70 



NATURE 



[May 15, 1879 



therefore under the necessity of lecturing twice a day. His 

 retirement from Glasgow College has opened the way for a 

 young botanist of great promise, Dr. I. Bayley Balfour, son of 

 the veteran professor at Edinburgh, who has been appointed by 

 the Crown to the vacant chair. Dr. Balfour took the degree of 

 Doctor of Science in Botany some years ago with great distinc- 

 tion at Edinburgh. He was selected by the Council of the 

 Royal Society to accompany the recent Transit of Venus Expe- 

 dition to Rodriguez for the purpose of making a scientific exa- 

 mination of that island. As the result of his researches, besides 

 the report on the natural history, which he has sent in to the Royal 

 Society, he has produced an excellent paper on the genus //a/o- 

 phila. Having had considerable experience in class-work under 

 his father, as well as under Professors Huxley and Sir Wyville 

 Thomson, he enters on his new duties with many advantages. 

 Whether as an original Investigator or as a successful teacher, 

 he will, we doubt not, fuUy sustain the reputation of the Glasgow 

 University. 



We are glad to notice that the School Board for London have 

 decided that it would be expedient to include the elements of 

 natural science among the recognised subjects of class examina- 

 tion. The object of this resolution is to transfer what is called 

 elementary science from the categoi-y of specific subjects into the 

 category of class subjects. At present there is little inducement 

 for pupils to take science subjects, nor will there be until it be 

 included in the regular course of instruction in elementary schools. 

 We hope the memorial which the Board is to prepare will^ be 

 treated with the attention it deserves. 



SOCIETIES AND ACADEMIES 

 London 



Royal Society, May 8. — ■" On the Sensitive State of 

 Electrial Discharges through Rarefied Ga«es." By William 

 Spottiswoode, P.K.S., and S. Fletcher Moulton, late Fellow 

 of Christ's College, Cambridge. 



It has frequently been remarked that the luminous column 

 produced by electric discharges in vacuum tubes sometimes 

 displays great sensitiveness on the approach of the finger, or 

 other conductor, to the tube. This is notably the case when 

 with an induction coil a very rapid break is used, or when with 

 any constant source of electricity an air-spark is interposed in the 

 circuit leading to the tube. The striking character of the 

 phenomena, and the opportunity which they showed for affecting 

 the discharge from the outside during its passage, led the 

 authors of this paper to consider that a special examination of 

 this sensitive state would be desirable. 



All the circumstances under which sensitiveness is produced 

 appear to agree in requiring, first, that there should be a rapid 

 intermittance in the current leading to the tube ; and secondly, 

 that the individual intermittent discharges should be small in 

 quantity and extremely brief, if not instantaneous, in duration. 

 Both these requirements are fulfilled by the methods used in the 

 present investigation, viz. , a Holtz machine with a suitable air- 

 spark between the machine and the tube, and a small coil with 

 a rapid break. 



If a conductor be made to approach a tube conveying a 

 sensitive discharge, due to an air-spark in the positive branch of 

 the circuit, a series of effects is'produced, of which the feeblest 

 and the strongest are the most pronounced. The transition 

 from one to the other is so rapid that the intermediate phases 

 may be easily overlooked. In the first case, the luminous 

 column is repelled by the conductor ; in the second it is broken 

 into two parts which stretch out in two tongues towards the 

 point on the tube (?) nearest the conductor, while a negative 

 halo appears between them. 



That these effects are due to the inductive action of the 

 conductor, or more particularly to re-distributions of electricity 

 in it, co-periodic with the air-spark, and not to any permanent 

 charge, is shown by the following experiments. A non-conductor, 

 whether charged or not, is without effect. The effect of a con- 

 ductor increases with its size or capacity, and with its proximity to 

 the ttibe, until the fullest effect (viz., that given by an earth con- 

 nexion) is produced. That the effects are not due to electro- 

 dynamic, or to magnetic action, is shown by the fact that a coil 

 of wire produces the same result, whether the ends be joined or 

 not. The effects of an iron core and helix with open ends are 

 often comparable'with, and sometimes equal to, those when the 

 ends, being connected with a battery, the whole becomes an 

 electro-magnet. The effect upon the interior is, in fact, due to 



the relief given by the conductor to the electric tension on the 

 outer surface of the tube and the space around it, caused by the 

 individual discharges. 



Instead, however, of connecting a point (P) on the tube with 

 a large conductor or with earth, we may connect it with one or 

 other terminal of the tube. And a further study of the .'ubject 

 shows that all the phenomena due to action from without may be 

 produced by means of one or other of theseconnexions. Connexion 

 with the non-air- spark terminal gives the relief effects described 

 above ; connexion with the air-spark terminal gives another set 

 of effects. Of these the feeblest has the appearance of attrac- 

 tion, while the strongest shows an abrupt termination of the 

 positive column in the neighbourhood of the point (P), followed 

 by a negative halo, and then by a recommencement of the 

 positive column in the direction of the negative terminal. Each 

 of these sectional discharges is in fact independent and complete 

 in itself, and they are due to impulses of positive electricity 

 thrown into the tube from the air-spark. At the positive 

 terminal these impulses are thrown directly in ; at the points of 

 connexion they are due to induction, ab extra. The negative 

 part of what was originally neutral meets the positive column, 

 and satisfies it as it arrives, while the positive leaps forward to 

 meet the negative due from the negative terminal. 



The effects above described need not be confined to a single 

 patch or ring of conducting material placed upon the tube : but 

 they may be produced many times over in the same tube by a 

 series of rings arranged at suitable distances. By this means the 

 column may be broken into a series of sections, all terminating 

 with well-defined configurations towards the negative end, and 

 having greater or less length, according to the position of the 

 rings. In the paper itself, arguments are there brought for'.vard 

 showing that these sectional discharges represent strise not 

 merely in their appearance, but also in their function and struc- 

 ture. But the discussion could hardly be produced within the 

 limits of an abstract. 



Returning from the digression about striae, the authors next 

 give evidence, derived mainly from the revolving mirror, and 

 from the discharges of a partially charged Leyden jar, for the 

 following conclusion : That the passage of the discharge occupies 

 a time sufficiently short in comparison with the interval between 

 the discharges to prevent any interference between successive 

 ptilses. Certain experiments are then described which indicate 

 that the discharge is effected, under ordinary circumstances, by 

 the passage through the tube from the air-spark terminal of 

 free electricity, of the same name as the electricity at that 

 terminal. In the case of an induction coil, where the air-spark 

 must be considered as existing at both terminals, there is evidence 

 of a neutral zone, where the sensitiveness disappears. The 

 position of this zone may be altered by damping the impulses at 

 either terminal ; or it may be abolished by connecting one 

 terminal with earth. The impulses may even be so distributed 

 as to divide electrically a single tube into three sections, the two 

 extremes presenting visible discharges, with a dark section 

 between them. 



Looking at all these phenomena from an opposite point of 

 view, we may, by means of the relief effects, determine the 

 terminal from which a discharge proceeds, and the distance to 

 which it reaches without provoking a response from the other. 

 And through these considerations, together with others detailed 

 in the paper, the authors are led to the conclusion that the dis- 

 charges at the two terminals of a tube are in the main indepen- 

 dent, and that they are each determined primarily by the condi- 

 tions at their own terminal, and only in a secondary degree by 

 those at the opposite terminal. 



In illustration of this view, an account is then given of the 

 production of unipolar, positive, or negative discharges in a 

 tube. In such cases, the discharge being insufficient of itself to 

 pass through the tube, returns by the way by w hich it entered. 



This closes a series of experiments, the result of which is that 

 the discharges from the two terminals can be made of equal 

 intensity, orof any required degree of inequality; or the discharge 

 can be made to issue from one terminal only, the other acting only 

 receptively ; or it can be made to return into its own terminal, 

 while the other takes no part in the discharge ; or, finally, the 

 two terminals can be made to pour out independent discharges 

 of the same name, each of which returns to its own terminal. 



Having traced the relation between the two parts of the dis- 

 charge, and having found means for controlling their range and 

 influence, the authors were led to inquire whether there be any 

 experimental evidence of the state of the tube during the occur- 

 rence of the discharge. Some experiments with two pieces of 



i 



