122 
units, this gives 10® as the rate of variation of displacement 
de 
The current intensity C is measured by 
where v 
de 
is the velocity of light, and for ^ we put as just found 10® 
We thus obtain C - lO'^^® nearly. This means that the total 
current through a surface of a million square kilometers is 
equal to unity. Charges such as described by Sir Wm. 
Thomson, if occurring simultaneously over a surface of 600 
miles squared, would thus be equivalent to a unit current. 
If occurring over an area about 20 per cent larger than 
Ireland, therefore, the current would be equivalent to one 
Ampere. 
Let us ask next how large the current through an area 
like that of Ireland would have to be in order to show 
itself in magnetic observations. A deflection of one minute 
of arc in the declination, if recurring periodically would, no 
doubt, show itself. This means in our latitudes a force of 
5 X 10”® C.G.S. Assuming for simplicity’s sake the surface to 
be a circular disc of radius 16 X 10®, we get 400 units of current 
or 4000 Amperes; which is 4000 times stronger than the dis- 
placement currents observed by Sir Wm. Thomson. We 
may then leave out of account the variations of electrification 
on the earth’s surface, for although the conditions are some- 
what more favourable over larger surfaces, and especially 
near the equator, the possible effects seem to me always 
to fall outside the observable limits. It is difficult to esti- 
mate the possible values of actual discharge, especially in 
the polar or equatorial regions. It must, therefore, be one 
of our first objects to find out whether the line integral of 
magnetic force does or does not vanish when taken round a 
closed curve taken on the earth’s surface. In our latitudes 
we should almost certainly find that it does vanish ; but the 
observation of Sabine near the magnetic pole tend to show 
an appreciable effect. 
