Jtine 19, 1879] 



NATURE 



175 



Weit Middlesex Water Company, the head of water being : attached, through a cock, to a four-way -junction-piece F, 

 106 feet ; it produces a vacuum to within half-an-inch | provided with three more co:ks, communicating : — one to 

 (0-47 in. = 13 millims.) of the height of the barometer, one end of the tube T, one to the last drying bottle of the 

 The pipe leading to it is so marked in the drawing ; it is | gas generator G g', and one to a mercurial gauge. The 



2200 a — h Volumes 



1^5 



Fig. ^. 



Other end of the vacuum tube T communicates by 

 means of a Y-piece to both, an Alvergniat mercurial 

 pump, on the right of the figure, and a Sprengel 

 pump, on the left. After the trompe has done its 

 work, the Alvergniat is used for rapid exhaustion, and 

 then shut off by means of the glass cock C, leaving 

 the exhaustion to be completed by the Sprengel ; we 

 have thus obtained, by the pumps alone, in tubes 32 

 inches long and 2 inches in diameter, vacua of only 0002 

 millimetre pressure, equal to 2'6 millionths of an atmo- 



sphere — a vacuum so perfect that the current of 8040 cells 

 would not pass. The apparatus is in connection with a 

 McLeod gauge, by means of which pressures to oooooj 

 mm. can be determined. Besides this gauge, the 

 Sprengel and Alvergniat pumps have their own gauges, 

 which read to a millimetre. M is a rotating mirror con- 

 sisting of a four-sided prism mounted on a horizontal axis 

 and provided with a multiplying wheel ; on each face of 

 the prism is fastened a piece of looking-glass. The reflec- 

 tion of the tube in the mirror enables one to examine 



Fig. 5. 



whether an apparently nebulous discharge is simply 

 nebulous or consists really of stra.a, also whether and in 

 what direction there is a flow of s lata which may appear 

 quite steady to the eye. The observations are facilitated 

 by covering the tube with a half cylinder of cardboard 

 having a slit in the direction of its axis about -^(j inch 

 wide. R is a radiometer attached to the Sprengel ; 

 li, d, a drying tube containing sticks of potash used 

 when gas is introduced from a reservoir through the 

 Alvergniat. 

 The resistance of vacuum tubes does not depend solely 



or mainly on the distance between the terminal, but it 

 does greatly on their diameter. 



In order to test how much of this depends on the 

 length of any constriction, we had made two tubes, 154 

 and 155, Fig. 6, of nearly the same length (16 inches), and 

 internal diameter JJths of an inch, the residual gas in each 

 case being carbonic acid, COj. From results obtained 

 with these tubes where the constricture varied in length 

 in the ratio of 125 to i, it became evident that the main 

 effect is due to the simple constricture of the tube. 



The diagram Fig. 7, shows the arrangement by which. 



