ON ELECTRICAL DISCHARGES THROUGH RAREFIED GASES. 
179 
the tinfoil were joined to earth; if, on the other hand, it be small and near to the 
tube, the effect will be much less. The same remarks .apply to cases in which, 
though the tinfoil is not absolutely disconnected from earth, yet its connexion is 
electrically very imperfect. Thus, if the wire from the tinfoil be allowed just to touch 
the finger, it will give less effect than if it be tightly held, and similiarly for other 
imperfect connexions. Even in the case of the telephone in circuit between the tin- 
foil and the earth, if the telephone be not too far from the tube a marked diminution 
in loudness will be observed when the connexion between the telephone and the earth 
is severed, so that the displaced electricity, no longer able to escape to the earth, is 
compelled to re-arrange itself in the telephone and the wire connected with it. 
There is no need to place the conducting body in actual contact with the glass in 
order to affect the sensitive discharge. If the body be near the tube it will produce 
nearly the same effect as though it were in contact, as in Plate 15, fig. 5 ; if it is 
farther off it will produce less; and, as in the former case, if it is large and capable of 
permitting displaced electricity to remove to a considerable distance from the tube, it 
will produce a much greater effect than if the contrary be the case. In short, it may 
generally be stated that the completeness of the effect of a conducting body upon the 
sensitive discharge depends on the completeness of the capability it possesses of 
permitting a displacement of electricity within it (caused by the electric disturbances 
in the tube) without any material alteration of its potential. 
Such experiments as those of the telephone in circuit with the tinfoil and the earth 
may not unnaturally suggest the idea that these effects may be due to electro-magnetic 
and not to electro-static induction. Blit careful examination negatives this supposition. 
In the first place it is not necessary that the tinfoil should have any considerable length 
in the direction of the tube, which is presumably the direction of the current through 
it. A narrow ring of tinfoil round the tube will give all the results above described. 
Again, if a coil of insulated -wire be placed near or round the tube, no difference is 
produced by joining the ends of the wire. In fact, no difference of effect is observable 
whether the conductor be made up of insulated wire or be a mere mass of metal; the 
whole seems to owe its effect to its being a conductor of considerable size. No form 
of conductor possesses greater effect than a narrow ring of tinfoil connected to earth 
by a single fine copper wire perpendicular to the tube, and no conducting system could 
be devised more unfavourable to the production of electro-magnetic inductive currents 
by the original current in the tube. Moreover, the effect is far too great to be 
attributable to electro-magnetic induction in circuits so unfavourable to its action. In 
short, every feature of the case, upon examination, tends to render it less probable that 
any of the electric phenomena in the external conducting system are due to electro¬ 
magnetic induction; and the supposition has been noticed here rather with the view 
of showing that it has not escaped consideration, than because it was considered to 
have any valid claims to notice. 
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