476 



Mr. Warren De La Rue 



[Jan. 21, 



have a pressure of 0" 00000000000000000000018 millimetre, or 

 0-00000000000000000024 millionth, and even at 124-15 miles in 

 height the atmospheric pressure would be only • 00000001 millimetre, 

 or • 00001 millionth. It is highly improbable that a display of aurora 

 would occur at a height even of 124 miles, and it is difficult to conceive 

 that an electrical potential could possibly exist necessary to overcome 

 the enormous resistance that would be offered at 281 miles, the tenuity 

 of the air being 54,000,000,000,000 (54 million million) times as 

 great as at 124 miles. 



Fig. 13. 



There are some phenomena connected with the discharge from 

 the voltaic battery which I will bring under notice before we 

 proceed to the study of the discharge in vacuum tubes. I have 

 already spoken of the difference in the length of the discharge 

 between points and discs ; and I have now to call your attention to 

 the influence of the form of the point on the length of the spark. At 

 first it would naturally be supposed that the 

 longest discharge would occur with the sharpest 

 point, but this is not the case ; a great number 

 of experiments with various forms of points have 

 shown that a point in the form of a paraboloid 

 gives the longest spark ; and longer in the pro- 

 portion 1 • 29 to 1 than one in the form of a cone 

 of the same length and diameter at the base. 

 It is difficult to account for this difference in 

 the length of the spark, but it is evident the 

 potential must be greater at the extremity of a 

 paraboloidal point than it is at the extremity 

 of a conical one (Fig. 13). 



If a point and a disc be used together as 

 terminals and the point be made alternately positive and negative, 

 the spark is longest when the point is negative for low tensions up 

 to 3000 cells, and longest when the point is positive beyond that 

 number. 



