erent spectra.

The structure of the discharge is much more easily studied when the
pressure of the gas is low, as the various parts which make up the
discharge are more widely separated from each other. We have already
described the general appearance of the discharge through gases at low
pressures (see p. 657). There is, however, one form of discharge which
is so striking and beautiful that it deserves more detailed
consideration. In this type of discharge, known as the striated
discharge, the positive column is made up of alternate bright and dark
patches known as _striations_. Some of these are represented in fig. 17,
which is taken from a paper by De la Rue and Muller (_Phil. Trans._,
1878, Pt. 1). This type of discharge only occurs when the current and
the pressure of the gas are between certain limits. It is most
beautifully shown when a Wehnelt cathode is used and the current is
produced by storage cells, as this allows us to use large currents and
to maintain a steady potential difference between the electrodes. The
striations are in consequence very bright and steady. The facts which
have been established about these striations are as follows: The
distance between the bright parts of the striations is greater at low
pressures than at high; it depends also upon the diameter of the tube,
increasing as the diameter of the tube increases. If the discharge tube
is wide at one place and narrow in another the striations will be
closer together in the narrow parts than in the wide. The distance
between the striations depends on the current through the tube. The
relation is not a very simple one, as an increase of current sometimes
increases while under other circumstances it decreases the distance
between the striations (see Willows, _Proc. Camb. Phil. Soc._ 10, p.
302). The electric force is not uniform along the striated discharge,
but is greater in the bright than in the dark parts of the striation. An
example is shown in fig. 16, due to H. A. Wilson, which shows the
distribution of electric force at every place in a striated discharge.
In experiments made by J. J. Thomson (_Phil. Mag._, Oct. 1909), using a
Wehnelt cathode, the variations in the electric force were more
pronounced than those shown in fig. 16. The electric force in this case
changed so greatly that it actually became negative just on the cathode
side of the bright part of the striation. Just inside the striation on
the anode side it rose to a very h