404 Mr. W. Spottiswoode on [Mar. 30, 



netic polarity. This effect is particularly noticeable if the magnetic pole 

 be inserted only a short distance beyond the ring. The discharge is then 

 seen to spread itself out sheetwise on the ring in the direction in which 

 rotation would take place. The edge of the sheet is in the form of a 

 helix. 



The object of the following observations is to bring out the character 

 of this phenomenon by making it a principal instead of a secondary 

 feature of the experiment. 



The arrangement here described consisted in using the poles of an 

 electromagnet as the terminals of a discharge from an induction-coil, and 

 in observing the effect on the form of the discharge caused by exciting 

 the electromagnet. Tor this purpose the movable poles were insulated 

 from the main body of the magnet by interposing a sheet of ebonite thick 

 enough to prevent the passage of the discharge, but not thicker, in order 

 as little as possible to diminish the strength of the magnetic poles. The 

 discharge was then effected either in the open air or in a closed chamber. 

 The latter was constructed of a short cylinder of glass, say 3 inches 

 in length and 2 in diameter, having conical ends pointed inwards, so as 

 to receive the poles of the magnet. The chamber was also furnished with 

 a pipe and stopcock for the purposes of exhaustion. 



The discharge from an induction-coil taken in air or other gas at 

 atmospheric pressure consists, as is well known, primarily of the spark 

 proper or bright line, irregular in form and instantaneous in duration. 

 But beside this, when the primary wire is thick and the battery-current 

 strong, the spark is enveloped in a bright cloud, or rather flame, which is 

 capable of being thrown on one side, although not entirely detached from 

 the spark, by a current of air. This, when examined in a revolving 

 mirror, is found to be subsequent in time to the spark proper, and may 

 be considered to be due to the gas in the neighbourhood of the spark 

 becoming sufficiently heated to conduct part of the discharge, and to the 

 consequent combustion of any extraneous matter floating in the medium. 

 Such a view is supported by the fact that the colour of this flame depends 

 partly upon the nature of the gas in which the discharge takes place, and 

 partly upon that of any volatilizable matter which may be introduced near 

 the poles. 



The exciting of the magnet produces upon the spark proper no appre- 

 ciable effect ; but as soon as the flame is submitted to its action it is 

 spread out into a sheet, which arranges itself in a helicoid right-handed 

 or left-handed according to the direction of the current and of the mag- 

 netic polarity in obedience to Ampere's law. 



Effects substantially the same are produced whether the discharge be 

 taken in gas at atmospheric or at a less pressure. But in the former 

 case the helix has a lower, in the latter a steeper gradient ; that is to say, 

 in the former case it presents a greater, in the latter a less number of 

 turns for a given interval between the poles. 



