Secondary Cathode Rays from gases. 543 
A comparatively small difference of potential, say 4 or 5 volts, 
was sufficient to produce a marked diminution in the brightness 
of the luminosity just above the gauze without destroying it 
altogether, the appearance showed that the particles emitted 
under the action of the primary rays did not all pass through 
the top of the box with the same velocity : the measurements 
made had for their object the determination of the maximum 
velocity with which the secondary cathode rays were emitted, or 
rather the maximum velocity possessed by a number sufficiently 
large to produce an appreciable effect upon the luminosity. The 
method adopted was as follows: between one half, D, of the gauze 
and the box a large potential difference was established, so large 
that it stopped all the secondary cathode particles before they 
reached the gauze; that it was sufficient to do this was ascertained 
by still further increasing the potential and observing that there 
was no further diminution in the luminosity below the gauze: 
there was always a just perceptible luminosity in this region due 
to the light from the discharge in the tube coming through the 
window at the top of the box. The potential between E, the other 
half of the gauze, and the box, which at the beginning of the 
experiment was the same as that between D and the box, was 
gradually diminished until the luminosity below the gauze E was 
appreciably greater than that below D, indicating that some 
particles were projected with enough energy to surmount the 
potential difference between the box and E. To facilitate the 
determination of the point where the increase of luminosity 
began, a switch was inserted so that the potentials of D and E 
could rapidly be interchanged ; it was found easier to be sure of 
difference of luminosity when the light and dark parts could 
be rapidly interchanged than when they kept always to one 
side. The method has also the advantage of eliminating the 
effect of any irregularities in the distribution of light over the 
window at the top of the box. 
The results of the experiments with the lime covered cathode 
are given in the following table; the first column contains the 
potential difference in volts between the lime covered cathode and 
the anode in the discharge tube, the second the greatest potential 
difference between the gauze and the walls of the box which 
allowed any luminosity to penetrate below the gauze. This 
potential difference multiplied by the charge on a corpuscle 
gives the greatest kinetic energy with which the secondary 
cathode rays are projected from the molecules of the gas when 
struck by the primary cathode rays emitted by the lime cathode, 
or more strictly the greatest kinetic energy of the secondary rays 
which are emitted in sufficient numbers to produce appreciable 
luminosity in the gas through which they pass. 
