DISCHARGE OF ELECTRICITY. 245 



;iii oxliiiusted tube is placHMl, while a pipe Iroiii tlie outer vessel leads 

 to a mercury ])uiiip and enables us to alter tlu' pressure at will. The 

 primary coil, L, M. is wouiul round the outer tube. When the air in 

 the outer tube is at atmosi)heric })ressure, the discharge caused by tbe 

 action of the primary i)asses in the tube I'] inserted in A, B, C; but 

 when the pressure iu the outer tube is reduced until a discharge i)asses 

 through it, tlu' discharge in the inner one sto]>s; the discharge in the 

 out(n- tube has thus sliielded the inner tube from the action of the 

 primary. It the exhaustion of tiie outer tul)e is carried so tar that the 

 discliarge through it ceases, tliat in the inner tube begins again. It re 

 (piires very high exhaustion to do this, and as on account of the joints 

 it is uusafe to niaivc the vessel very liot during the ])umping, I have 

 found it impossible ro keep a vacuum good enougli to show this ettect 

 for more thau from half to tiiree (piarters of an hour; in that time suffi- 

 cient gas seems to have escaped from the sides of the vessel to make 

 the pressure too high to show this effect, and It then takes from two to 

 three hcmrs' ])unii)ing to get the tube back again into its former state. 

 An interesting feature of this experiment is that for a small range of 

 ])ressure, just greater than that at which the discharge first ai)i»ears in 

 the outer tube, there is no discharge iu either of the tubes; thus the 

 action of the primary is screened off from the inner tube, though there 

 is no luminosity visible in the outer one; this shows that a discharge 

 ecpiivalent in its effects to a current can exist in the gas without suffi- 

 cient luminosity to be visible even in a darkened room. We shall have 

 occasicHi to mention other cases in which the existence of a discharge 

 non luminous througiiout the whole of its course is rendered evident iu 

 a similar way. 



Another experiment by which the screening can be effectively shown 

 is to i)lace the primary coil inside a bell-Jar which is connected with a 

 mercury i)umi), the electrical connexions with the primary being led 

 through mercury joints. An exhausted bulb is placed inside the pri- 

 mary, the bulb being considerably smaller than the primary, so that 

 there is an air-space between the two. Before the bell-jai' is exhausted 

 the discharge passes through the bulb, but when the ])ell-jar is ex- 

 hausted sufficiently to allow of the discharge ])assing through the gas 

 outside the bulb the discharge in the bulb ceases, and the oidy dis- 

 charge is tliat oHtside. I ha\ e never Ixhmi able to exhaust the bulb 

 sufficiently well to get the discharge outside the bell-jar to cease, and 

 that in the bulb to appear again, as in the preceding experiment. In 

 this experiment, as in tiie ])receding one, there was a range of pressure 

 when neither the bulb nor the bell-Jar was luminous, showing again the 

 existence of currents in the gas which are not accompanied by any 

 appreciable luminosity. 



A curious beiiding-in of tiie discharge which takes i)lace in a stiuare 

 tube i)rovided with a bulb can, I think, Ix; ex])lained by the princii)le 

 of shielding. The discharge in the bulb does not, unless very long 



