ositive column," sometimes (as in fig. 15, a) continuous,
sometimes (as in fig. 15, b) broken up into light or dark patches called
"striations." The dimensions of the Faraday dark space and the positive
column vary greatly with the current passing through the gas and with
its pressure; sometimes one or other of them is absent. These
differences in appearances are accompanied by great difference in the
strength of the electric field. The magnitude of the electric force at
different parts of the discharge is represented in fig. 16, where the
ordinates represent the electric force at different parts of the tube,
the cathode being on the right. We see that the electric force is very
large indeed between the negative glow and the cathode, much larger than
in any other part of the tube. It is not constant in this region, but
increases as we approach the cathode. The force reaches a minimum either
in the negative glow itself or in the part of the Faraday dark space
just outside, after which it increases towards the positive column. In
the case of a uniform positive column the electric force along it is
constant until we get quite close to the anode, when a sudden change,
called the "anode fall," takes place in the potential.

[Illustration:
_Discharge in Hydrogen
Pressure 2.25 m.m. Current 0.568.10^-3 ampere_
FIG. 16.]

The difference of potential between the cathode and the negative glow is
called the "cathode potential fall" and is found to be constant for wide
variations in the pressure of the gas and the current passing through.
It increases, however, considerably when the current through the gas
exceeds a certain critical value, depending among other things on the
size of the cathode. This cathode fall of potential is shown by
experiment to be very approximately equal to the minimum potential
difference. The following table contains a comparison of the
measurements of the cathode fall of potentials in various gases made by
Warburg (_Wied. Ann._, 1887, 31, p. 545, and 1890, 40, p. 1), Capstick
(_Proc. Roy. Society_, 1898, 63, p. 356), and Strutt (_Phil. Trans._,
1900, 193, p. 377), and the measurements by Strutt of the smallest
difference of potential which will maintain a spark through these gases.

+---------+-----------------------------------------+-----------------+
| | Cathode fall in Volts. |Least potential |
| Gas. +-----------------------------+-----------+ difference