CHAPTER IV 

 MEASUREMENT OF CURRENT AND E.M.F. 



1. It is required to measure 5542 amperes by means of a resist- 

 ance of 5 X 10~ 6 ohms and a voltmeter; draw a diagram of the 

 connections, and show what would be the reading of the voltmeter 

 and what power would be lost in the shunt. Why not use a 0.001- 

 ohm resistance? (5 min.) 



2. Five lamps having resistances of 193, 203, 207, 197 and 200 

 are put in series across a railway circuit, and a 150- volt voltmeter 

 of infinite resistance is connected around the 200-ohm lamp; it 

 reads 97 volts; what is the pressure on the circuit? What would 

 be the reading for this pressure if the resistance of the voltmeter 

 were only 500? How low a resistance in the voltmeter could be 

 left out of the calculation for an accuracy of \ per cent? (17 min.) 



3. It is desired to use a 50-millivolt meter as a 200-ampere 

 meter on a 500-volt circuit. What resistance must the shunt 

 have? Same for a 0.2-ampere meter. What will be the voltage 

 on the instrument if the shunt is opened between the millivolt- 

 meter terminals? (3 min.) 



4. How low could the resistance of the millivoltmeter used in 

 each case of problem 3 be, without introducing an error of more 

 than T V per cent? (6 min.) 



5. A 150-volt meter having a resistance of 16,000 ohms is to 

 be used with a multiplier as a 600-volt instrument. What resist- 

 ance must the multiplier have? (3 min.) 



6. If the readings on a 150-volt meter can be estimated accu- 

 rately to 0.2 of a division, what per cent of accuracy can be obtained 

 at 1.5, 15 and 150 volts respectively? (2 min.) 



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