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EFFEC T A T LO W PRESSURES 19

entirely changes. At these low pressures It Is exceedingly
difficult to get the discharge to pass through tubes of
moderate size when the cathodes are made of aluminium or
any of the metals ordinarily used for this purpose, and there Is
great danger of sparks passing through the glass and breaking
the tube. This can be avoided to a great extent by facing
the cathode with a thin layer of calcium, or smearing the face
of the cathode with the liquid alloy of sodium and potassium.
This reduces considerably the difficulty of getting the dis-
charge to pass and diminishes the risk of perforating the tube.
The appearance at these low pressures when hydrogen or air
is In the tube Is shown In Fig. 9. It will be noticed that the
straight bands of phosphorescence have
almost disappeared and that most of
phosphorescent light Is concentrated Into
two parabolic curves which are connected
with the undeflected spot by straight faintly
luminous lines. The value of ejm for one
parabola is io⁴, that for the other 5 x io³ *

so that they are due to the atom and _FlG< _g

molecule of - hydrogen respectively. At
these low pressures the luminosity In the negative direction
disappears. But both at the low and higher pressure there
is, even when the magnetic and electric fields are in action,
an appreciable amount of luminosity at the position occupied
by the undeflected spot

There is considerable advantage in using very large glass
vessels for the discharge tubes when studying positive rays;
with large vessels the pressure can be made very small before
the tube offers great resistance to the passage of the discharge
through It The increase in the difficulty of getting the
discharge to pass comes in at the pressure when the dark
space round the cathode reaches the walls of the tube. When



	*EFFEC* T A T LO W PRESSURES 19 entirely changes. At these low pressures It Is exceedingly difficult to get the discharge to pass through tubes of moderate size when the cathodes are made of aluminium or any of the metals ordinarily used for this purpose, and there Is great danger of sparks passing through the glass and breaking the tube. This can be avoided to a great extent by facing the cathode with a thin layer of calcium, or smearing the face of the cathode with the liquid alloy of sodium and potassium. This reduces considerably the difficulty of getting the dis- charge to pass and diminishes the risk of perforating the tube. The appearance at these low pressures when hydrogen or air is In the tube Is shown In Fig. 9. It will be noticed that the straight bands of phosphorescence have almost disappeared and that most of phosphorescent light Is concentrated Into two parabolic curves which are connected with the undeflected spot by straight faintly luminous lines. The value of ejm for one parabola is io⁴, that for the other 5 x io³ * so that they are due to the atom and _FlG< _g molecule of - hydrogen respectively. At these low pressures the luminosity In the negative direction disappears. But both at the low and higher pressure there is, even when the magnetic and electric fields are in action, an appreciable amount of luminosity at the position occupied by the undeflected spot There is considerable advantage in using very large glass vessels for the discharge tubes when studying positive rays; with large vessels the pressure can be made very small before the tube offers great resistance to the passage of the discharge through It The increase in the difficulty of getting the discharge to pass comes in at the pressure when the dark space round the cathode reaches the walls of the tube. When