596 ELECTRICAL MEASUREMENTS 



in direct-current instruments of the moving-coil type and in 

 wattmeters over the larger part of the scale. Alternating- 

 current ammeters and voltmeters have scales on which the 

 graduations are crowded together at the lower end and possibly 

 the upper end also, so this precision of reading may be ob- 

 tained between 25 per cent and about 90 per cent of the full- 

 scale reading. 



Under commercial conditions, one must expect irregular fluc- 

 tuations in the readings. If the fluctuations are not too great, 

 the readings may be averaged mentally, but it is generally best 

 to record a series of readings taken at regular intervals and 

 calculate the average. In industrial testing, it is surprising how 

 closely the averages from various runs made under the same 

 general conditions will check one another, even though there are 

 great momentary fluctuations. 



In selecting instruments for industrial testing, attention must 

 be given to the damping (often very defective in alternating- 

 current instruments), otherwise the swinging of the pointer in 

 its own natural period will be superposed on the deflection due 

 to the load, and will render it quite impossible to obtain the true 

 reading. If several instruments have to be read simultaneously, 

 their times of vibration and damping should be such that they 

 will keep step as the load varies. 



MECHANICAL ERRORS 



Friction. The effects of friction at the jewels and pivots should 

 be reduced to a minimum. This means that the construction 

 must be of the best and that the ratio of torque to total weight 

 of the moving parts must be high. Various writers assign values 

 ranging from 3/ to % for the ratio, 



Torque in gram-centimeters, at full-scale deflection 

 Weight of moving element in grams 



Before taking any instrument from the laboratory for use on 

 a test, one should satisfy himself as to the pivot friction and free- 

 dom of motion of its movable element by putting it in circuit 

 and slowly carrying the reading over the whole scale, stopping 

 at several points. If there is undue friction, it will be made 



