1919.| The Sixth Indian Science Congress. evii 
‘The subject of this title is co-extensive with the whole 
range of the physics of imponderable agencies. For, if it is 
correct to say with Maxwell that all radiation is an electro- 
a as ; 
established by the operation, on the molecules of those bodies, 
of fields of force which are propagated in free space as radia- 
tion and in accordance with the laws of radiation from one 
body to the other.”’ 
As the simplest illustration of an electro-magnetic field, 
we may take the region in the neighbourhood of a wire carry 
ing current (as well as inside it). There is flow of electriciiy, 
, attendant 
effects, essentially associated with it. And even without know- 
ing the intimate nature of electricity, flow of electricity, mag- 
netism, and magnetic effects, we may well call this an electro- 
magnetic field. 
Another, a less simple one, would be the field due to a con- 
denser, charged in the usual way by a frictional machine or a 
voltaic cell, . in particular, the dielectric region between 
the two conducting surfaces. The sum total of effect is to stati- 
cally charge the condenser, but when the intimate nature of the 
eres is analy zed, one is constrained to admit that there 
is a “movement” across the dielectric, in the samé sense 
as there i vement in a wire carrying current. For, 
if the difference of potential between the plates is_ suffi- 
ciently increased, there is set up a measurable current with 
or without luminosity (an electric discharge, silent or lumin- 
ous). Across the dielectric, therefore, one may describe the 
effect as that of peice displacement, completing the electric 
circuit, as in an inary wire circuit. It follows, according- 
ly, that the siahis effect observed is of the nature of a convec- 
tive equilibrium, so that a charged condenser is to be regarded 
also on this view as giving rise to an electro-magnetic field. 
And if this is granted, there would be no reason for deny- 
ing that any so-called electrostatic field is also in reality an 
electro-magnetic field, even though no direct magnetic effect 
may be Npaesteegh in i 
nother illustration, we may again take a condenser, 
dain or : discharge e it, at intervals. It can be directly proved 
that the discharge is oscillatory, and the period of oscillation 
can be calculated ow, as we know, by suitable arrange- 
ments, the effect of this oscillatory discharge can be detected 
d - - 
is thus taken up by the field or the medium and induces oscil- 
tory movement, thousands of miles away, the practical'reali- 
zation of which represents the latest triumphs of wireless tele- 
graphy. 
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