.V(».u/-: COM r.Mi'oK.uo' .inr.ixci-s ix rnvsics vi 145 



i;nnv liol fni>ii>;li; if it docs, tlic callnHlf-l'.ill MuMi-iiK- Cdll.ipsrs. and 

 till- ^;lt)\v is liaMf to turn into an arc. Mxpiilsion of t.-lfrlrons from cold 

 metals l)y ions strikini; tlu-m lias hccn separately studied, l)iil not 

 siilTicientK'. 



On the other haiul, there is good e\ idenci- ill, it the (rookis d.irk 

 space, like those tlark sheaths scooped out in the i)ositi\i' cohm;:! 



Faraday Crookes 



dark space Negative glow dark space 



2 4 6 8 10 12 14 16 18 20 

 d 



Negative 

 Uniform positive coljmn. Faraday darl< space, glow 



12 K 16 18 20 



Fi^. 9 — Sketches of the (,'li)W in rarefud nitrogen ,it two pressures (the higher below) 



with curves showing the trenri of field strength along the discharge. (Grahnni, 



Wiedemanns Annalen) 



of the mercur>- arc by intruding a negatively-charged wire, is a region of 

 predominantly positive space-charge, in which positive ions advance 

 towards the cathode in a manner controlled by some such equation 

 as (16) or (17). F"or example, (iunther-Schulze proposed (16) to de- 

 scribe the state of affairs in the Crookes dark space in the condition 

 of normal cathode-fall; that is, he assumed that the ions fail unim- 

 peded from the edge of the negative glow to the cathode surface. Xo 



