ELECTRICITY METERS 



485 



A practical difficulty has been that, in time, the mercury 

 is very likely to become contaminated and cause an increase in 

 the friction of the meter. 



The Mercury Ampere-hour Meter. Aside from special uses, 

 some of which will be referred to later, ampere-hour meters are 

 intended for use on constant-potential circuits, for if the potential 

 be kept constant their readings form as just a basis for that part of 

 the charges which are dependent on the amount of energy 

 furnished as do those of the watt-hour meter. When the ampere- 

 hour meter is so used, the register is graduated to read in kilowatt- 

 hours, the voltage of the circuit having some definite value. 



In America the watt-hour meter is now used almost exclusively 

 in lighting installations, but in Great Britain the ampere-hour 

 meter is extensively employed. 



Fio. 278. Working parts of Ferranti ampere-hour meter. 



Ferranti Ampere-hour Meter. The Ferranti ampere-hour 

 meter was one of the earliest forms of electricity meters, its 

 development having been begun as early as 1883. 



Fig. 278 shows the essential parts of this meter as at present 

 constructed. The motor is of the Faraday disc type. 



