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[Illustration:
VI
]
28. DAVY, BANCALARI AND QUET’S EXPERIMENTS. ELECTRIC ARC, MAGNETISM AND
FLAME. SOUND PRODUCED. PRACTICAL APPLICATION OF ELECTRIC ARC. _Phil.
Mag._, 1801.—When the electric arc, for example between two carbon
electrodes, occurs, in a powerful magnetic field, it is violently drawn
to one side as first shown by Sir Humphry Davy, as if the wind were
blowing it and sometimes it is broken into two parts. Fig. VI. Again a
loud noise is produced. § 9. Without the magnet, the appearance is as at
the left. With the energized magnet, the arc and light, as a whole, are
as shown at the right.
29. DE LA RIVE’S EXPERIMENT. ROTATION OF LUMINOUS EFFECT BY MAGNET.
APPLICATION TO EXPLAIN AURORA BOREALIS. _Phil Trans._, vol. 137, 1847.
_Pynchon_, p. 471. _Ganot_, Sect. 928.—An oval discharge tube was
employed, having a highly exhausted atmosphere (for those days) of
spirits of turpentine. A cylindrically shaped pole of a magnet extended
into the bulb half way, Fig. 4, p. 17. The inner end of the magnetic
pole formed one electrode of the tube, and the other electrode was a
ring within the vacuum at the foot of the magnetic pole. A fountain of
light extended from one end of the magnet pole to the other, and
remained stationary, while the magnet was not energized; but the light
was condensed into an arc and travelled around the magnet pole when a
current was passed through the coils of the magnet. For similar action
of magnet on a flexible and movable wire carrying a current, see
experiments of Spottiswoode and Stokes, _Proc. R. So._, 1875. The aurora
borealis rotates around the pole of the earth, and therefore, De La Rive
thought that the phenomenon in his laboratory and in nature were but one
and the same thing and different only in degree. He also extinguished an
arc in open air by means of a powerful magnet.
[Illustration:
VII
]
30. PLÜCKER AND HITTORF’S EXPERIMENTS. ACTION OF MAGNET ON CATHODE
COLUMN OF LIGHT. _Pogg. Ann._, 1858 and 1869. Plücker found that the
magnet acts on the cathode light in a rarefied atmosphere in a different
manner from that on the anode light. In the former the light follows the
magnetic curves and strike the side of the bulb, according to position
of the poles, see Fig. VII. “Where the discharge is perpendicular to the
line of the poles, it is separated into two distinct parts, which can be
referred to the different action exerted by the electro-magnet on the
two extra currents produced in the discharge.” _Ganot._ § 925.
31. THOMSON’S EXPERIMENT. A DISCHARGE RETARDED ACROSS AND ACCELERATED
ALONG THE LINES OF MAGNETIC FORCE. _Nature_, Lon., Jan. 31, 1895, p.
333. _Lect. Royal Inst._—Prof. J. J. Thomson, F. R. S., performed an
experiment which illustrates that the electrical discharge is retarded
in flowing across the lines of magnetic force and accelerated in flowing
with or parallel to such lines. As illustrated in Fig. 20, p. 17, he
employed a large electro-magnet adapted to be cut in and out of circuit.
He had two air chambers, one a bulb, indicated by a circle, and the
other a tube bent into a rectangle, indicated by the dotted square.
Between these, was an adjustable coil having its terminals connected to
the outside coatings of Leyden jars. When the discharge took place
between the poles of the magnet, that is, in the direction of the lines
of force, the discharge was helped along by the magnetic field, but when
it took place across the bulb, that is, across the lines of force, the
discharge was retarded. “The coil can be adjusted so that when the
magnet is ‘off’ the discharge passes through the bulb, but not round the
square tube; when, however, the magnet is ‘on,’ the discharge passes in
the square tube but not in the bulb.”
[Illustration:
SOME EXPERIMENTS PRIOR TO LENARD’S.
]
32. THOMSON’S EXPERIMENT. RESISTANCE OFFERED TO STRIAE BY A THIN
DIAPHRAGM. _Lect. Royal Inst. Nature_, Lon. Jan. 31, ’95, p. 333.—It has
often been remarked that lightning always takes the easiest path. The
same has been noticed with references to the artificial electric spark.
Prof. J. J. Thomson, F.R.S. performed an experiment, which not only
confirms this principle but does so in an emphatic manner, and proves it
true in reference to the electric discharge in rarefied gases. He
arranged a very thin platinum diaphragm so as to divide a Geissler tube
into two compartments, Fig. 19, p. 17. He then formed a passage way
around the diaphragm, which could be opened and closed by mercury, by
respectively lowering and raising the lower vessel of mercury along the
barometer tube. When the passage way is opened around the diaphragm, the
luminosity extends through the passage way in preference to going
through the diaphragm. When the passage way is closed by mercury, the
discharge goes through the thin metal plate. The same was found to occur
when the platinum leaf was replaced by a mica scale.
33. SIR DAVID SOLOMON’S EXPERIMENT IN 1894. _Proc. Royal So._, June 21,
’94. _Nature_, Lon. Sept. 13, ’94, p. 490.—With a tube having a
perforated diaphragm, he noticed a “forcing effect” at and near the
hole. The striae had the appearance of being pushed through from the
longer part of the tube—the diaphragm not being in the centre. There was
no passage way around the diaphragm—only through the small puncture. §
19.
[Illustration]
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