gion as fast as they are sent into it from the cathode. If,
however, there is no production of ions in the region between the
negative glow and the anode, all the ions in this region will have come
from near the cathode and will be negatively charged; this negative
electrification will diminish the electric force on the cathode side of
it and thus tend to stop the discharge. This back electric field could,
however, be prevented by a little ionization in the region between the
anode and glow, for this would afford a supply of positive ions, and
thus afford an opportunity for the gas in this region to have in it as
many positive as negative ions; in this case it would not give rise to
any back electromotive force. The ionization which produces these
positive ions may, if the field is intense, be due to the collisions of
corpuscles, or it may be due to radiation analogous to ultra-violet, or
soft Rontgen rays, which have been shown by experiment to accompany the
discharge. Thus in the most simple conditions for discharge we should
have sufficient ionization to keep up the supply of positive ions, and
an electric field strong enough to keep the velocity of the negative
corpuscle equal to the value it has when it emerges from the negative
glow. Thus the force must be such as to give a constant velocity to the
corpuscle, and since the force required to move an ion with a given
velocity is proportional to the pressure, this force will be
proportional to the pressure of the gas. Let us call this force ap; then
if l is the distance of the anode from the negative glow the potential
difference between these points will be alp. The potential difference
between the negative glow and the cathode is constant and equals c;
hence if V is the potential difference between the anode and cathode,
then V = c + alp, a relation which expresses the connexion between the
potential difference and spark length for spark lengths greater than the
critical distance. It is to be remembered that the result we have
obtained applies only to such a case as that indicated above, where the
electric force is constant along the positive column. Experiments with
the discharge through gases at low pressure show the discharge may take
other forms. Thus the positive column may be striated when the force
along it is no longer uniform, or the positive column may be absent;
the discharge may be changed from one of these forms to another by
altering the current. The re