y2ine 3, 1880] 



NATURE 



nected with outer terminals. The poles a and b were 

 connected with the induction coil, an earth wire was 

 brought near the idle pole <■, and a gold leaf electroscope 

 was brought near d. 



On passing the current at inferior exhaustions, when 

 the dark space is about 8 millims. from the negative pole, 



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Electroscope 



no movement of the gold leaves takes place whether a ox b 

 is negative, and whether c is connected with earth or is 

 insulated. 



At a good exhaustion, when the green phosphorescence 

 of the glass is strong, the gold leaves are only slightly 

 affected whichever way the current passes. 



On increasing the exhaustion to a very high point, so 

 that the green phosphorescence gets weaker and the spark 

 has a difficulty in passing, the gold leaves are violently 

 affected. When the pole a is negative and b positive, the 

 leaves diverge to their fullest extent. On examining their 

 potential it is found to be positive. The coil was stopped 

 and the gold leaves remained open. A touch with the 

 finger caused them to collapse. They then gradually 

 opened again, but not to the original extent. The finger 

 again discharged them, when they reopened slightly a 

 third time. Experiment ihowed that the electrical excite- 

 ment took many minutes to recover equilibrium. A 

 Leyden jar put to the idle pole d was charged positively. 



The earth wire and electroscope remaining, as shown 

 in the figure, the direction of current was reversed so as 

 to make a positive and /' negative. The gold leaves were 

 now less strongly affected ; they opened a httle, and re- 

 mained quivering, as if under the influence of rapidly- 

 alternating currents. 



The wires were rearranged as shown in Fig. 8, b and d 



being connected with the coil. When (/ was made nega- 

 tive, faint sparks about l miUim. long could be drawn by 

 the finger from c ; but when d was made positive the 

 sparks from c wore 10 millims long. The same results 

 are obtained when the finger is brought near a, so long as 

 c remains insulated. If, however, c be connected with 

 earth by a wire, no sparks can be got from, a, whichever 

 way the current passes between b and d. Connecting a 



with earth diminishes the length of the sparks, which can 

 be drawn from c by about one-half. 



The poles « and if' heing connected with the coil and 

 the idle poles c and d having loose wires hanging from 

 them, the wires were strongly repelled from each other. 



The above experiments show that an idle pole in the 

 direct line between the positive and the negative poles, 

 and consequently receiving the full impact of the mole- 

 cules driven from the negative pole, has a strong positive 

 charge. 



It now became of interest to ascertain whether the tra- 

 jectory of the molecules suflered anydefiection in passing 

 an idle pole when it was suddenly uninsulated by an earth 

 contact. For this purpose I used the tube described in a 

 former paper,' where the shadow of an aluminium star 

 was projected on a plate of phosphorescent glass. So 

 long as the aluminium star is insulated, the shadow is 

 sharp, as already described ; but on touching the star to 

 earth, the shadow widens out, forming a tolerably well- 

 defined penumbra outside the original shadow, which can 

 still be seen unchanged in size and intensity. On re- 

 moving the earth connection, the penumbra disappears, 

 the umbra remaining as before. The same penumbra 

 is produced by connecting the idle pole with the negative 

 pole through a very high resistance, such as a piece of 

 wet string, instead of connecting it with earth. On 

 bringing a magnet near the negative pole, the shadow of 

 the (insulated) star is much increased in definition, the 

 adjacent luminous parts of the screen becoming more 

 luminous. Touching the star now brings a large, some- 

 what blurred, penumbra round the original image. The 

 penumbra obeys the magnet the same as the umbra. 



The aluminium star was now made the positive pole, 

 the other pole remaining unchanged. The shadow of the 

 star was projected on the phosphorescent plate of the 

 same sharpness and almost the same intensity of light and 

 shade as if the positive pole had been the one ordinarily 

 used as such. The image obeyed the magnet as usual. 

 With this arrangement the penumbral action could not be 

 tested. 



This, therefore, confirms the above-described results — 

 that the idle pole, the shadow of which is cast by the 

 negative pole, has strong positive charge. Now the 

 stream of molecules must be assumed to carry negative 

 electricity ; when they actually strike the idle pole they 

 are arrested, but tbose which graze the edge are attracted 

 inwards by the positive electricity, and form the shadovv. 

 When the idle pole is connected with earth its potential 

 would become zero were the discharge to cease ; but, 

 inasmuch as a constant positive charge is kept up from 

 the passage of the current through the tube, we must 

 assume that the potential of the uninsulated idle pole is 

 sull sufficiently positive to neutralise the negative charge 

 which the impinging molecules would give it, and leave 

 some surplus of positive. The effect of alternately unin- 

 sulating and insulating the idle pole is therefore to vary 

 its positive electricity between considerable limits, and 

 consequently its attractive action on the molecules which 

 graze its edge.- 



Experiments were tried with an idle pole and shadow 

 tube whilst the exhaustion was going on. At such a 

 rarefaction that the shadow can ju^t be made out, it is 

 quite sharp ; touching the idle pole causes a small 

 penumbra to appear round its shadow. When the 

 exhaustion is at the best point for obtaining the green 

 phosphorescence on the glass, the shadow is very sharp 

 and well defined ; and connecting the idle pole with earth 

 gives a much wider penumbra, the width of the penumbra 

 increasing with the degree of rarefaction. When the 



Phil. Trans.. 1879. vol. 170, p. 147. 



; that the theory which makes these effects of deflection de- 

 >static attractions and repulsions is open to some grave objec- 



^;; still it was that which in a great measure guided me in my experiments, 



and it could not well be omitted without reducing the description of them to 

 a drj' record of apparently unconnected facts. 



pend c 



