Vol. 6, 1920 
PHYSICS: L. PAGE 
119 
A short calculation gives 
dc 1 
Therefore, 
(f X c) X c. 
dt 
E = -^|c+I (f Xc)Xc|. (3) 
So far no use has been made of the relativity transformations. In 
order to get the electric intensity due to a moving charge, recourse must 
be had to these transformations, resulting in the following expression, 
^— V + {f X (c— v)} X c|, (4) 
where 
('-?)' 
From the definition of magnetic intensity, it follows that 
H 
0-7) 
^A-"-^ + ~cX({fX{c-v) \ Xc)l.(5; 
For fields specified by these expressions, it is easily shown that 
V H = 0, (6) 
and 
V X E = — - H. (7) 
c 
These are the two remaining field equations, the second being the mathe- 
matical expression of Faraday's law of current induction for the case 
where a current is induced by varying the magnetic flux through a sta- 
tionary circuit. As these laws are linear in E and H, they hold as well 
for the resultant of the simple fields due to a number of charged particles 
as for each of these fields individually. 
The last of the electromagnetic' equations gives the force on a charge 
moving through electric and magnetic fields. Let, unprimed letters in 
figure 4 refer to quantities as measured in a so-called stationary system, 
and primed letters to the same quantities as measured in a system moving 
to the right with velocity v. On account of the dependence of time on 
position required by the relativity transformations, the line of force 
PE in the stationary system has the different direction PE' in the moving 
