Discharge of Electricity from Cylinders and Points. 795 



It will be seen that nearly all the results are within five 

 per cent, of the mean in each case. It is remarkable that the 

 results are in such good agreement considering the variation 

 which must have been produced by the chemical action of 

 the discharge on the gas. From the mean results curves 

 have been drawn showing the relation between aXj. and ap 

 over the range ap — '05 to ap = 20 (figs. 3 & 4). 



Watson's results may be represented in a similar manner. 

 In his experiments eight different sized wires were used, and 

 it is seen from figure 5 that the points lie very well on a 

 curve, which covers the range from ap = 2Q to op = 350. 



6. In the present investigation there was observed in 

 general a marked difference between the sparking potentials 

 when the wire was charged positively and when the wire was 

 charged negatively. For this reason separate curves are 

 given for the values of ap less than 20. At the smaller 

 values of ap the sparking potential was found to be lower 

 when the wire was negative than when it was positive ; this 

 is in accordance with Meservey's * results on the discharges 

 from wires in gases at low pressures. But for the larger 

 values of ap investigated, that is to say with ap about 5 to 20, 

 a lower potential was required to produce a positive dis- 

 charge. This is contrary to the generally accepted view ; 

 also, Watson observed no material difference between the two 

 cases ; however, most of his measurements were made with 

 much larger values of ap. 



The theory indicates that there should be some similar 

 phenomenon in the case of the discharge of electricity from 

 blunt points. A large number of measurements have been 

 made by Zeleny f of the sparking' potentials of points to 

 planes, the exact shape of the points being defined, as, for 

 example, cylindrical wires with ends rounded approximately 

 to hemispheres. His measurements were nearly all made at 

 atmospheric pressure. With the discharges from the rounded 

 ends of fine wires the sparking potential with the wire 

 negatively charged was lower than with the point positively 

 charged. However, as the size of the wire was increased, 

 this difference decreased regularly, and with a wire *05 cm. 

 in diameter the sparking potentials were the same for the 

 two cases. But Zeleny gives no results for negative dis- 

 charges from points larger than '05 cm. diameter. 



7. As this required further investigation the discharges 

 from the ends of wires of diameters *05, '1, and *15 centi- 

 metre were investigated. 



* A. P. Meservey, Phil. Mag-. [6] xxi p. 479. 

 t Zeleny, Phys. Rev. xxv. p. 305 (1907). 



