16 



of electrostatic units of electric charge in one electromagnetic unit of the same. 

 There is sometimes a question as to whether electric current is to be expressed 

 in electrostatic or electromagnetic units, since it has both electric and magnetic 

 attributes. It is usually expressed in electrostatic units in the Gaussian system. 

 It may be observed from the dimensions of K given in Table 2, part 3, that 

 [I/Kfi] = [L 2 /T 2 ] which has the dimensions of a square of a velocity. This 

 velocity was found experimentally to be equal to that of light, when K and /* 

 were expressed in the same system of units. Maxwell proved theoretically that 

 1/V Kfi is the velocity of any electromagnetic wave. This was subsequently 

 proved experimentally. When a Gaussian system is used, this equation becomes 

 c/\/ Kn = v. For the ether K=\ in electrostatic units and fi=\ in electromag- 

 netic units. Hence c — v for the ether, or the velocity of an electromagnetic 

 wave in the ether is equal to the ratio of the cgs electromagnetic to the cgs 

 electrostatic unit of electric charge. This constant c is of primary importance 

 in electrical theory. Its most probable value is 2.99776 x 10 10 centimeters per 

 second. 



Part 3. — Electrical and magnetic units 



Absolute ("practical") electromagnetic system (1948). — This electro- 

 magnetic system is based upon the units of 10° cm, 10" 11 g, the sec and fi of 

 the ether. The principal quantities are the resistance unit, the ohm=10 9 emu 

 units; the current unit, the ampere = 10 _1 emu units; and the electromotive 

 force unit, the volt= 10 8 emu units. (See Table 6.) 



The International electric units. — The units used before January 1, 

 1948, in practical electrical measurements, however, were the "International 

 Units." They were derived from the "practical" system just described, or as 

 the latter is sometimes called, the "absolute" system. These international units 

 were based upon certain concrete standards that were defined and described. 

 With such standards electrical comparisons can be more accurately and readily 

 made than could absolute measurements in terms of the fundamental units. 

 Two electric units, the international ohm and the international ampere, were 

 chosen and made as nearly equal as possible to the ohm and ampere of the 

 "practical" or "absolute" system. 10 



QUANTITY OF ELECTRICITY 



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

 quantity of electricity necessary to liberate 1 gram equivalent in electrolysis. 

 It is equivalent to 96,488 absolute coulombs (Birge). 



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 used for capacity is the microfarad or the one-millionth of the farad, 

 which is the capacity of a condenser that is charged to a potential of 1 volt by 

 1 coulomb of electricity. Capacities are commonly measured by comparison 

 with standard capacities. The values of the standards are determined by 



10 There was, however, some slight error in these values that had to be taken into 

 account for accurate work. (See Table 5.) 



SMITHSONIAN PHYSICAL TABLES 



