(75) 
ON THE DAILY EANGE OF THE ATMOSPHEEIC POTENTIAL 
GEADIENT AT BLOEMFONTEIN, AND THE INFLUENCE 
OF DUST-STOEMS. 
By W. a. Douglas Eudge, M.A. 
It has long been known that a strong electrostatic field surrounds the 
earth, and many theories have been advanced to account for the presence 
of the electric charge. In fine weather the charge present in the atmo- 
spheric layer is positive, and there seems to be a constant current of 
positive ions moving towards the earth. According to one theory the 
earth itself behaves as though it were a negatively charged sphere and 
the potential gradient in the atmosphere surrounding it is due to a stream 
of positive ions being drawn towards it by the electrostatic attraction. If 
this is the case there must be some means by which the negative charge 
can be maintained, or else it would tend to become smaller and finally 
would be reduced to zero. That the atmosphere itself is strongly ionized 
admits of no doubt ; and if one set of ions were brought down to the 
earth more readily than the others, then of course the charge on the 
earth might be maintained. The experiments of G. T. E. Wilson 
and others have shown that the negatively charged ions are more 
readily removed from a mixture of ions, by condensation of moisture, than 
is the case with the positive ions, and thus a constant field might be 
established between the earth and the atmospheric layer surrounding it, 
provided the ionization could be maintained. During rain, hail, and snow 
storms charged masses of water are continuously rained upon the earth, 
and any charges the particles possess are naturally given up to it. The 
experiments of Simpson show, however, that whilst the amount of posi- 
tive electricity brought down by rain was 3-2 times that of the negative, 
the amount of negative brought down by snow was 3*6 that of the posi- 
tive, so that on the whole it appears possible that the annual gain of 
electrification of the earth from this source might be zero. Observations 
taken in the Orange Free State during the past three years have, however, 
shown that rain charged with negative electricity only has fallen. The 
