1880.] on Electricity in Transitu. 429 



nearer, the column is severed into two, while from the point under that 

 where the finger rests there issues a halo similar to that which sur- 

 rounded the negative terminal before the air-si)ark was introduced. 

 The explanation of this phenomenon is to bo sought in the fact that the 

 positive electricity coming from the machine accumulates at the air- 

 spark interval until it has acquired sufficient tension to make the 

 leap. It then passes jjcr saltum into the tube, giving to the latter an 

 instantaneous charge of the same name as that appertaining to the 

 air-spark terminal. The conductor outside, througli a redistribution 

 of the electricity on its surface, is able to supply to the tube by 

 induction the electricity which it needs, and forms a quasi terminal 

 immediately within the point of contact. In the case in question, 

 tlie quasi terminal is a negative one ; the repulsion is the equivalent 

 of the blank space, and the blue discharge that of the negative halo. 

 On account of the fact that this inductive supply of electricity from 

 the outside relieves the charge upon the tube due to the impulse from 

 the air-spark terminal, we have called the effects in question the 

 relief effects. They are by their very nature intermittent and co- 

 periodic with the discharge whose needs are thus supplied. 



The same is the case, mutatis mutandis, with a negative air-spark. 

 The relief consists in a series of jDositive discharges, which arc 

 characterized by an attraction of the luminous column within the 

 tube, and by the commencement of similar luminosity immediately 

 within the point of contact. The effects with a negative air-spark 

 are not so marked as those with a positive, for reasons to be men- 

 tioned hereafter. 



It is further to be noticed that the completeness of these effects 

 depends upon the capacity of the conductor. In the case of an earth 

 connection, the capacity is infinite and the relief complete. If instead 

 of the earth we take simply a reel of wire insulated bodily, then, 

 when the wire is coiled up close to the tube, the relief which it 

 affords is very small, viz. we find only moderate repulsion ; but as 

 the wire is uncoiled and led away at right angles to the tube, the 

 capacity of the system increases, and the relief becomes more and 

 more complete, viz. we have stronger repulsion, and when the relief 

 is sufficient we have the blue discharge also. 



If contact with the outside be made with a ring of tinfoil wrapped 

 round the tube the effect will be more striking. In the case of negative 

 relief, the positive impulses will start in the form of a hollow cone in 

 the direction of the negative terminal, while the negative electricity 

 left free beneath the tinfoil goes to meet and to satisfy the positive 

 impulses arriving from the positive terminal; and in so doing, it 

 truncates the positive column. 



It will doubtless have occurred to some of my audience that, if the 

 explanation of these effects be correct, it ought to be possible to 

 imitate them by coimccting a point on the outside of the tube with the 

 opposite or non air-spark terminal, because we should then be sup- 

 plying exactly what was wanted, viz. impulses of electricity of the 



