Force of the Electric Discharge through Gases. 441 



discharge in this case was intensely bright, and gave the 

 usual candle-spectrum of the gas in the discharge-tube. 



As the gas was pumped out of the tube the appearance of 

 the discharge changed gradually from being very bright, 

 white, and sharply defined, to being dull, reddish coloured for 

 air, and " fuzzy/' its deflexion by magnetic force and also its 

 duration increasing at the same time. 



This discharge was separated both by the magnet and by 

 the rotating mirror into two parts, the first being very bright, 

 least deflected by the magnet, and lasting a very short time ; 

 the next not so bright, reddish coloured for air, deflected and 

 broadened by the magnet, and of much longer duration than 

 the first. 



The discharge as deflected by magnetic force is shovn in 

 fig. 14; the heavy line represents the bright part of the 

 discharge, the hatched portion of the figure the fainter but 



Fig. 14. 



more deflected and broadened part. The same figure would 

 represent the undeflected discharge as seen in the rotating 

 mirror if the heavy line were straight instead of curved. 



As the pressure of the gas in the discharge-tube was lowered 

 still further the discharge became uniform in appearance, both 

 in the mirror and in the magnetic field, while the deflexion as 

 well as the broadening and the duration of the discharge 

 increased. 



(4) In this case no deflexion could be observed at any 

 pressure when the spark-gap was of sufficient length to ensure 

 the whole discharge passing at once ; for very low pressures 

 of the gas in the discharge-tube the capacity of the jars had 

 to be increased, as changing the length of the spark-gap alone 

 was not sufficient to prevent the discharge trailing out. 



In any of the above experiments a discharge that could be 

 sensibly broadened in the rotating mirror was always deflected 

 by the magnetic force. 



The frequency of the discharge was about fifteen per second. 



In an experiment arranged as in case (3) above the pressure 

 of the gas was gradually reduced, until the coil was just able 

 to send a discharge that was drawn out a little in the rotating 

 mirror, and that had a slightly " fuzzy " appearance ; but when 

 the discharge passed for a little time it suddenly changed into a 

 bright sharp discharge, not visibly broadened in the rotating 



