Electric Discharge In Vacuum Tubes

Friday, July 17, 1891
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14 THE ELECTRICAL ENGINEER. [Julyl,1891. ELECTRIC DISCHARGE IN VACUUM TUBES. HY IN Tim ELEc'r1>.1cAL Eucmscn of Juno 10, I have noted the description of some experiments of Prof. J. J. Thom- son, on the “Electric Discharge in Vacuum Tubes,” and in your issue of June 24 Prof. `lihu Thomson describes an experiment of the same kind. The fundamental idea in these experiments is to set utp an slectromotivc force in a vaouum tube-preferably evoid of any electrodes-by means of electro-magnetic induction, and to excite the tube in`this manner. As I view the subject, I should think that to any experi- menter who has carefully studied the roblem confronting us and who has attempted to End a soliition of it, this idea must present itself as naturally, as, for instance, the idea of replacing the tinfoil coatings of a Leyden jar by rarciied gas and exciting] luminosity in the condenser thus obtained by repeatedly c arging and discharging it. The idea being Fic. 1. obvious, whatever merit there is in this line of investiga- tion must depend upon the completeness of the stud of the subject and the correctness of the observations. 'lllie fol- lowing lines are not penned with any desire on my part to put myself on record as one who has performed similar experiments, but with a desire to assist other experiment- ers by pointing out certain peculiarities of the phenomena observed, which, to all appearances, have not been noted by Prof. J. J. Thomson, who, however, seems to have gone about systematically in his investigations and who has been the first to make his results known. These peculiarities noted by me would seem to be at variance with the views of Prof. J. J. Thomson, 'and present the phenomena in a different light. ~ My investigations in his line occupied me principally during the winter and spring of the past year. During this time many different experiments were performed, and in my exchanges of ideas on this subject with Mr. Alfred S. Brown, of the Western Union 'lelegraph Company, various different dispositions were suggested which were carried out by me in practice. Fig. 1 may serve as an ex- ample of one of the many forms o apparatus used. This consisted of a large glass tube sealed at one end and pro- jecting into an ordinary incandescent lamp bulb. 'I‘he primary, usually consisting of a few turns of thick, well- insulated copper sheet was inserted within the tube, the inside space of the bulb furnishing the secondary. This form of apparatus was arrived at after some experimenting and was used principally with the view of enabling me to place a polished rellecting surface in the inside of the tube, and for this purpose the last turn of the primary was covered with a thin silver sheet. In all forms of apparatus used there was no special difliculty in exciting a luminous circle or cylinder in proximity to the primary. As to the number of turns, I cannot quite understand why Prof. J. J. Thomson should think that a few turns were “ mtuite suilicient,” but lest I should im nte io him an opinion e may not have, I will add that lp have gained . Pins ow @ 5 is Lf ss CUND. Fic. 2. this impression from the reading of the published abstracts of his lecture. Clearly, the number of turns which gives the best result in any case, is dependent on the dimensions of the apparatus, and, were it not for various iuusidern- tions, one turn would always give tho best result. l have found that it is preferable to use in thess experi- ments an alternate current machine giving a moderate number cf alternations per second to excite the induction coil for charging the Leyden jar which discharges through the primary-shown diagrammatically in Fig. 2,-as in such case, before the disruptive disc arge takes place the tube or bulb is slightly excited and the formation of the luminous circle is decidedly facilitated. But I have also used a Wimshurst machine in some experiments. Prof. J. J. 'I`homson’s view of the phenomena under con- sideration seems to be that they are wholly due to electro- magnetic action, I was, at one time, of the same opinion, but upon carefully investigating the subject I was led to the conviction that they are more of an electrostaiic nature. It must be remembered that in these experiments we have to deal with primary currents of an enormous frequency or rate of change and of a high potential, and that the secondary conductor consists of a rareiied gas, and that under such conditions electrostatic effects must play an im- portant part. In support of my view I will describe a few experiments made by me. To excite luminosity in the tube it is not 'fi "" " ’ VW tlll , Flu. 3. absolutely necessary that the conductor should be closed. For instance, if an ordinary exhausted tube (preferably of large diameter] be surrounded by a spiral of tllirk copper wire serving as the primary, a feebly luminous spiral may be induced in the tube, roughly shown in Fig. 3. ln one of these experiments a curious phenomenon was observed; namely, two intensely luminolls circles, each of them close to a turn of the' rimary spiral, were formed inside of the tube, and I attributed this phenomenon to the existence of nodes on the primary. The circles were connected hy a faint luminous spiral parallel to the primary and in close proximity to it. To produce this effect I ha\e found it necessary to strain the jar to the utmost. The turns of the spiral tend to close and form circles, but this, of course,

-l“lY1»1391-l THE ELECTRICAL ENGINEER. 15 would be expected, and does not necessarily indicate an electro-magnetic effect ; whereas the fact that a glow can be produced along the primary in the form of an open spiral argues for an electrostatic effect. In using Dr. I.odge’s rccoil circuit, the electrostatic action is likewise apparent. The arrangementis illustrated = yy ” ll ll: Fm. 4. in Fig. 4. In his experiment two hollow exhausted tubes H u were slipped over the wires of the recoil circuit and upon discharging the jar in the usual manner luminosity was excited in the tubes. Another experiment performed is illustrated in Fig, 5, In this case an ordinary lamp-bulb was surrounded by one or two turns of thick copper wire P and a luminous circle L excited in the bulb b discharging the jar through the pri- mary. The lamp-bufb was provided with a tinfoil c0at~ ing on the side opposite to the primary and each time the tinfoil coating was connected to the ground or to a large 'object the luminosity of the circle was considerably in- creased. This was evidently due to electrostatic action. , In other experiments I have noted that when the primary touches the glass the luminous circle is easier produced and is more sharply defined ; but I have not noted that,gener- ally speaking, the circles induced were very sharply defined, as Prof. J. J. Thomson has observed ; on the contrary, in my experiments they were broad and often the whole of the bulb or tube was illuminated ; and in one case I have observed an intensely purplish glow, to which Prof. J. J. Thomson refers. But the circles were always in close proximity to the primary and were considerably easier pro- (/li ____ .I ,t fl l Fm. 5. duced \vhcn the latter was very close tc the glass, much more so than would be expected assuming the action to be electromagnetic and considering the distance; and these facts speak for an electrostatic effect. Furthermore I have observed that thcro is a molecular bonibardrnent in tho plane of thc luminous circlc at right angles to the glass-supposing the circle to be in tho plane of the primary-this bombardment being evident from the rapid heating of thc glass near the primary, \Vere the bombardment not at right angles to the glass the heating could not bc so rupid. lf there is a circumferential move- ment ofthe molecules constituting the luminous circle, I have thought that it might be rendered manifest by plac- ing within the tube or bulb, radially to thc circle, a thin plate of mica coated with some phosphorescent material, and another such plate tangentially to the circle. If the molecules would move circumferentially, the former plate would be rendered more intensely phosphorescent. For want of time I have, however, not been able to perform the experiment, Another observation made by me was that when the specific inductive capacity of the medium between the primary and secondary is increased, the inductive effect is augmented. This is roughl illustrated in Fig. 6. In this case luminosity was excited’ in an exhausted tube or bulb B and a glass tube T slipped between the rimary and the bulb, when the edect pointed out was noted. \Vere the action wholly electromagnetic no change could possibly have been observed. I have likewise noted that when a bulb is surrounded by awire closed upon itself and in the plane of the primary, the formation of. the luminous circle within the bulb is not prevented. But if instead of the wire a broad strip of tin- foil is glued upon the bulb, the formation of the luminous band was prevented because then the action was distrib- uted over a greater surface. The effect of the closed tin- fcil was no doubt of an electrostatic nature, for it pre~ ra .... _ulhlttllrihilhln nl "U \ ll* T Qllllllllllllllllllllll!lllI!!l'»V/I @ spHl|r1||l|rgl|.@._- I - »lrHll!lll||lll|l mm Illlilll `f | \ p I Fic. 6. sented a much greater resistance than the closed wire and produced therefore a much smaller electrom$netic effect. Some of the experiments of Prof. J. J. homson also would seem to show some electrostatic action. For in- stance, in the ex eriment with the bulb enclosed in a bell jar, I should thinb that when the latter is exhausted so far that the gas enclosed reaches the maximum conductivity, the formation of the circle in the bulb and jar is prevented because of the space surrounding the primary being highly conducting; when the jar is further exhausted the cou- ductivity of the space around the primary diminshes and the circles appear necessarily first in the bell jar as the rare. fied gas is nearer to the primary. But were the inductive effect very powerful they would probably appear in the bulb also. If, however, thc bell jar were exhausted to the highest degree they would very likely show themselves in the bulb only, that is, supposing the vacuous space to be non-conducting. On the assumption that in thevsephe- nomcna electrostatic actions are concerned we find it easily cxplicable why the introduction of mercury or the heating of the bulb prevents the formation of the luminous band or shorten the after-glow ; and also why in some cases a platinum wire may prevent the excitation of the tube. Nevertheless some of the experiments of Prof. J. J. Thom- son would seem to indicate an electro-magnetic effect. I may add that in one of my experiments in which a vnonum was srodnccd in the 'l‘oricellian method, I was unable to produce the luminous band, but this may have been due to-the weak exciting current employed. My principal argument is the following : have experi- mentally proved that if the same discharge which is barely snflicicnt to excite a luminous band in the bulb when passed through the primary circuit bc so directed as to exalt the electrostatic inductive effect-namely, by converting upwards-an exhausted tube, devoid of electrodes, may be excited at a distance of several feet.