1. Lord Kelvin, in discussing the origin of magnetic storms, came to the conclusion that they could not be due to a direct solar action on account of the enormous energy which would have to be supplied by the sun. This verdict was generally accepted until recently, when the theory of a direct solar action has been revived in a form, which is assumed to he free from the objection raised, the magnetic action being supposed to be due to a swarm of electrified corpuscles ejected by the sun. The question of energy has not, so far as I know, been discussed in this case, and it seems to be taken for granted that the total energy of the magnetic field due to a swarm of corpuscles is equal to the sum of the energies of each, calculated as if the others were not present. If the corpuscles are sufficiently far apart, this is approximately correct; but in that case the magnetic field itself would have to be negligible, except within molecular distance from each particle. How far we may go wrong by treating the energy as if it could be obtained by a process of addition becomes apparent when we consider that such treatment would lead to coefficients of self-induction which are proportional to the length of a circuit and independent of its shape. 2. If A
1
, A
2
, A
3
represent the components of electro-kinetic potential and
u
1
,
u
2
,
u
3
the current components at any point, the total electro-magnetic energy is given by T = ½ ∭(A
1
u
1
+ A
2
u
2
+ A
3
u
3
)
dx dy dz
.
The origin of magnetic storms
