ELECTRICITY METERS 495 



differing by 0.5 volt. Therefore, when it is used on a test, it 

 is necessary only to observe the applied voltage in order to de- 

 termine the load on the meter. It is to be noted that a 1 per 

 cent, error in the voltage reading will cause a 2 per cent, error 

 in the watts. 



The resistor in the box, shown in Fig. 283, consists of four 

 units, two having a resistance of approximately 220 ohms each, 

 and two having a resistance of about 22 ohms each. The follow- 

 ing loads may be obtained: 



At 110 volts At 220 volts 



25 watts approximately 100 watts approximately 



50 1,000 " 



100 " 



250 " 



500 " 



1,000 " 



The connecting cables are included in the measurement when 

 the box is calibrated. They should be composed of a large num- 

 ber of fine wires so that they may be very flexible and in order 

 that the effect of a break in any individual wire may be small. 



When such a load box is used for routine tests, it is accompanied 

 by a calibration card which gives the watts corresponding to 

 various applied voltages. The advantage of this method of 

 testing in direct-current work is that it is necessary to provide and 

 to read only one instrument. 



Portable Standard Watt-hour Meters. Routine tests are fre- 

 quently much facilitated by using a standard watt-hour meter 

 instead of an indicating wattmeter and a stop watch. The 

 standard is a portable watt-hour meter with a special register, 

 readable to 0.01 of a revolution, which allows the number of 

 revolutions of the movable element to be read with precision. 

 This register must be so arranged that it may be promptly started 

 and stopped by the use of a push button. 



In case such an instrument is used, after having connected its 

 current coils in series and its potential coils in parallel with those 

 of the meter under test, one has only to compare the number of 

 revolutions made by the standard during a certain time with 

 the number made by the meter under test during an equal time, 

 for example, that required for a definite number of revolutions 



