480 ELEMENTS OF ELECTRICAL ENGINEERING. 



where k is a constant. Now, in a direct-reading instrument each scale division corre- 

 sponds to a certain constant fractional part of an ampere or volt which may be repre- 

 sented by dx. Then the length of a scale division of such a direct-reading instrument 

 expressed in degrees is given by the value of dy, which according to the above 

 equation is proportional to x, which is the reading in amperes or volts of the direct- 

 reading instrument. 



Problems 129, 130, and 131 touch upon a matter of very great practical impor- 

 tance and two simple examples covering the entire ground may be worth while. A 

 standardized direct current ammeter (or voltmeter) which indicates 100 amperes (or 

 volts) with a certain degree of precision will indicate 50 amperes (or volts) with the 

 same actual error or with twice the percentage error. A standardized alternating cur- 

 rent ammeter (or voltmeter) which indicates 100 amperes (or volts) with a certain 

 degree of precision will indicate 50 amperes (or volts) with twice as large an actual 

 error or with four times as large a percentage error. 



132. A wattmeter is connected as shown in Fig. 121, Chapter 

 VII. The resistance of the coil B is 5 ohms. The wattmeter 

 indicates 55 watts when used to measure the power delivered to 

 a lamp which takes one ampere of current. What is the true 

 power delivered to the lamp ? Ans. 50 watts. 



133. A wattmeter is connected as shown in Fig. 122, Chapter 

 VII. The resistance of the circuit AR is 1,200 ohms. The 

 wattmeter indicates 60 watts when used to measure the power 

 delivered to a iio-volt glow lamp. What is the true power 

 delivered to the lamp ? Ans. 49.92 watts. 



134. A Thomson watt-hour meter without a starting coil starts 

 on a 7 5 -watt load. The meter is adjusted to give a true watt- 

 hour record when run on a 5OO-watt load. What will the instru- 

 ment indicate after running for 4 hours on a constant load of 200 

 watts, running friction being assumed to be equal to half of start- 

 ing friction. See note to problem 136. Ans. 702.8 watt-hours. 



135. The watt-hour meter specified in problem 134 is provided 

 with a starting coil so as to start, on I lo-volt mains, when the 

 power delivered to the receiving circuit is 40 watts. At what 

 load will the meter start on 5 5 -volt mains ? See note to problem 

 136. Ans. 66.25 watts. 



136. The watt-hour meter of problem 135 is adjusted to record 

 a 5oo-watt load correctly on i lo-volt mains. At what load will 

 it record correctly on 55-volt main? Ans. 5,750 watts. 



