656 



Mr. C. G. Barkla on the Velocity of 



of the most consistent results, temporary diminutions of in- 

 tensity being then inappreciable in the resnlt of the whole 

 period. 



The spark-gap was 3 or 4 millimetres in length, so that the 

 discharges were not of the most violent nature, and hence not 

 so liable to produce sudden changes on the surfaces between 

 which sparking occurred. It was found, however, that im- 

 mediately after these surfaces were repolished there was a 

 rapid change in their character during sparking, and the 

 effect on the detector-needle of two consecutive periods 

 was altogether different, though everything else remained 

 unchanged. 



Platinized balls were not an obvious improvement. 



The ordinary hammer interrupter also was replaced by the 

 Wehnelt interrupter, but for these long periods of sparking 

 the results were not so good. 



The only satisfactory readings were obtained after sparks 

 had been passing for a considerable time, and the faces of 

 the knobs had lost their polish, so that further sparking pro- 

 duced little or no change on them. The constancy of the 

 detector's indications during some of these occasional steady 

 states was remarkable, as the following example shows. 



During a series of observations made with the bridge in 

 different positions, alternate readings were taken with the 

 bridge in a standard position, so that corrections might be 

 made for changes in the intensity of sparking. These standard 

 readings, which are given below, were taken as closely as 

 possible to tenths of a millimetre on the magnetometer-scale. 

 As will be seen, the variations, with one exception, were 

 almost imperceptible. The table also shows the final read- 

 ings to be independent of the small variations in the saturation 

 values. 



Table I. 



Position of 

 bridge. 



Zero reading. 



Saturation 

 deflexion. 



Final 

 deflexion. 



Standard 







2031 mm. 



156-3 mm. 



" 



>3 



202-85 „ 



156-2 „ 





if 



202-3 „ 



156-1 „ 





„ 



202-5 „ 



156-1 „ 



" f 



ff 



203-1 „ 



156-05 „ 



»» 



JS 



203-3 



156-1 „ 



^ 





202-05 „ 



156-05 „ 





tt 



201-3 „ 



#155-3 „ 





J) 



201-25 „ 



15605 „ 







201-85 „ 



156 



" 



" 



200 



156-1 „ 



