332 M. I. Pupin—Studies in the Electro-magnetic Theory. 
together, constitute the physical meaning of the perfect sim- 
ilarity in form between the two fundamental laws of Maxwell’s 
electro-magnetic theory.* Rowland’s classical experiment 
which proved the existence of a magnetic field produced by 
a moving electrostatic charge, and Rontgen’s detection of the 
magnetic effect of a varying dielectric polarization, may be 
considered as the only direct experimental evidences ever 
obtained. , 
2. Second part of Maxwell's theory. 
A search for a source of indirect experimental evidences 
leads to the second and most interesting, but at the same time 
most difficult, part of Maxwell’s electro-magnetic theory. Zhe 
principal feature of this part is the second characteristic fea- 
ture of this theory. Lt is the importance of the function which 
as performed by the physical constants of the field, namely, by 
the specific inductive capacity, the magnetic permeability, and 
the resistwity. 
These constants Maxwell defines by the law of flux, which 
in the well known notation is expressed as follows: 
ac Cline gh a cae 
LT ag A aR agi 
y yu iu for isotropic 
© geal eee ae Picton d 
xX Ww! Z | 
C= aye ar ee J 
t for anisotropic 
| media. 
Wo po : 
cA Saal J 
It should be observed now that the first part of this law, 
referring to isotropic media, is a statement of experimental facts 
as long as we confine ourselves to constant and slowly-varying 
forces ; itis an hypothesist in every other case. The second part, 
referring to anisotropic media, was a pure hypothesis at the time 
when Maxwell first stated it. He justified it by pointing out 
the close connection between the optical and the electro-mag- 
netic constants. Hence just as double refraction of light in a 
substance proves that the optical constants of the substance 
are different in different directions, so the double refraction of 
* Lodge, Phil. Mag., June, 1889. 
+ See Goldhammer, Wied. Ann. xlvi, p. 99, 1892. 
