xliv INTRODUCTION 



"Some writers * on the theory of electricity prefer to use what is called a Gaus- 

 sian system, a combination of electrostatic units for purely electrical quantities 

 and electromagnetic units for magnetic quantities. There are two such Gaus- 

 sian systems in vogue, — one a combination of c.g.s. electrostatic and c.g.s elec- 

 tromagnetic systems, and the other a combination of the two corresponding 

 Heaviside systems. 



"When a Gaussian system is used, caution is necessary when an equation 

 contains both electric and magnetic quantities. A factor expressing the ratio 

 between the electrostatic and electromagnetic units of one of the quantities 

 has to be introduced. This factor is the first or second power of c, the number 

 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 i that \_i/ Kfi] 

 = [L 2 /^ 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 fx were ex- 

 pressed in the same system of units. Maxwell proved theoretically that i/V K^ 

 is the velocity of any electromagnetic wave. This was subsequently proved 

 experimentally. When a Gaussian system is used, this equation becomes c/\/ K/a 

 = v. For the ether K = i in electrostatic units and p, = i in electromagnetic 

 units. Hence c = v for the ether, or the velocity of an electromagnetic wave in 

 the ether is equal to the ratio of the c.g.s. electromagnetic to the c.g.s. electro- 

 static unit of electric charge. This constant c is of primary importance in elec- 

 trical theory. Its most probable value is 2.9979 x io 10 centimeters per second. 



" Practical " Electromagnetic System. — This electromagnetic system is 

 based upon the units of io 9 cm, io~ u gram, the sec. and /j. of the ether. It is 

 never used as a complete system of units but is of interest as the historical basis 

 of the present International System. The principal quantities are the resistance 

 unit, the ohm = io 9 c.g.s. units; the current unit, the ampere = io -1 c.g.s. units; 

 and the electromotive force unit, the volt = io 8 c.g.s. units. 



The International Electric Units. — The units used in practical measurements, 

 however, are the "International Units." They were derived from the "practical" 

 system just described, or as the latter is sometimes called, the "absolute" sys- 

 tem. These international units are based upon certain concrete standards pres- 

 ently to be 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. 



1 For example, A. G. Webster, "Theory of Electricity and Magnetism," 1897; J. H. Jeans, 

 " Electricity and Magnetism," 191 1; H. A. Lorentz, "The Theory of Electrons," 1909; and 

 O. W. Richardson, "The Electron Theory of Matter," 1914. 



