In particular cases in fact, a low inherent sensitivity may actu- 

 ally tend to reduce the absolute amount of the variance as in pre- 

 cision balances, where the advantages gained by the use of a low 

 angular sensitivity and high magnification with the resulting 

 quick period of oscillation, are well known. 



The factors of maximum or mean inaccuracy (or accuracy), 

 sensitivity, variancy, and specific set (the amount by which the 



