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[Illustration]
97_a_. HERTZ’ EXPERIMENTS. ELECTRIFIED BODIES DISCHARGED BY ULTRA-VIOLET
LIGHT OF A SPARK AND BY OTHER SOURCES OF LIGHT. _Berlin Akad._ II., p.
487, ’87. _Wied Ann._ XXXI, p. 983. English translation of the above.
Lon. and N.Y. Macmillan, p. 63, ’93. From notes by Mr. N. D. C.
Hodges.—This is the all-important initial work of H. Hertz. The source
of light was a spark, and the great discovery resulted from a
combination of circumstances and was unsought; but by studying and
testing the matter, he found the cause. Two induction coils, _a_ and
_b_, having interrupter _d_, were included in the same circuit, as shown
in the figure. The sparking of the active one (A) increased the length
of the spark of the passive (B) § 10. He sought the cause. The discharge
was more marked as the distance between the sparks was reduced. Sparks
between the knobs had the same effect as those between points; but the
effect was best displayed when the spark B was between knobs. The
relation between the two sparks was reciprocal. The discharging effect
of the active spark (A) spread out on all sides, according to the laws
of light, first suggesting that light was the cause. Most solid bodies
acted as screens, s. Liquid and gases served more or less as screens.
The intensity of the action increased by the rarefaction of the air
around the passive spark, __i.e.__, in a discharge tube. The radiations
from the spark, A, reflected from most surfaces, according to the laws
of light, and refracted according to the same laws, caused the
discharge. The ultra-violet light of the spark A was inferred to be the
active agent in producing the discharge. The same effect was produced by
other sources of light than the electric spark. The conclusions were
afterwards confirmed by many, and subordinate discoveries originated. §
98—99T.
97_b_. WIEDEMANN AND EBERT’S EXPERIMENT. LIGHT DISCHARGES CATHODE, BUT
HAS NO INFLUENCE UPON ANODE, NOR AIR-GAP. DIFFERENT GASES AND DIFFERENT
PRESSURES. _Wied. Ann._ XXXIII, p. 241. 1888. From notes by N. D. C.
Hodges.—The arc light was used in place of the active spark of Hertz.
Principal result was that the effect depended on the illumination of the
cathode (§ 99.) The illumination of the anode or of the spark-gap did
not influence the discharge. The very character of the charge was
altered by the action of light upon the cathode. The influence of the
illumination of the cathode did not consist solely at the starting of
the spark, but lasted as long as the sparks continued to pass. With
decreasing pressure of surrounding gas, the effect first increased (§
97_a_) to a maximum, and then decreased (§ 54). The illumination had an
effect on the path of the sparks, the path being perpendicular to the
rays of light. The best results were obtained with carbonic acid gas.
Hydrogen was next, and then air. They were contained in the tubes
surrounding the poles. The character of the gas also had an influence on
the rays which would produce the effect, with carbonic acid gas the
effect showing itself even with the visible rays.
98. ELSTER AND GEITEL’S EXPERIMENT. NEGATIVELY CHARGED BODIES DISCHARGED
BY LIGHT. _Wien. Berichte._ Vol. CI, p. 703, ’92. _Wied. Ann._ Vols.
XXXVIII, XXXIX, XLI, XLII, XLIII, XLIV, XLVI, XLVII, LII. _Nature_,
Lon., Sept. 6, ’94, p. 451.—The elements employed for carrying on the
experiment consisted of a delicate electroscope and certain metals,
including aluminum, amalgamated zinc, magnesium, rubidium, potassium and
sodium. Some of the experiments were made on the top of Mount Sonnblick,
the same being 3,100 m. high, where the discharging power of light was
found to be about twice as great as at Wolfenbuttel, which was at the
level of 80 m. The whole time for the discharge was only a matter of a
few seconds. The greater rapidity of discharge at the higher level was
attributed to the greater proportion of ultra-violet rays (Hertz), which
are the most easily absorbed by the atmosphere, according to Langley.
All metals are not discharged alike by the action of light. The law
follows the electro-positive series in such a way that the more
electro-positive the metal, the longer the wave length of light
necessary to produce the discharge. In experiments with potassium,
sodium and rubidium, they made them successively, the cathode in a bulb
of rarefied hydrogen. In this case it was found that the light of a
candle, even at so great a distance as 7 m., would cause the discharge.
Rubidium was sensitive in this respect to the red light from a heated
rod of glass. Elster and Geitel were able also to discharge, by light,
some non-metallic bodies, like calcic sulphide, when so prepared that it
had the property of phosphorescing, and also darkly colored fluorites.
Independently, the phenomenon is of importance, because Elster and
Geitel determined that there was some common cause as to the discharge
of bodies of light and the discharge from the earth’s surface. A series
of experiments lasting three years, consisted in investigating the
relation of the ultra-violet rays from the sun simultaneously to the
quantity of charge in the atmosphere. The results acted as evidence of
the explanation of the daily and annual variation of atmospheric
potentials. These experiments are of importance in connection with
X-rays, because Röntgen and Prof. J. J. Thomson subsequently, and
possibly others independently, discovered that X-rays produce, not only
a like, but a more extended action in that there is not so great a
difference between their power to discharge negatively and positively
electrified bodies. § 90_a_. In the further developments of their ideas,
they tried the action of diffused day-light upon a Geissler tube
traversed by vibrations which were produced by a Hertz vibrator (see
recent book on Hertzian waves), the tube having an electrode of metal of
the alkaline group. They were able to adjust the combination so that the
presence of a little day-light would initiate a luminous discharge,
while in the dark such a charge ceased. § 14_a_.
99. ELSTER AND GEITEL’S EXPERIMENT. EFFECT OF POLARIZED LIGHT UPON THE
CATHODE. _Berlin Akad._ ’95. _Nature_, Lon., March 28, ’95, p. 514.
_Proc. Brit. Asso._, Aug. 16, ’94; Aug. 23, ’94, p. 406.—The X-rays have
properties similar to those of light, and have their source in
electricity. Quincke discovered that light which has been polarized
perpendicularly to the plane of incidence is greatly increased as to its
power of penetrating metals. Elster and Geitel used the following
apparatus to determine the relation between polarized light and
electricity. The current varied according to the angle of incidence and
the plane of polarization. The apparatus comprised the following
elements: An exhausted bulb, provided with a platinum anode, and a
cathode consisting of potassium and sodium, combined in the form of a
liquid alloy having a bright surface of reflection. The source of light
was an oxyhydrogen flame, which played upon zircon instead of lime; a
lens changed the diverging rays to parallel rays, which were polarized
by a Nichol prism and allowed to fall upon the cathode. The electrodes
of the vacuum bulb were connected to the poles of a generator of a
current of about 400 volts. “The strength, of the current was greatest
when the plane of polarization was perpendicular to the plane of
incidence—__i.e.__, when the electric displacements constituting light,
took place in the plane of incidence, and when the angle of incidence
was about 60°, __i.e.__, the polarizing angle of the alloy itself.”
Prof. Sylvanus P. Thompson confirmed these results by experiment. The
rate of discharge was greatest, he said, when the plane of polarization
was such that the Fresnellian vibration “chopped into” the surface.
Polarized light, he reminded them, produced similar results upon
selenium.
Although the domain of this book is necessarily limited to the
consideration of phenomena connected with the internal and external
energy of a discharge tube, yet if any other one subject is of special
interest and utility in connection with the consideration of X-rays, it
is that concerning the relation between the electric discharge and
light, which has been thoroughly studied only during the past few years,
and the accounts of the researches recorded in various periodicals and
academy papers. Those readers, however, who desire to study this
exceedingly interesting and novel branch of science, which in connection
with the action of the internal cathode rays and X-rays upon electrified
bodies, tends to uphold Maxwell’s theory as developed by mathematics and
based upon early known facts and predicted discoveries, may find volumes
upon this subject by referring to the citations below, named by Mr. N.
D. C. Hodges and obtained by him by a search in the archives of the
Astor Library. Of especial interest are those of Branly, § 99_I_, 99_J_,
99_Q_, 99_S_, 99_T_. Some notion as to the contents of the citations are
given here and there.
99_A_. KOCH’S EXPERIMENT. THE LOSS OF ELECTRICITY FROM A GLOWING
ELECTRIFIED BODY. _Wied. Ann._, XXXIII., p. 454, ’88.
99_B_. SCHUSTER AND ANPENIUS’ EXPERIMENT. THE INFLUENCE OF LIGHT ON
ELECTROSTATICALLY CHARGED BODIES. _Proc. R. So._, Lon., LXII., p. 371,
’87; _Proc. Swedish Acad._, LXIV., p. 405, ’87.—Many recent periodicals
have set forth that ultra-violet light will discharge only negatively
charged bodies. While this is practically or sometimes the case, yet
these experimenters found that a positive charge was dissipated very
slowly. They confirmed the results that the ultra-violet rays played the
principle part in the removal of a negative charge. Polishing the
surface accelerated the action. § 99, near beginning.
99_C_. RIGHI’S EXPERIMENT. SOME NEW ELECTRIC PHENOMENA PRODUCED BY
LIGHT. Note 2-4, _Rend. R. Acad. die Lincei_, May 6, 20, and June 3,
’88.
99_D_. RIGHI’S EXPERIMENT. SOME NEW ELECTRIC PHENOMENA PRODUCED BY
ILLUMINATION. _Rend. R. Acad. die Lincei._ VI., p. 135, 187,
’88.—Confirmation of the results of other physicists, and a quantitative
measurement determining that the E. M. F. between copper and selenium
was increased 25 per cent. by illumination by an arc light. The selenium
was in the form of crystals mounted upon a metal plate.
99_E_. STOLSTOW’S EXPERIMENT. ACTINO-CURRENT THROUGH AIR. _C. R._, CVI.,
pp. 1593 to 95, ’88.—Liquids tested. Greatest absorbents of active rays
most quickly discharged.
99_F_. RIGHI AND STOLSTOW’S EXPERIMENTS. KIND OF ELECTRIC CURRENT
PRODUCED BY ULTRA-VIOLET RAYS. _C. R._, CVI, pp. 1149 to 52, ’88.—The
discharge was accelerated by using a chemically clean surface. The
burning of metals, for example, aluminum, zinc or lead in the arc light
increased the discharging power.
99_G_. BICHAT & BLONDOT’S EXPERIMENT. ACTION OF ULTRA-VIOLET RAYS ON THE
PASSAGE OF ELECTRICITY OF LOW TENSION THROUGH AIR. _Comptes Rendus._
CVI, pp. 1,349 to 51. ’88.—They employed arc lamps whose carbons had
aluminum cores.
99_H_. NACARRI’S EXPERIMENT. THE DISSIPATION OF ELECTRICITY THROUGH THE
ACTION OF PHOSPHOROUS AND THE ELECTRIC SPARK. _Atti di Torino._ XXV, pp.
252 to 257. ’90.—The loss of charge was eighteen times less rapid in the
dark through the air in a bottle, than when a piece of luminous
phosphorous was placed in the bottle. The introduction of turpentine,
which checked the glowing of the phosphorous, retarded the loss of
charge.
[Illustration:
FROM SCIAGRAPH OF FROG, THROUGH SMALL HOLE IN DIAPHRAGM, AS IN FIG. 1,
p. 100.
]
99_I_. BRANLY’S EXPERIMENT. PHOTO-ELECTRIC CURRENT BETWEEN THE TWO
PLATES OF A CONDENSER. _C. R._ CX, pp. 898 to 901. ’91.—A positive
charge was dissipated, and by a peculiar arrangement of the plates,
screens, etc., and with particular materials, he was able to show that
the rates of loss of a positive and negative charge were about equal.
Numerous tests were instituted. If he is not mistaken, how closely
related are X-rays and light. § 90. Those who wish to more thoroughly
investigate this matter and verify the same, should study these
experiments more in detail in connection with Schuster’s and Anpenius’
experiments (§ 99_B_), whose arrangement of the plates was the same as
those of Branly.
99_J_. BRANLY’S EXPERIMENT. LOSS OF BOTH ELECTRICITIES BY ILLUMINATION
WITH RAYS OF GREAT REFRANGIBILITY. _C. R._ CX, pp. 751 to 754. ’90.
99_K_. RIGHI’S EXPERIMENT. ELECTRIC PHENOMENA PRODUCED BY ILLUMINATION.
_Luer’s Rep._ XXV, pp. 380 to 382. ’89.
99_L_. BORGMANN. ACTINO-ELECTRIC PHENOMENA. _C. R._ CVIII, p. 733. ’89.
_Jour. d. Russ. Phys. Chan. Ges._ (2) XXI, pp. 23 to 26. ’89.—The
photo-electric effect not instantaneous. A telephone served in the place
of the galvanometer to detect the discharge.
99_M_. STOLSTOW’S EXPERIMENT. ACTINO-ELECTRIC INVESTIGATIONS. _Jour. d.
Russ. Phys. Chan. Ges._ (7-8) XXI, pp. 159 to 207.—It is necessary that
the rays of light should be absorbed by the charged surface before
having the discharging influence. § 99_E_. All metals are subject to the
action, and also the aniline dyes. Two plates between which there is a
contact difference of potential generate a current so long as the
negative plate is illuminated. The effect is increased with the increase
of temperature and is only found in gases, and is therefore of the
nature of convection. He determined these principles by continuous work
for two years. It should be remembered that in all these researches, the
arc light is preferable, because the ultra-violet spectrum is six times
as long as that given by the sun.
99_N_. MEBIUS’ EXPERIMENT. AN ELECTRIC SPARK AND A SMALL FLAME EMPLOYED.
_Bihang till K. Svenska Vet.-Akad. Hand._ 15, _Afd._ 1, No. 4, p. 30,
’89.
99_O_. WORTHINGTON’S EXPERIMENT. DISCHARGE OF ELECTRIFICATION BY FLAMES.
_Brit. Asso. Rep._, ’90, p. 225.
99_P_. FLEMING’S EXPERIMENT. DISCHARGE BETWEEN ELECTRODES AT DIFFERENT
TEMPERATURES IN AIR AND IN HIGH VACUA. § 99_M_, near end. _Proc. Ro.
So._, LXVII., p. 118.
99_Q_. BRANLY’S EXPERIMENT. HALLWACH AND STOLSTOW’S EXPERIMENT. LOSS OF
ELECTRIC CHARGE. _Lum. Elect._, LXI., pp. 143 to 144, ’91.—Branly
obtained quantitative results. Hallwach found with the use of the arc
light, a very small loss of positive electricity at high potentials;
Stolstow, no such loss at potentials under 200 volts. Branly, with a 50
element battery and an arc light as the source of illumination, caused a
discharge and thereby a constant deflection of 124 degrees of the
galvanometer needle. The action of the light upon a positive disk caused
a deflection of only three degrees by the same battery. With aluminum in
the electrodes, the deflections were about 1400 and 24 respectively. Is
it not sufficiently fully established that ultra-violet light will
discharge not only negative but positive electricity? He experimented
with substances heated to glowing or incandescence. Glass lamp chimneys
at a dull, red heat, when covered with aluminum, oxide of bismuth, or
lead oxides, withdraw positive charges. In the same way, for example,
behaves a nickel tube in place of the lamp chimney.
99_R_. WANKA’S EXPERIMENT. A NEW DISCHARGE EXPERIMENT. _Abk. d. Deuts.
Math. Ges. in Rrag._, ’92, pp. 57 to 63.—He confirms the principle that
the ultra-violet rays are the most powerful. A glass plate, which, as
well known, cuts off most of the ultra-violet rays, was properly
interposed and then removed and the difference noted.
99_S_. BRANLY’S EXPERIMENT. DISCHARGE OF BOTH POSITIVE AND NEGATIVE
ELECTRICITY BY ULTRA-VIOLET RAYS. _C. R._, CXIV., pp. 68 to 70, ’92.—He
further proves that ultra-violet rays of light will dissipate a positive
charge. The experiments in this connection seem to prove more and more
that the discharging power is only a matter of sufficiently high
refrangibility of the rays of light.
99_T_. BRANLY’S EXPERIMENT. LOSS OF ELECTRIC CHARGE IN DIFFUSE LIGHT AND
IN THE DARK. _C. R._, CXVI., pp. 741 to 744. ’93.—A polished aluminum
sheet was attached to the terminal of an electroscope properly
surrounded by a metal screen. After a few days, the plate acted like any
other metal plate polished or unpolished; it lost its charge very
slowly, positive or negative alike, independently of the illumination.
If it is then again polished, as for example, with emery paper and
turpentine, it loses its charge rapidly in diffused light, which has
passed through a pane of window glass, for example. Therefore, the
ultra-violet rays are not alone effective, although most effective. The
longer the time elapsing, after polishing, the slower the discharge
takes place. Zinc behaved likewise, only more slowly. Other metals were
tried. Bismuth acted differently from most metals. Whether charged
positively or negatively, they exhibited rapid loss in the dark, in dry
air under a metal bell, independently of the state of the polish.
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