452 Prof. Chattock and Mr. Tyndall on Pressure of the 



The difficulty that the plate could not be perforated right 

 up to the side of the discharge-tube was met by putting 

 extra holes as near the edge as possible. 



Di and D 2 are connected by the tube G ; and the gas is in- 

 troduced through G by way of a glass spiral (not shown) 

 sufficiently slender to admit of the tilting of the manometer. 



The discharging point P, of finest platinum wire cut 

 obliquely with sharp scissors, and sheathed almost to its end 

 with glass, is supported by a thin steel rod sliding in a 

 mercury-tight stuffing-box. This rod is so protected by a 

 column of mercury, C, that no part of its surface which has 

 been exposed to the air passes into the discharge-vessel when 

 the point is lowered. 



Stuffing-box and connecting tube, G, are attached to an 

 ebonite plug which closes the top of D ; the joint with D 

 being made gas tight by an indiarubber band (not shown) 

 protected from the air by mercury. 



Below the perforated plate hangs a small tube of phos- 

 phorus pentoxide. 



The internal diameter of D 2 > the tube we chiefly used, is 

 4*47 cm. One turn of the tilting-screw corresponds with a 

 pressure of 9*07 dyne-cm."~ 2 The pressure of the gas was. 

 adjusted to 75 cm. The temperature was that of the room. 



The principle of the wind-pressure method may thus be 

 described. If a point discharges a current of strength c 

 through a uniform tube of cross-section a against a plate at 

 a distance z from the point, the specific velocity of the ions 

 V is given by the expression 



c I dp 



a J dz> 



where p is the average pressure of the electric wind per unit 

 area of the plate ; it being assumed that the plate fills the 

 tube, and that ions of one sign only are present in the dis- 

 tance dz. 



The measurements were made in the same way as those 

 described in the 1899 paper (loc. cit.). 



Discussion of Results. 



Earlier measurements by the wind-pressure method * led 

 to values of V, which are in satisfactory agreement with 

 those obtained by other methods. For dry hydrogen the 

 values of V+ and V— were respectively 5*4 and 7*43 cm.- 

 sec. -1 in a field of 1 volt-cm. -1 , the hydrogen containing a 



* Phil. Mag. 1901 [6] vol. i. p. 79. 



