when considering the spark discharge given a general descrip- tion of some of them; there are, however, some features which require further discussion.
Starting from the cathode we find a thin layer of luminous gas, the colour of which depends on the kind of gas through which the discharge is passing. In most gases the light appears to reach right up to the cathode, but in helium Aston has shown it is separated from it by an exceedingly thin dark space. This luminous layer is sometimes called " Goldstein's first layer "; next to this we have a region where there is comparatively little luminosity called " Crookes' dark space," the boundary of this space being approximately the surface traced out by normals to the surface of the cathode of con- stant length. The thickness of the dark space, which is of the order of the critical spark length, depends upon the pressure of the gas, varying approximately as the reciprocal of the pressure for air; at the pressure of I mm. of mercury the thickness of the dark space is about 2 mm., so that at atmospheric pressure the thickness would not be much more than about 1/400 of a millimetre. If the pres- sure remains constant and the current through the tube is increased, the thickness of the dark space remains unaltered until the current is large enough to cover the whole of the cathode with the lumi- nous glow ; after this stage is reached any further increase in the cur- rent causes a diminution in the thickness of the dark space. Start- ing from the boundary of the dark space there is a brightly lumi- nous region called " the negative glow." The function of the parts of the discharge from the cathode to the negative glow is to pro- duce the supply of electrons from the neighbourhood of the cathode necessary to keep the discharge going. The dimensions of this part of the discharge are independent of the distance between the cathode and anode; at very low pressures this part may occupy a length of several centimetres, but at atmospheric pressure they are crowded into a very small fraction of a millimetre and as far as length goes occupy a negligible portion of the sparks at such pressures. The Crookes' dark space, though it appears dark in contrast to the nega- tive glow, is not devoid of luminosity; indeed Seeliger, who has made a spectroscopic examination of the dark space, finds that there are some lines, such as the Balmer series lines, which are almost as bright in the dark space as in the negative glow. But many lines are much stronger in the negative glow than in the dark space.
Beyond the negative glow there is another comparatively dark region called the " Faraday dark space "; the length of this is very variable even when the pressure is constant, as it is sensitive to any change in current. Beyond this and reaching right up to the anode is a luminous column, called the positive column. The luminosity in some cases is fairly uniform in intensity, but when the pressure and current are between certain limits this column may exhibit remarkable alternations of dark and bright spaces called striations, such as are shown in fig. 13. Under some circumstances a dark space round the anode has been detected by several observers.
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When the distance between the electrodes is considerable and the pressure not very low, the positive column forms by far the greater part of the discharge ; thus at atmospheric pressures all but a frac- tion of a millimetre of the discharge next the cathode will consist of the positive column.
Distribution of the Electric Force along the Discharge. The electric force is very large indeed in the part of the dark space next the cathode, but diminishes rapidly towards the negative glow. In the negative glow itself it is smaller than in any other part of the discharge; passing the negative glow, the electric force increases in the Faraday dark space, until the positive column is reached. When the positive column is of uniform luminosity the electric force in the column is constant until quite close to the anode, when there is an abrupt change of potential of about 20 volts, called the anode fall of potential. When the positive column is striated, the alternations of luminosity in the positive column are accompanied by alternations in the intensity of the electric force, the maxima of the electric force occurring at the bright parts of the striae, the minima at the dark. From the equation
/7V
j- = 4Jrp, where X is the electric force in the direction of x and
p the density of the electrification, we see that there is an excess of positive electricity in the cathode dark space and of negative in the Faraday dark space; in a uniform positive column
Gas
Pt
Hg
Ag
Cu
Sc
In
Al
Mg
Na
Na-K
K
O 2
^60
H 2
N 2 He
300 232 >->fi
226
295
280
230
213
190
1 68
207
185 178 80
169
125
78-s
172 170 60
Arg Cl
167 705
IOO
Br
^ss
I
377
there is no appreciable excess of electricity of one sign over that
of the opposite, while in the striated positive column there is an
excess of negative electricity on the cathode side of a bright
part of a striation and of positive on the anode side.
Cathode Fall of Potential. Until the glow next the cathode covers the whole of the electrode the difference of potential be- tween the cathode and the negative glow is constant, depending on the gas and the material of which the cathode is made, but being independent of the pressure of the gas and the strength of the current. This constant difference of potential is called the " cathode fall of potential," and there is evidence to show that it is equal to the minimum potential that can produce a spark through the gas. Its value, as determined by Mey for different gases and different electrodes, is given in the following table, which includes also Matthies' results for Cl2, Br2, 12:
When the current is so large that the luminous glow com- pletely covers the cathode, the potential difference between the cathode and the negative glow increases as the current increases, while the thickness of the dark space diminishes. Mr. Aston, as the result of experiments made with very large parallel plate electrodes, found the following relations between V the cathode fall of potential, i the intensity of the current, D the thickness of the dark space, and p the pressure of the gas
These relations are empirical, and must not be taken to imply that the dark space would increase indefinitely if the current were diminished without limit. Aston also found that the thick- ness of the dark space as well as the cathode fall of potential depended upon the material of which the cathode is made. If the space round the cathode is restricted so that the dark space has not room to develop (for example, if the cathode is placed in a narrow tube), then, as soon as the dark space reaches the walls of the tube, the cathode fall begins to increase, and increases very rapidly as the pressure diminishes and the thickness of the free dark space exceeds more and more the space available round the cathode. This is due to the same cause as that which makes the spark potential increase rapidly when the spark length falls below the critical value. This result is utilized to make " elec- tric valves," i.e. tubes through which a current will only pass in one direction. For if electrodes are put in a tube which is nar- row at one end and very wide at the other, the development of the negative glow will be restricted when the cathode is at the nar- row, but not when it is at the wide end of the tube: a discharge through the bulb will pass much more easily when the wide end is cathode than when it is anode, so that even if the electrodes are made alternately positive and negative the discharge through the tube will only be in one direction.
A very important question in connexion with the cathode fall of potential is whether the fall is continuous throughout the dark space or whether an appreciable fraction of it occurs abruptly at the surface of the cathode: Aston, who measured the distribu- tion of potential near a very large flat cathode, came to the con- clusion that there was no abrupt fall at the cathode. Westphal, on the other hand, found in his experiments an abrupt fall of potential quite close to the cathode amounting to 20% or more of the total cathode fall. The question is important in connexion with the mechanism of the discharge, for if the fall is so abrupt that it occurs within molecular distances the electric force on the surface of the cathode might be so great that the electrons would
