xliv INTRODUCTION. 



difference which exists between the terminals of a resistance of one international 

 ohm when the latter carries a current of one absolute ampere. The emf of the 

 Weston normal cell may be taken as 1.01821 semi-absolute volts at 20 C. 



QUANTITY OF ELECTRICITY. 



The international unit of quantity of electricity is the coulomb. The faraday 

 is the quantity of electricity necessary to liberate i gram equivalent in electroly- 

 sis. It is equivalent to 96,500 coulombs. 



Standards. There are no standards of electric quantity. The silver voltam- 

 eter may be used for its measurement since under ideal conditions the mass 

 of metal deposited is proportional to the amount of electricity which has flowed. 



CAPACITY. 



The unit generally used for capacity is the international microfarad or the 

 one-millionth of the international farad. Capacities are commonly measured 

 by comparison with standard capacities. The values of the standards are de- 

 termined by measurement in terms of resistance and time. The standard is 

 some form of condenser consisting of two sets of metal plates separated by a 

 dielectric. The condenser should be surrounded by a metal shield connected to 

 one set of plates rendering the capacity independent of the surroundings. An 

 ideal condenser would have a constant capacity under all circumstances, with zero 

 resistance in its leads and plates, and no absorption in the dielectric. Actual 

 condensers vary with the temperature, atmospheric pressure, and the voltage, 

 frequency, and time of charge and discharge. A well-constructed air condenser 

 with heavy metal plates and suitable insulating supports is practically free from 

 these effects and is used as a standard of capacity. 



Practically air condenser plates must be separated by i mm or more and so 

 cannot be of great capacity. The more the capacity is increased by approach- 

 ing the plates, the less the mechanical stability and the less constant the capac- 

 ity. Condensers of great capacity use solid dielectrics, preferably mica sheets 

 with conducting plates of tinfoil. At constant temperature the best mica con- 

 densers are excellent standards. The dielectric absorption is small but not quite 

 zero, so that the capacity of these standards with different methods of measure- 

 ment must be carefully determined. 



INDUCTANCE. 



The henry, the unit of self-inductance, is also the unit of mutual inductance. 

 The henry has been known as the " quadrant" and the "secohm." The length 

 of a quadrant or quarter of the earth's circumference is approximately io 9 cms. 

 and a henry is io 9 cms. of inductance. Secohm is a contraction of second and 

 ohm; the dimensions of inductance are [TR] and this unit is based on the 

 second and ohm. 



Inductance Standards. Inductance standards are measured in international 

 units in terms of resistance and time or resistance and capacity by alternate- 



