CHAPTER XIII 

 THE CALIBRATION OF INSTRUMENTS 



This chapter deals chiefly with indicating electrical instruments 

 intended for use in engineering work. 



Accuracy and Precision. Every experimenter must form his 

 own estimate of the accuracy, or approach to the absolute truth, 

 obtained by the use of his instruments and processes of measure- 

 ment. He must remember that a high precision, or agreement of 

 the results among themselves, is no indication that the quantity 

 under measurement has been accurately determined. For 

 instance, under favorable conditions one may be certain of the 

 reading of a voltmeter (at about 100 volts) to Ko of 1 per cent. 

 At the same time the instrument may be several per cent, in 

 error, and the true value of the measured P.D. will remain 

 unknown until the instrument has been calibrated. 



Generally speaking, in any piece of experimental work one 

 should aim at as high a degree of accuracy in the final result as 

 can be attained without undue labor and expense. This conduces 

 to discipline among the observers, as it discourages slipshod 

 methods of observation. It also aids in the collection of 

 reliable engineering data for use on other occasions. But it is 

 to be remembered that the expense and the labor increase very 

 rapidly as the required degree of accuracy is raised. Conse- 

 quently the determination of what the required degree of ac- 

 curacy shall be, becomes an economic problem. 



Clear ideas concerning the distinction between accuracy and 

 precision are especially important to those beginning experimental 

 work. 



During the progress of many tests the obvious thing, and there- 

 fore the factor on which the beginner's attention is likely to be 

 fixed, is the uncertainty in determining the best representative 

 values of the readings of his instruments, for frequently the cir- 

 cuit conditions are fluctuating so that close attention in reading 

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