430 Mr. Spottisiooode [May 21, 



opposite name and coperiodic with tliose projected into the interior 

 from the air-spark terminal. And this proves to be the case. 



It may then be fairly asked, what will be the effect of leading to 

 the outside of the tube impulses from the air-spark terminal itself ; 

 i. e. imjDulses coperiodic with those inside, but of the same name. 

 These effects, which to distinguish them from the relief effects we have 

 called special, are really what might have been anticipated ; viz. 

 special effects with a positive air-spark, or more briefly positive special 

 effects are like relief effects with a negative air-spark, or negative 

 relief effects, and vice versa. This then completes the four possible 

 combinations of air-spark with coperiodic inductive impulses, ab 

 extra. 



In all the cases of special effects it will be observed that the 

 impulses conveyed by the wire outside have always arrived at the 

 point of contact in time to produce their effect on the electricity 

 advancing within the tube ; in other words, that electricity, whether 

 positive or negative, is conveyed along a conductor at least as quickly 

 as along the gas. We shall in the sequel show reasons for thinking 

 that it travels more quickly along a conductor. 



Having exhausted the effects due to a single air-spark, we are 

 naturally entitled to inquire what will be the effect if two air-sparks 

 be used, the one positive the other negative. The experiment can be 

 made with the same apparatus as that hitherto used, viz. a Holtz 

 machine and an air-spark interval in each wire leading to the tube ; 

 but it is more easily effected with a small induction coil giving small 

 but rapid impulses, which are equivalent to an air-spark at each ter- 

 minal. In this case it will be found, on testing the tube by means 

 of its relief or its special effects, that one half of the tube is 

 charged positively, the other half negatively ; and that between the 

 two there is a neutral zone, showing no signs of charge whatever. 

 By attaching a little condenser, e. g. a thunderplate, to one terminal 

 or to the other, the impulses at that terminal become so toned down 

 that the neutral zone is brought nearer to the attached terminal, in 

 proportion to the capacity of the condenser. If either of the terminals 

 be connected to earth, the neutral zone is brought close to the con- 

 nected terminal ; i. e. the tube is charged throughout with electricity 

 of the opposite name. 



It will have been noticed that through these processes, whether 

 relief or special, whether positive or negative, when sufficiently 

 energetic, the discharge is severed into two ; although the exact con- 

 figuration near the point of severance differs in the various cases. 

 In other words, the tube has been divided into two parts, each of 

 which presents the features of a complete discharge. Now, if this 

 process be repeated at several points on the tube, the discharge will 

 be subdivided into as many smaller but complete discharges as there 

 are points of contact or of relief, the negative terminal itself being 

 counted as one. These being sufficiently numerous, or at all events 

 sufficiently near together, we have a complete artificial production of 



