Referring to a footnote to my Royal Institution lecture, on page 11,
Messrs. Elster and Geitel have been good enough to call my attention to
a great deal of work done by them in the same direction. To make amends
for my ignorance of this work at the time of my Royal Institution
lecture, and to make it better known in this country, I make abstract
of their Papers as follows:--
_Wiedemann’s Annalen, 38, p. 40.--“On the Dissipation of Negative
Electricity by Sun- and Daylight.”_
With a view to Arrhenius’ theory concerning atmospheric electricity,
we arranged experiments on the photo-electric power of sunlight and
diffuse daylight at Wolfenbüttel from the middle of May to the middle
of June, 1889. Hoor alone had observed the effect of sunlight; other
experimenters had failed to find it, but we find a discharging effect
even in diffuse daylight.
We take an insulated zinc dish, 20 cm. diameter, connect it to a
quadrant electrometer or an Exner’s electroscope, and expose it in
the open so that it can be darkened or illuminated at pleasure.
Sunlight makes it lose a negative charge of 300 volts in about 60
seconds. A positive charge of 300 volts is retained. The dissipation
of negative electricity ceases in the dark, and is much weakened by
the interposition of glass. But light from the blue sky has a distinct
effect. Fill the dish with water, or stretch a damp cloth over it, and
the action stops. A freshly-scrubbed plate acquires a positive charge
of 2½ volts, which can be increased by blowing.
With freshly-cleansed wires of zinc, aluminium, or magnesium attached
to the knob of the electroscope, a permanent negative charge is
impossible in open sunlight. Indeed, magnesium shows a dissipating
action in diffuse evening light. Such wires act like glowing bodies.
Exposing an electroscope so provided in an open space it acquires
a positive charge from the atmosphere. No abnormal dissipation of
positive electricity has been observed.
_Wied. Ann., 38, p. 497.--Continuation of Same Subject._
Our success last time was largely due to the great clearness of the sky
in June, and we wished to see if we could get the same effect at the
beginning of the winter.
The following is our summary of results:
Bright fresh surfaces of the metals zinc, aluminium, magnesium were
discharged by both sun- and daylight when they were negatively charged;
and they spontaneously acquired a positive charge, whose amount could
be increased by blowing.[35] A still more notable sensitiveness to
light is shown by the amalgams of certain metals, viz., in the order of
their sensitiveness, K, Na, Zn, Sn. Since pure mercury shows no effect,
the hypothesis is permissible that the active agent is the metal
dissolved in the mercury. If so, the following are the most active
metals:--
[35] A fact noticed by Bichat and Blondlot.
K, Na (Mg, Al), Zn, Sn.
All other metals tried, such as Sn, Cd, Pb, Cu, Fe, Hg, Pt, and gas
carbon, show no action. The same is true of nearly all non-metallic
bodies; but one of them--namely, the powder of _Balmain’s luminous
paint_--acted remarkably well in sunlight. Of liquids, hot and cold
water, and hot and cold salt solution were completely inactive;
consequently wetting the surface of metals destroys their sensibility
to light.
The illumination experiments can be arranged in either of two ways. For
experiments in free space we use zinc, aluminium, or magnesium wires,
or small amalgamated spheres of zinc provided with an iron rod. With
these it can be easily shown that the illuminated surface of certain
metals act in the same way as a flame collector.
For demonstration experiments the apparatus described[36] is better,
and with this we show the following:--
Amalgamated zinc, negatively charged, discharges almost instantly in
sunlight; and if near a positively-electrified body charges itself
positively.
The same thing happens, though more slowly, in diffuse daylight. Red
glass stops the action, but the following let some through:--Selenite,
mica, window glass, blue (cobalt) glass.
[36] In this apparatus the mercury amalgams of K and Na are run through
a fine funnel, so that the freshly-formed surface of the drops may be
illuminated. Under these circumstances, while pure mercury fell from
-185 to -175 volts in 30sec., amalgam of zinc fell from -195 to -116 in
15sec., amalgam of sodium fell from -195 to 0 in 10sec., and an amalgam
of potassium fell from -195 to 0 in 5sec.
[Illustration: FIG. 59.
_Explanation of Fig. 59._--B′ is a brightly polished amalgamated zinc
plate attached to the negative pole of a Holtz machine, with the
positive knob from 6 to 10 centimetres distant. The source of a light
is a strip of burning magnesium ribbon 30 to 50 centimetres away.
Whenever the spark is able just to choose the path B B′, light shining
on the zinc plate checks it and transfers the spark to A A′.]
_Wied. Ann., 39, p. 332.--On a Checking Action of Illumination on
Electric Spark and Brush Discharge._
If sparks are just able to occur between a brass knob and a clean
amalgamated zinc cathode, illumination of the latter by ultra-violet
light tends to check them. (This is a curious inversion of Hertz’s
fundamental experiment on the subject. It is an effect I have not yet
observed; but Elster and Geitel’s arrangement differs from mine[37] in
that the surfaces are at a steady high potential before the spark, so
that light can exert its discharging influence, whereas in mine the
surfaces were at zero potential until the spark-rush occurred. Hertz’s
arrangement was more like mine, inasmuch as he illuminated the knobs
of an induction coil on the verge of sparking. It appears, then, that
whereas the action of light in discharging negative electricity from
clean oxidisable metallic surfaces is definite enough, its influence
on a spark discharge differs according to the conditions of that
discharge--in cases of “steady strain” it tends to hinder the spark; in
cases of “sudden rush” it tends to assist it.--O. J. L.)
[37] _See_ Fig. 7, page 9.
_Wied. Ann., 41, p. 161.--On the use of Sodium-Amalgam in
Photo-electric Experiments._
Elster and Geitel have repeated some of Righi’s experiments on the
discharge of negative electricity from metals in rarefied air, and
find, in agreement with him, that a reduction of pressure to about one
millimetre increases the discharge velocity about six or seven times.
They proceed to try sodium-amalgam exposed to daylight in exhausted
tubes, and describe apparatus for the purpose. Such an arrangement
simply cannot hold a negative charge in bright daylight, even although
it be unprovided with quartz windows. Even paraffin lamps and sodium
flames exert some action.
They observe that under the action of light the boundary surface of the
metal and glass changes, and the metal begins to cling to the glass.
They suppose that Warburg’s vacuum tubes of pure sodium may behave
similarly, and show photo-electric sensibility.
_The Same, p. 166.--On a Checking Action of Magnetism on Photo-Electric
Discharge in Rarefied Gases._
The authors point out analogies between the above effects and those
they had observed in the action of glowing bodies in air, and they
mention Lenard and Wolf’s experiments (_Wied. Ann._ XXXVII., p.
443), tending to show that the effect is due to a disintegrating
or evaporative effect of light on surfaces. Elster and Geitel had
observed that the discharging power of glowing bodies was diminished by
application of a magnetic field, the effect being the same as if the
temperature was lowered; and they proceed to try if the discharge of
negative electricity from illuminated surfaces in highly-rarefied gas
could also be checked or hindered by a magnetic field. They find that
it can.
[Illustration: FIG. 60.
_Explanation of Fig. 60._--The sodium and mercury are introduced
through the tube S into the globe K. The tube S is then closed, a pump
applied to X, and exhaustion carried on for some days. T is an open
funnel sealed into the tube (as is done in some vacuum tubes made by
Holtz) to show a curious unilateral conductivity of rarefied gas. The
object of this funnel is to permit metal from the interior, free from
scum, to be introduced from K to D when the whole is tilted. Thus
a bright surface is exposed to the earth ring R. It can be charged
negatively, and its leak under illumination be measured, through the
terminal D. Sometimes the tube is inverted, so that the active surface
may be at D′, further from the earth wire.]
Using the light from sparks admitted through a quartz window into the
vacuum tube when a negatively charged amalgamated zinc surface was
exposed near an earth-connected platinum ring, and between the poles of
a small electromagnet, they found that when the tube was full of air
at 10 mm. pressure the magnet had but little effect, but that at 0·15
mm., whereas without the magnet the charge of -270 volts disappeared
completely in five seconds, when the magnet was excited it only fell
about half that amount in the same time. With hydrogen at 0·24 mm. the
result was much the same, and at either greater or less pressure in
both cases the magnet had less effect. In oxygen the loss of charge
was not quite so rapid; and, again, at a pressure of O·1mm., the
magnet more than halved the rate. But in CO₂ the rapidity of loss was
extreme.[38] Either at 1·1 mm. or at 0·005 mm. the charge of 270 volts
leaked away completely in two seconds when the magnet was not excited;
but in the latter case (low pressure) exciting the magnet reduced the
speed by about one-half. At the pressure of 1·1 mm. the magnet did not
seem to produce an effect. With daylight the results are similar.
[38] Corresponding to the activity of this gas as found by Wiedemann
and Ebert (_Wied. Ann._, XXXIII., p. 258), in their researches on the
influence of light on ease of sparking.
[Illustration: FIG. 61.
_Explanation of Fig. 61._--P is the plate of amalgamated zinc, and R
is the earth ring, as before. Ultra-violet light is introduced through
a quartz window Q from a spark gap _r_. The vessel has a joint at the
middle, so that the sensitive plate can be got at and changed. Magnet
poles are applied outside this vessel in various positions.]
The authors then discuss the meaning of the result, and its bearing
on the opposition hypotheses of Lenard and Wolf and of Righi. Lenard
and Wolf’s view is that the loss of negative electricity is due to
dust disintegrated from the surface by the action of light, but whose
existence they consider is established by an observed effect on steam
jets. Righi, on the other hand, believes that gas molecules themselves
act the part of electric carriers. Elster and Geitel consider that the
magnetic effect observed by them supports this latter view, it being
known that a magnet acts on currents through gases; and they surmise
that the impact of light vibrations may directly assist electric
interchange between a gas molecule and the surface, by setting up in
them syntonic stationary vibrations, something like resonant Leyden
jars. It is to be remembered that phosphorescent substances, such
as Balmain’s paint powder, exhibit marked photo-electric effect in
daylight.
[Illustration: FIG. 62.
_Explanation of Fig. 62._--A simpler arrangement, like the one above
(Fig. 61), whereby clean liquid alkali metals can be introduced into
the experimental chamber B, from the preliminary chamber A, through a
cleansing funnel, F, which dips its beak into the interior.]
The unilateral character of the electric motion, and the charging of
neutral surfaces by light, require special hypotheses, concerning an
E.M.F. at the boundary of gases and conductors, such as Schuster and
Lehmann have made.
_Weid. Ann. 42, p. 564.--Note on a New Form of Apparatus for
Demonstrating the Photo-electric Discharging Action of Daylight._
A vacuum tube suitable for experiments with sodium-amalgam or pure
sodium, or the liquid sodium-potassium alloy, is described, with the
aid of which a current (shown by the charge of an electroscope) can be
maintained by a dry pile through the rarefied gas above the metal when
it is illuminated from ordinary windows.
_Wied. Ann. 43, p. 225.--On the Dependence of the Discharging Action of
Light on the Nature of the Illuminated Surface._
Experiments also on differently-coloured lights. Summary of results.
The photo-electrically active metals arrange themselves in the
following order--Pure K, alloys of K and Na, pure Na. Amalgams of Rb,
K, Na, Li, Mg (Tl, Zn); the same as their voltaic order. With the most
sensitive term of the series a candle six metres off can be detected,
and the region of spectral red is not inactive. The later terms of the
series demand smaller waves, and even for potassium blue light gives a
much greater effect than red. No discharge of positive electricity is
observable with these substances.
_Wied. Ann. 44, p. 722.--On the Dissipation of Electric Charge from
Mineral Surfaces by Sunlight._
Hitherto only Balmain’s paint powder has been observed to be active
among non-metallic substances. Now they try other phosphorescent
bodies, and arrive at the following results:--
Fluorspar is conspicuously photo-electric, both in sunlight and
daylight, especially the variety of fluorite called stinkfluss.
Freshly-broken surfaces discharge much more rapidly than old surfaces.
Blue waves, and not alone the ultra-violet, have a perceptible effect
on fluorspar.
In a vacuum the mineral loses its photo-electric sensibility and its
conductivity too. Contact with damp air restores its sensibility.
Moistening with water weakens but does not destroy the sensitiveness.
On the other hand, igniting the mineral destroys both its
photo-electric power and its exceptional phosphorescent property.
Distinct traces of photo-electric power are shown by the following
minerals also: Cryolite, heavy spar, celestine, arragonite,
strontianite, calcspar, felspar and granite.
The hypothesis that the power of phosphorescing when illuminated is
approximately a measure of the discharging power of light has been
verified in many cases; the exceptions can probably be explained by
the influence which the electrical conductivity of the illuminated
substance exerts on the rate of discharge of electricity from its
surface. This agreement confirms the view expressed by us on the
occasion of experiments with Balmain’s paint, that, during electrical
discharge by light, actions take place which are analogous to those of
resonance. Messrs. Wiedemann and Ebert had previously been led by other
considerations to the same conclusion.
We are compelled by the results of the present experiments to conclude
that a more rapid discharge of electricity into the atmosphere takes
place in sunlight than in darkness from the surfaces of the earth,
which is composed of mineral particles charged, as the positive sign of
the slope of atmospheric potential indicates, with negative electricity.
[Illustration: FIG. 63.
_Explanation of Fig. 63._--Arrangement used by Elster and Geitel
for exposing various phosphorescent minerals to daylight, while
under inductive charge. They were put in powder in the tray P, and
the transparent wire-gauze N above them was charged positively from
a battery. The metal cover M M′ could be removed and replaced at
pleasure, and the effect on a delicate quadrant electrometer connected
to P observed. By this method considerable tension can be got up on the
mineral surface, notwithstanding that it is close upon zero potential.
The light effect depends on tension, not potential.]
It seems to us evident that there exists a direct electric action of
sunlight upon the earth, and that we have given experimental evidence
in favour of the theory put forward by von Bezold and Arrhenius,
according to which the sun acts on the earth, not by electrostatic or
electro-dynamic action-at-a-distance, which would involve difficulties
of a theoretical character, but through the medium of the electrical
forces of light waves. We hope soon to establish the consequences of
this theory in meteorology in another Paper, giving the results of two
years’ observations on the intensity of the most refrangible rays of
sunlight and of the slope of atmospheric potential.
_Wied. Ann., 48, p. 338.--Experiments on the Gradient of Atmospheric
Potential and on Ultra-Violet Solar Radiation._
Elster and Geitel describe the observations they have made for two
years on solar radiation, at observing stations of low and high
altitude, as tested by its electrical discharging power; and they plot
curves of such effective radiation for days and months along with the
curves of atmospheric potential observed at the same places. These
curves are of much interest, and need study. Incidentally they find
that, of the whole effective solar radiation, 60 per cent. was absorbed
at altitudes above 3,100 metres; 23 per cent. of the remainder was
absorbed in the layer between this and a station at 1,600 metres; and
47 per cent. was absorbed between this and 80 metres above sea level.
Or, in other words, of 236 parts which enter the atmosphere 94 reach
the highest observing station (Sonnblickgipfel), 72 the middle one
(Kolm-Saigurn), and 38 the lowest (Wolfenbüttel). They discuss the
question as to how far the daily variation of terrestrial magnetism is
due to electrical currents in the atmosphere excited by sunshine and
other meteorological matters.
(The Paper and plates are worthy of reproduction in full in the
_Philosophical Magazine_.)
_Weid. Ann., 46, p. 281.--On the Behaviour of Alkali Metal Cathodes in
Geissler Tubes; On Photo-Electric Discharge in a Magnetic Field; and On
the Measure of Photo-Electric Currents in Potassium Cells by means of a
Galvanometer._
Results:--The resistance of a Geissler tube provided with a cathode
surface of pure alkali metal is diminished by the light from the sparks
of an induction coil; especially when the pressure is ·1 to ·01 mm.
of mercury. The resistance which rarefied gas opposes to an electric
current in a magnetic field is greatest in the direction normal to
the magnetic lines. The changes of resistance effected by any kind
of light in a vacuum tube with alkali metal cathode can be measured
galvanometrically. (A Daniell cell gives 100 divisions on a Rosenthal
galvanometer when coupled up through such an illuminated tube, each
division meaning about 10⁻¹⁰· ampere.)
[Illustration: FIG. 64.
_Explanation of Fig. 64._--A vacuum tube of rarefied hydrogen
containing alkali metal as cathode, say the liquid K-Na alloy, or solid
K or Na. A spark gap at S serves as alternative path, and a stream
of sparks can occur to the plate P in the dark. But when light falls
on the surface A this stream of sparks can cease, showing that the
resistance of the vacuum tube is diminished.]
[Illustration: FIG. 65.
_Explanation of Fig. 65._--Showing position of magnetic poles with
respect to the vacuum tube discharge. With the poles _across_ the
line of discharge, as in Fig. on left, excitation of the magnet
opposes the leak from the surface. With the poles as in Fig. on right,
the discharge is not much affected, it is even sometimes slightly
increased.]
[Illustration: FIG. 66.
_Explanation of Fig. 66._--Potassium vacuum bulbs containing
⅓ millimetre of hydrogen mounted and connected to battery and
galvanometer, and arranged as a photo-electric photometer.]
_Wied. Ann., 48, p. 625. On the Photo-Electric Comparison of Sources of
Light._
Attempts to make such a potassium cell into a photometer.
_Wied. Ann. 52, p. 433. Further Photo-Electric Experiments._
Plates of platinum, silver, copper need exceedingly ultra-violet light
before they show any photo-electric power; zinc, aluminium, magnesium
show it for visible violet and blue light; the alkali metals, in an
atmosphere of rarefied hydrogen, advance their range of sensibility
into the spectral red; while under the most favourable conditions they
show a sensibility only inferior to that of the eye itself. The authors
now use galvanometric methods of measuring the effect, instead of only
electrometers, and they arrive at the following results:--
(1) The three alkali metals Na, K, Rb, have different sensibility for
differently-coloured lights. For long waves their order of sensibility
is Rb, Na, K; though rhubidium is far exceeded by the other two metals
in white light.
(2) Illumination of a plane alkali metal cathode surface with polarised
light causes greatest discharge if the plane of polarisation is normal
to plane of incidence; and least, if the two coincide.
(This is a most remarkable observation. Its probable meaning is that
the electric oscillations of light are photo-electrically effective
in so far as they are normal to the surface on which they act; while
electric oscillations tangential to the surface are scarcely operative.
Different angles of incidence must be tried before the proof is
complete.--O. J. L.)
(3) Electric oscillations of very short period, such as are given by
a Hertz oscillator, are commutated by illumination in the presence of
alkali metals in rarefied gas, so as to be able to set up a constant
electric tension in the gas.
(A Zehnder tube[39] was used, and the momentary phases of the
oscillation during which the metal is negatively charged are apparently
taken advantage of by the illumination.)
(4) The photo-electric dissipation showed by powdered fluorspar is
dependent on the colour of the mineral, in such a way that the deepest
blue, violet or green specimens are the most sensitive.
[39] See Fig. 13, p. 15.