[shaw] THE KELVIN-VARLEY SLIDE POTENTIOMETER 101 



that a calibration of this kind must be repeated from time to time in 

 order to maintain the degree of accuracy in the use of the instrument. 

 The slow alterations in the coils can attain considerable magnitude 

 and ultimately render old calibrations quite useless. The changes are 

 due, usually, to an average effect rather than to especial weakness in 

 some particular coil or coils. It is thus possible to get a good indica- 

 tion of the condition of the whole box by taking and comparing read- 

 ings for several ratios measured first from one end of the slide and 

 then from the other. At the same time it is at once apparent in such 

 a test, if the variation happens to be a purely local one. For measure- 

 ments requiring an accuracy of more than one part in ten thousand 

 we should not rely upon a calibration which is older than a year. 



