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The Cr»K)kcs dark space (or catluxle dark spare, or Mitlorf dark 

 space as it is called in (ierinaiu) extends from the cath<Mle to the 

 boundary of the bright luminous cloud which is the ncRatise glow. 

 The boundary is generally so well-defined and distinct that an observer 

 finds it easy to juilgc when a soundinj;-wirc just touches it, or the 

 cross-hair of a telescope coincides with its image; "in the case of oxy,-- 

 gen," Aston s;ud, "the sharpness was simply amazing; even with so 

 large a dark space as 3 cm., the sighter coukl be set (to the boundary) 

 as accurately as to the cathode itself, i.e., to about 0.01 mm." I re- 

 produce some of Aston's i)hotograplis in Figure 8, although ho says 

 that for reasons of perspective the boimdary of the negative glow ap- 

 pears more diffuse than it really is." The electric field strength within 

 the CrcMikes dark space is greater, often very much greater, than in 

 any of the other di\'isions of the discharge; almost the whole of the 

 voltage-rise from cathode to ancnlc is comprised within it, and the re- 

 mainder, although spread across all the brilliant parts of the glow, is 

 inconsiderable unless the tube is made unusually long. The behavior 

 of the dark space when the current through the tube is varied (by 

 varjing a resistance in series with the tube) is curious and instruc- 

 tive. If the current is small and the cathode large (a wide metal 

 plate) the negative glow overarches a small portion of the cathode 

 surface, lying above it like a canopy with the thin dark sheath be- 

 neath it. When the current is increased the canopy spreads out, 

 keeping its distance from the metal surface unaltered, but increasing 

 its area proportionally to the current; the thickness of the Crookes 

 dark space and the current-density a'cross it remain unchanged. If 

 the experimenter continues to increase the current after the cathode 

 is completely overhung by the glow, the dark space thickens steadily, 

 and the current-density across it rises. 



The changes in the voltage across the Crookes dark space which 

 accompany these changes in area and thickness are very important. 

 The voltage is measured with a souniling-wire, like the cathode-fall 

 in thq arc; but since the boundary of the dark space is so sharply 

 marked, the experimenter can set the sounding-wire accurately to it 

 instead of merely as close as pnjssible to the cathode. So long as the 



"Adjacent to the cathode a thin perfectly dark stratum can be distinguished 

 (especially in the picture on the right l. The P.D. across this thin black space is, as 

 nearly as it can be guessed from the width of the space, of alK)Ut the magnitude of 

 the ionizing-potential of the gas. In fact .-Xston estiniatc<l it for helium (to which 

 the pictures refer) as 30 volts, a good anticipation of the value 24.5 assigned years 

 later to the ionizing potential. It seems therefore that the outer edge of the very 

 dark space is at the level where the electrons coming from the cathode first acquire 

 energy enough to ionize. 



