494 



ELECTRICAL MEASUREMENTS 



In carrying out the test the meter should be timed for as much 

 as 60 sec. if accurate results are desired. This tends to reduce 

 the errors due to the personal equation in timing and counting 

 as well as the errors due to the stop watch. 



Great care must be exercised in the purchase and in the main- 

 tenance of the stop watches, for they are the weakest element in 

 this method of testing. A watch may keep good time but be in- 

 accurate as a stop watch. It is important that the indicating 

 hand start and stop promptly without jumping and reset to ex- 

 actly zero. The starting and stopping errors are of great im- 

 portance. In order that one may be sure that the watch is in 

 good condition, it should be tested at several points before be- 

 ginning work. 



The index hand of a stop watch moves forward by a succes- 

 sion of jumps separated by intervals during which the hand is at 

 rest, so that though the watch beats J^ sec., the hand is in 



motion only about ^f oo sec. at each 

 beat, that is, while the escapement is 

 in action. There is thus a possibility 

 of an error of nearly % sec. due to 

 the peculiar mechanism of the watch. 

 In timing for 30 sec., this might give 

 rise to an error of about two-thirds 

 of 1 per cent., in addition to all the 

 other errors due to the imperfect 

 mechanical action of the mechanism 

 and the personal equation of the 

 observer. 



Load Boxes. A calibrated load box 

 is frequently used in testing meters of 

 small size. Such an arrangement is 

 shown in Fig. 283. 



The coils should be wound non- 



FIG. 283. Load box for 

 meter testing. 



inductively, so that the box is applicable to both direct and 

 alternating-current circuits. The resistance material should have 

 a very low temperature coefficient. The switches should be of 

 such a construction that contact resistances are reduced to a 

 minimum. The box is tested in the laboratory with different 

 combinations of switches and under a series of applied voltages 



