The following pages (up to page 42) are the Notes of a Lecture
delivered by Dr. O. J. Lodge before the Royal Institution of Great
Britain on Friday evening, June 1, 1894. These notes have been revised
by Dr. Lodge, and prepared for publication in the form here presented.
After page 42 an account is given of the later applications of Hertzian
wave experiments to wireless telegraphy, and a series of Appendices are
also given.
_Introductory._--1894.
The untimely end of a young and brilliant career cannot fail to strike
a note of sadness and awaken a chord of sympathy in the hearts of his
friends and fellow-workers. Of men thus cut down in the early prime
of their powers there will occur to us here the names of Fresnel, of
Carnot, of Clifford, and now of Hertz. His was a strenuous and favoured
youth; he was surrounded from his birth with all the influences that
go to make an accomplished man of science--accomplished both on the
experimental and on the mathematical side. The front rank of scientific
workers is weaker by his death, which occurred on January 1, 1894, the
thirty-seventh year of his life. Yet did he not go till he had effected
an achievement which will hand his name down to posterity as the
founder of an epoch in experimental physics.
In mathematical and speculative physics others had sown the seed. It
was sown by Faraday, it was sown by Thomson and by Stokes, by Weber
also doubtless, and by Helmholtz; but in this particular department
it was sown by none more fruitfully and plentifully than by Clerk
Maxwell. Of the seed thus sown Hertz reaped the fruits. Through his
experimental discovery, Germany awoke to the truth of Clerk Maxwell’s
theory of light, of light and electricity combined, and the able army
of workers in that country (not forgetting some in Switzerland, France,
and Ireland) have done most of the gleaning after Hertz.
This is the work of Hertz which is best known, the work which brought
him immediate fame. It is not always that public notice is so well
justified. The popular instinct is generous and trustful, and it is
apt to be misled. The scientific eminence accorded to a few energetic
persons by popular estimate is more or less amusing to those working on
the same lines. In the case of Hertz no such mistake has been made. His
name is not over well-known, and his work is immensely greater in every
way than that of several who have made more noise.
His best known discovery is by no means his only one, and no less
than eighteen Papers were contributed to German periodicals by him,
in addition to the papers incorporated in his now well-known book on
electric waves.
In closing these introductory and personal remarks, I should like to
say that the enthusiastic admiration for Hertz’s spirit and character
felt and expressed by students and workers who came into contact with
him is not easily to be exaggerated. Never was a man more painfully
anxious to avoid wounding the susceptibilities of others; and he was
accustomed to deprecate the prominence given to him by speakers and
writers in this country, lest it might seem to exalt him unduly above
other and older workers among his own sensitive countrymen.
Speaking of the other great workers in physics in Germany, it is not
out of place to record the sorrow with which we have heard of the
recent death of Dr. August Kundt, Professor in the University of
Berlin, successor to Von Helmholtz in that capacity.
When I consented to discourse on the work of Hertz, my intention was to
repeat some of his actual experiments, and especially to demonstrate
his less-known discoveries and observations. But the fascination
exerted upon me by electric oscillation experiments, when I, too,
was independently working at them in the spring of 1888,[1] resumed
its hold, and my lecture will accordingly consist of experimental
demonstrations of the outcome of Hertz’s work rather than any precise
repetition of portions of that work itself.
[1] _Phil. Mag._, XXVI., pp. 229, 230, August, 1888; or “Lightning
Conductors and Lightning Guards,” pp. 104, 105; also _Proc. Roy. Soc._,
Vol. 50, p. 27.
In case a minority of my audience are in the predicament of not knowing
anything about the subject, a five minutes’ explanatory prelude may be
permitted; and the simplest way will be for me hastily to summarise our
knowledge of the subject before the era of Hertz.
[Illustration: FIG. 1.--Oscillations of Dumb-bell Hertz Vibrator (after
Bjerknes).]
Just as a pebble thrown into a pond excites surface ripples, which
can heave up and down floating straws under which they pass, so a
struck bell or tuning fork emits energy into the air in the form of
what are called sound waves, and this radiant energy is able to set up
vibrations in other suitable elastic bodies.
If the body receiving them has its natural or free vibrations violently
damped, so that when left to itself it speedily returns to rest (Fig.
1), then it can respond fully to notes of almost any pitch. This is
the case with your ears and the tones of my voice. Tones must be
exceedingly shrill before they cease to excite the ear at all.
If, on the other hand, the receiving body has a persistent period of
vibration, continuing in motion long after it is left to itself (Fig.
2) like another tuning fork or bell, for instance, then far more
facility of response exists, but great accuracy of tuning is necessary
if it is to be fully called out; for if the receiver is not thus
accurately syntonised with the source, it fails more or less completely
to resound.
[Illustration: FIG. 2.--Oscillation of Ring-shaped Hertz Resonator
excited by Syntonic Vibrator (after Bjerknes).]
[Illustration: FIG. 3.--Oscillation of Ring Resonator not quite
syntonic with Radiator. (For method of obtaining these curves see Fig.
14.)]
Conversely, if the _source_ is a persistent vibrator, correct tuning
is essential, or it will destroy at one moment (Fig. 3) motion which
it originated the previous moment. Whereas, if it is a dead-beat or
strongly-damped exciter, almost anything will respond equally well or
equally ill to it.
What I have said of sounding bodies is true of all vibrators in a
medium competent to transmit waves. Now a sending telephone or a
microphone, when spoken to, emits waves into the ether, and this
radiant energy is likewise able to set up vibration in suitable bodies.
But we have no delicate means of directly detecting these electrical or
etherial waves; and if they are to produce a perceptible effect at a
distance, they must be confined, as by a speaking-tube, prevented from
spreading, and concentrated on the distant receiver.
This is the function of the telegraph wire; it is to the ether what a
speaking-tube is to air. A metal wire in air (_in function_, not in
details of analogy) is like a long hollow cavity surrounded by nearly
rigid but slightly elastic walls.
_Sphere charged from Electrophorus._
Furthermore, any conductor electrically charged or discharged with
sufficient suddenness must emit electrical waves into the ether,
because the charge given to it will not settle down instantly, but will
surge to and fro several times first; and these surgings or electric
oscillations must, according to Maxwell, start waves in the ether,
because at the end of each half-swing they cause electrostatic, and at
the middle of each half-swing they cause electromagnetic effects, and
the rapid alternation from one of these modes of energy to the other
constitutes etherial waves.[2] If a wire is handy they will run along
it, and may be felt a long way off. If no wire exists they will spread
out like sound from a bell, or light from a spark, and their intensity
will decrease according to the inverse square of the distance.
[2] Strictly speaking, in the waves themselves there is no lag or
difference of phase between the electric and the magnetic vibrations;
the difference exists in emitter or absorber, but not in the
transmitting medium. True radiation of energy does not begin till about
a quarter wave length from the source, and within that distance the
initial quarter period difference of phase is obliterated.
Maxwell and his followers well knew that there would be such waves;
they knew the rate at which they would go, they knew that they would
go slower in glass and water than in air, they knew that they would
curl round sharp edges, that they would be partly absorbed but mainly
reflected by conductors, that if turned back upon themselves they
would produce the phenomena of stationary waves, or interference, or
nodes and loops; it was known how to calculate the length of such
waves, and even how to produce them of any required or predetermined
wave length from 1,000 miles to a foot. Other things were known about
them which would take too long to enumerate; any homogeneous insulator
would transmit them, would refract or concentrate them if it were of
suitable shape, would reflect none of a particular mode of vibration at
a certain angle, and so on, and so on.
All this was _known_, I say, known with varying degrees of confidence;
but by some known with as great confidence as, perhaps even more
confidence than, is legitimate before the actuality of experimental
verification.
Hertz supplied the verification. He inserted suitable conductors in the
path of such waves, conductors adapted for the occurrence in them of
induced electric oscillations, and to the surprise of everyone, himself
doubtless included, he found that the secondary electric surgings thus
excited were strong enough to display themselves by minute electric
sparks.
[Illustration: FIG. 4.--Experiment with Syntonic Leyden Jars (_cf._
page 21).]
_Syntonic Leyden Jars._
I shall show this in a form which requires great precision of tuning or
syntony, both emitter and receiver being persistently vibrating things
giving some 30 or 40 swings before damping has a serious effect. I take
two Leyden jars with circuits about a yard in diameter, and situated
about two yards apart (Fig. 4). I charge and discharge one jar, and
observe that the surgings set up in the other can cause it to overflow
if it is syntonised with the first.[3]
[3] See _Nature_, Vol. XLI., p. 368, where I first described this
experiment; or quotation in J. J. Thomson’s “Recent Researches,” p.
395.
A closed circuit such as this is a feeble radiator and a feeble
absorber, so it is not adapted for action at a distance. In fact, I
doubt whether it will visibly act at a range beyond the ¼λ at which
true radiation of broken-off energy occurs. If the coatings of the
jar are separated to a greater distance, so that the dielectric is
more exposed, it radiates better; because in true radiation the
electrostatic and the magnetic energies must be equal, whereas in a
ring circuit the magnetic energy predominates. By separating the coats
of the jar as far as possible we get a typical Hertz vibrator (Fig.
5), whose dielectric extends out into the room, and thus radiates very
powerfully.
[Illustration: FIG. 5.--Standard Hertz Radiator.]
_Ordinary Size Hertz Vibrator._
In consequence of its radiation of energy, its vibrations are rapidly
damped, and it only gives some three or four good strong swings (Fig.
1). Hence it follows that it has a wide range of excitation; _i.e._, it
can excite sparks in conductors barely at all in tune with it.
The two conditions, conspicuous energy of radiation and persistent
vibration electrically produced, are at present incompatible. Whenever
these two conditions coexist, considerable power or activity will,
of course, be necessary in the source of energy. At present they
only coexist in the sun and other stars, in the electric arc, and in
furnaces.
_Two Circular Vibrators Sparking in sympathy._
The receiver Hertz used was chiefly a circular resonator (Fig. 6), not
a good absorber but a persistent vibrator, well adapted for picking
up disturbances of precise and measurable wave length. Its mode of
vibration when excited by emitter in tune with it is depicted in Fig.
2. I find that the circular resonators can act as senders too; here is
one exciting quite long sparks in a second one.
_Electric Syntony._--That was his discovery, but he did not stop there.
He at once proceeded to apply his discovery to the verification of
what had already been predicted about the waves, and by laborious and
difficult interference experiments he ascertained that the previously
calculated length of the waves was thoroughly borne out by fact. These
interference experiments in free space are his greatest achievement.
[Illustration: FIG. 6.--Circular Resonator. (The knobs ought to nearly
touch each other.)]
[Illustration: FIG. 6A.--Any circular Resonator can be used as a sender
by bringing its knobs near the sparking knobs of a coil; but a simple
arrangement is to take two semi-circles, as in above figure, and make
them the coil terminals. The capacity of the cut ends can be varied,
and the period thereby lengthened, by expanding them into plates.]
He worked out every detail of the theory splendidly, separately
analysing the electric and the magnetic oscillation, using language
not always such as we should use now, but himself growing in theoretic
insight through the medium of what would have been to most physicists a
confusing maze of troublesome facts, and disentangling all their main
relations most harmoniously.
_Holtz Machine, A and B Sparks; Glass and Quartz Panes in Screen._
While Hertz was observing sparks such as these, the primary or exciting
spark and the secondary or excited one, he observed as a bye issue
that the secondary spark occurred more easily if the light from the
primary fell upon its knobs. He examined this new influence of light
in many ways, and showed that although spark light and electric brush
light were peculiarly effective, any source of light that gave very
ultra-violet rays produced the same result.
[Illustration: FIG. 7.--Experiment arranged to show effect on one spark
of light from another. The =B= spark occurs more easily when it can
see the =A= spark through the window, unless the window is glazed with
glass. A quartz pane transmits the effect: glass cuts it off.]
The above figure represents my way of showing the experiment. It
will be observed that with this arrangement the =B= knobs are at
the same potential up to the instant of the flash, and in that case
the ultra-violet portion of the light of the =A= spark assists the
occurrence of the =B= spark. But it is interesting to note what Elster
and Geitel have found (_see_ Appendix IV., Fig. 59), that if the
=B= knobs were subjected to steady strain instead of to impulsive
rush--_e.g._, if they were connected to the inner coats of the jars
instead of the outer coatings--that then the effect of ultra-violet
light on either spark gap would exert a deterrent influence, so that
the spark would probably occur at the other, or non-illuminated
gap. With the altered connections it is, of course, not feasible to
illuminate one spark by the light of the other; the sparks are then
alternative, not successive.
Wiedemann and Ebert, and a number of experimenters, have repeated and
extended this discovery, proving that it is the cathode knob on which
illumination takes effect; and Hallwachs and Righi made the important
observation, which Elster and Geitel, Stoletow, Branly, and others have
extended, that a freshly-polished zinc or other oxidisable surface, if
charged negatively, is gradually discharged by ultra-violet light.
[Illustration: FIG. 8.--Zinc Rod in Arc Light, protected by Glass
Screen. The lenses are of quartz, but there is no need for any lenses
in this experiment; leakage of electricity begins directly the glass
plate is withdrawn.]
It is easy to fail in reproducing this experimental result if the right
conditions are not satisfied; but if they are it is absurdly easy, and
the thing might have been observed nearly a century ago.
_Zinc discharging Negative Electricity in Light; Gold Leaf
Electroscope; Glass and Quartz Panes; Quartz Prism._
Take a piece of zinc, clean it with emery paper, connect it to a gold
leaf electroscope, and expose it to an arc lamp. (Fig. 8). If charged
positively nothing appears to happen, the action is very slow; but a
negative charge leaks away in a few seconds if the light is bright.
Any source of light rich in ultra-violet rays will do; the light from
a spark is perhaps most powerful of all. A pane of glass cuts off all
the action; so does atmospheric air in sufficient thickness (at any
rate, town air), hence sunlight is not powerful. A pane of quartz
transmits the action almost undiminished, but fluorspar may be more
transparent still. Condensing the arc rays with a quartz lens and
analysing them with a quartz prism or reflection grating, we find that
the most effective part of the light is high up in the ultra-violet,
surprisingly far beyond the limits of the visible spectrum[4] (Fig. 9,
next page).
[4] While preparing for the lecture it occurred to me to try, if
possible during the lecture itself, some new experiments on the effect
of light on negatively charged bits of rock and ice, because if the
effect is not limited to metals it must be important in connection with
atmospheric electricity. When Mr. Branly coated an aluminium plate with
an insulating varnish, he found that its charge was able to soak in and
out of the varnish during illumination (_Comptes Rendus_, Vol. CX., p.
898, 1890). Now the mountain tops of a negatively charged earth are
exposed to very ultra-violet rays, and the air is a dielectric in which
quiet up-carrying and sudden downpour of electricity could go on in a
manner not very unlike the well-known behaviour of water vapour; and
this perhaps may be the reason, or one of the reasons, why it is not
unusual to experience a thunderstorm after a few fine days. I have now
tried these experiments on such geological fragments as were handy, and
find that many of them discharge negative electricity under the action
of a naked arc, especially from the side of the specimens which was
somewhat dusty, but that when wet they discharge much less rapidly, and
when positively charged hardly at all. Ice and garden soil discharge
negative electrification, too, under ultra-violet illumination, but
not so quickly as limestone, mica schist, ferruginous quartz, clay,
and some other specimens. Granite barely acts; it seems to insulate
too well. The ice and soil were tried in their usual moist condition,
but, when thoroughly dry, soil discharges quite rapidly. No rock tested
was found to discharge as quickly as does a surface of perfectly
bright metal, such as iron, but many discharged much more quickly than
ordinary dull iron, and rather more quickly than when the bright iron
surface was thinly oiled or wetted with water. To-day (June 5, 1894)
I find that the leaves of Geranium discharge positive electrification
five times as quickly as negative, under the action of an arc light,
and that glass cuts the effect off while quartz transmits it. (For
Elster and Geitel’s experiments, and those of Righi, _see_ Appendices,
p. 115 _et seq._)
This is rather a digression, but I have taken some pains to show it
properly because of the interest betrayed by Lord Kelvin on this
matter, and the caution which he felt about accepting the results of
the Continental experimenters too hastily.
It is probably a chemical phenomenon, and I am disposed to express it
as a modification of the Volta contact effect[5] with illumination.
[5] _See_ B. A. Report, 1884, pp. 502-519; or _Phil. Mag._, Vol. XIX.,
pp. 267-352.
[Illustration: FIG. 9.--Zinc Rod discharging Negative Electricity in
the very Ultra-violet Light of a Spectrum formed by a Quartz Train. The
best discharging light is found far beyond the limits of the visible
spectrum.]
Return now to the Hertz vibrator, or Leyden jar with its coatings well
separated, so that we can get into its electric as well as its magnetic
field. Here is a great one giving waves 30 metres long, radiating while
it lasts with an activity of 100 H.P., and making ten million complete
electric vibrations per second. It is made of four large copper sheets
soldered together two and two, strung up by thin rope to a gallery,
and each pair connected with the other by several yards of No. 0 pure
copper rod, interrupted by a pair of sparking knobs (Fig. 10).
_Large Hertz Vibrator in action; Abel’s Fuse Detector; Vacuum Tube
Detector; Striking of an Arc._
Its great radiating power damps it down very rapidly, so that it does
not make above two or three swings; but nevertheless, each time it is
excited, sparks can be drawn from most of the reasonably elongated
conductors in the theatre of this Institution, and indeed from wire
fencing and iron roofs outside this building.
A suitably situated gas leak can be ignited by these induced sparks.
An Abel’s fuse connecting the water pipes with the gas pipes will blow
off; vacuum tubes connected to nothing will glow (this fact has been
familiar to all who have worked with Hertz waves since 1889), electric
leads, if anywhere near each other, as they are in some incandescent
lamp-holders, may spark across to each other, thus striking an arc
and blowing their fuses. This blowing of fuses by electric radiation
frequently happened at Liverpool till the suspensions of the theatre
lamps were altered. They had at first been held in position by wire
guides, which served as collectors of the Hertz waves or impulses.
[Illustration: FIG. 10.--Hertz Oscillator on reduced scale, ⅒th inch to
a foot.]
The striking of an arc by the little reverberating sparks between two
lamp-carbons connected with the 100-volt mains I incidentally now
demonstrate. An arc is started directly the large Hertz vibrator is
excited at a distance.
There are some who think that lightning flashes can do none of these
secondary things. They are mistaken.
_Specimens of Emitters and Receivers._
On the table are specimens of various emitters and receivers such
as have been used by different people; the orthodox Hertz radiator
(Fig. 5), and the orthodox Hertz receivers:--A circular ring (Fig. 6)
for interference experiments, because it is but little damped, and a
straight wire for receiving at a distance, because it is a much better
absorber. Beside these are the spheres and ellipsoids (or elliptical
plates), which I have myself introduced and mainly used (Fig. 19),
because they are powerful radiators and absorbers, and because their
theory has been worked out by Horace Lamb and J. J. Thomson. Also
dumb-bells (Fig. 11) without air gap, which must be excited by a
positive spark at one end and a negative spark at the other, and many
other shapes, the most recent of mine being the inside of a hollow
cylinder with sparks at ends of a diameter (Fig. 12); this being a
feeble radiator, but a very persistent vibrator,[6] and, therefore,
well adapted for interference and diffraction experiments. But, indeed,
spheres can be made to vibrate longer than usual by putting them into
copper hats or enclosures, in which an aperture of varying size can be
made to let the waves out (Figs. 20 and 21).
[6] J. J. Thomson, “Recent Researches,” 344.
[Illustration: FIG. 11.--A Small Dumb-bell Form of Radiator for
Impulsive Rush.]
[Illustration: FIG. 12.--Dr. Lodge’s Hollow Cylindrical Radiator,
arranged horizontally against the outside of a Metal-lined Box
containing the Spark-producing Apparatus. Half natural size. Emitting 3
in. waves.]
Many of these senders will do for receivers too, giving off sparks
to other insulated bodies or to earth; but besides the Hertz type of
receiver, many other detectors of radiation have been employed. Vacuum
tubes can be used, either directly or on the trigger principle, as by
Zehnder (Fig. 13),[7] the resonator spark precipitating a discharge
from some auxiliary battery or source of energy, and so making a feeble
disturbance very visible. Explosives may be used for the same purpose,
either in the form of mixed water-gases or in the form of an Abel’s
fuse. FitzGerald found that a tremendously sensitive galvanometer could
indicate that a feeble spark had passed, by reason of the consequent
disturbance of electrical equilibrium which settled down again through
the galvanometer.[8] This was the method he used in this theatre
four years ago. Blyth used a one sided electrometer, and V. Bjerknes
has greatly developed this method (Fig. 14), abolishing the need
for a spark, and making the electrometer metrical, integrating and
satisfactory.[9] With this detector many measurements have been made
at Bonn by Bjerknes, Yule, Barton and others on waves concentrated and
kept from space dissipation by guiding wires.
[7] _Wied. Ann._, XLVII., p. 77.
[8] FitzGerald, _Nature_, Vol. XLI., p. 295, and Vol. XLII., p. 172.
[9] _Wied. Ann._, 44, p. 74.
[Illustration: FIG. 13.--Zehnder’s Trigger Tube. Half Natural Size. The
two right-hand terminals, close together, are attached to the Hertz
receiver; another pair of terminals are connected to some source just
not able to make the tube glow until the scintilla occurs and makes the
gas more conducting--as observed by Schuster and others.]
Mr. Boys has experimented on the mechanical force exerted by electrical
surgings, and Hertz also made observations of the same kind.
_Various Detectors._
Going back to older methods of detecting electrical radiation, we have,
most important of all, a discovery made long before man existed, by
a creature that developed a sensitive cavity on its skin; a creature
which never so much as had a name to be remembered by (though perhaps
we now call it trilobite). Then, in recent times we recall the
photographic plate and the thermopile, with its modification, the
radiomicrometer; also the so-called bolometer, or otherwise-known
Siemens’ pyrometer, applied to astronomy by Langley, and applied to the
detection of electric waves in wires by Rubens and Ritter and Paalzow
and Arons. The thermal junction was applied to the same purpose by
Kolacek, D. E. Jones and others.
And, before all these, the late Mr. Gregory, of Cooper’s Hill, made his
singularly sensitive expansion meter, whereby waves in free space could
be detected by the minute rise of temperature they caused in a platinum
wire, a kind of early and sensitive form of Cardew voltmeter.
[Illustration: FIG. 14.--Bjerknes’ Apparatus, showing (1) a Hertz
vibrator connected to an induction coil; (2) a nearly closed circuit
receiver properly tuned with the vibrator; and (3) a one sided
electrometer for inserting in the air gap of 2. The receiver is not
provided with knobs, as shown, but its open circuit is terminated by
the quadrants of the electrometer, which is shown on an enlarged scale
alongside. The needle is at zero potential and is attracted by both
quadrants. By calculation from the indications of this electrometer
Bjerknes plotted the curves 1, 2 and 3 on pages 4 and 5. Fig. 1
represents the oscillations of the primary vibrator, rapidly damped by
radiation of energy. Fig. 2 represents the vibrations thereby set up in
the resonating circuit when the two are accurately in tune; and which
persist for many swings. Fig. 3 shows the vibrations excited in the
same circuit when slightly out of tune with the exciter. A receiver of
this kind makes many swings before it is seriously damped, though the
open plate radiator does not.]
Going back to the physiological method of detecting surgings, Hertz
tried the frog’s leg nerve-muscle preparation, which to the steadier
types of electrical stimulus is so surpassingly sensitive, and to which
we owe the discovery of current electricity. But he failed to get any
result. Ritter has succeeded; but, in my experience, failure is the
normal and proper result. Working with my colleague, Prof. Gotch, at
Liverpool, I too have tried the nerve and muscle preparation of the
frog (Fig. 15), and we find that an excessively violent stimulus of a
rapidly alternating character, if pure and unaccompanied by secondary
actions, produces no effect--no stimulating effect, that is--even
though the voltage is so high that sparks are ready to jump between the
needles in direct contact with the nerve.
All that such oscillations do, if continued, is to produce a temporary
paralysis or fatigue of the nerve, so that it is unable to transmit
the nerve impulses evoked by other stimuli, from which paralysis it
recovers readily enough in course of time.
[Illustration: FIG. 15.--Experiment of Gotch and Lodge on the
physiological effect of rapid pure electric alternations.
Nerve-muscle preparation, with four needles, or else non-polarisable
electrodes applied to the nerve. C and D are the terminals of a
rapidly-alternating electric current from a conductor at zero potential
(namely, the terminals of a derived circuit from a wire connecting
the outer coats of a pair of discharging Leyden jars), while A and
B are the terminals of an ordinary very weak galvanic or induction
coil stimulus only just sufficient to make the muscle twitch. The C D
terminals do not stimulate the nerve, though at very high alternative
potentials, but they gradually and temporarily paralyse it, so that the
test terminals A B produce no effect for a time.]
This has been expected from experiments on human beings, such
experiments as Tesla’s and those of d’Arsonval. But an entire animal is
not at all a satisfactory instrument wherewith to attack the question;
its nerves are so embedded in conducting tissues that it may easily be
doubted whether the alternating type of stimulus ever reaches them at
all. By dissecting out a nerve and muscle from a deceased frog after
the historic manner of physiologists, and applying the stimulus direct
to the nerve, at the same time as some other well known ¹/₁₀₀th a volt
stimulus is applied to another part of the same nerve further from the
muscle, it can be shown that rapid electric alternations, if entirely
unaccompanied by static charge or by resultant algebraic electric
transmission, evoke no excitatory response until they are so violent as
to give rise to secondary effects such as heat or mechanical shock.
Yet, notwithstanding this inaction, they gradually and slowly exert a
paralysing or obstructive action on the portion of the nerve to which
they are applied, so that the nerve impulse excited by the feeble just
perceptible ¹/₁₀₀th-volts stimulus above is gradually throttled on its
way down to the muscle, and remains so throttled for a time varying
from a few minutes to an hour after the cessation of the violence. [I
did not show this experiment at the lecture.]
_Air Gap and Electroscope charged by Glass Rod and discharged by the
Wave Impulse from a moderately distant Sphere excited by Coil._
[Illustration: FIG. 16.--Air gap for Electroscope. Natural size.
The bottom plate is connected to, and represents, the cap of an
electroscope; the “knob” above it, mentioned in text, is the polished
end of the screw, whose terminal is connected with the case of the
instrument or “earth.” The electroscope being charged to the verge of
overflow, the impact of weak electric waves collected by a bit of wire
sticking up from the left-hand binding screw precipitates the collapse
of the leaves.]
Among trigger methods of detecting electric radiation, I have spoken of
the Zehnder vacuum tubes; another method is one used by Boltzmann.[10]
A pile of several hundred volts is on the verge of charging an
electroscope through an air gap just too wide to break down. Very
slight electric surgings precipitate the discharge across the gap, and
the leaves diverge. I show this in a modified and simple form. On the
cap of an electroscope is placed a highly polished knob or rounded end
connected to the sole, and just not touching the cap, or, rather, just
not touching a plate connected with the cap (Fig. 16), the distance
between knob and plate being almost infinitesimal, such a distance as
is appreciated in spherometry. Such an electroscope overflows suddenly
and completely with any gentle rise of potential. Bring excited glass
near it, the leaves diverge gradually and then suddenly collapse,
because the air space snaps: remove the glass, and they rediverge
with negative electricity; the knob above the cap being then charged
positively, and to the verge of sparking. In this condition any
electrical waves, collected if weak by a foot or so of wire projecting
from the cap, will discharge the electroscope by exciting surgings in
the wire, and so breaking down the air gap. The chief interest about
this experiment seems to me the extremely definite dielectric strength
of so infinitesimal an air space. Moreover, it is a detector for Hertz
waves that might have been used last century; it might have been used
by Benjamin Franklin.
[10] _Weid. Ann._, 40, p. 399.
For to excite them no coil or anything complicated is necessary; it is
sufficient to flick a metal sphere or cylinder with a silk handkerchief
and then discharge it with a well-polished knob. If it is not well
polished the discharge is comparatively gradual, and the vibrations
are weak; the more polished are the sides of an air gap, the more
sudden is the collapse and the more vigorous the consequent radiation,
especially the radiation of high frequency, the higher harmonics of the
disturbance.
For delicate experiments it is sometimes well to repolish the knobs
every hour or so. For metrical experiments it is often better to let
the knobs get into a less efficient but more permanent state. This is
true of all senders or radiators. For the generation of the, so to
speak, “infra-red” long-period Hertz waves any knobs will do, but to
generate the “ultra-violet” short-period waves high polish is essential.
_Microphonic Detectors._
Receivers or detectors, which for the present I temporarily call
microphonic, are liable to respond best to the more rapid vibrations.
Their sensitiveness is to me surprising, though of course it does
not approach the sensitiveness of the eye; at the same time I am by
no means sure that the eye differs from them in kind. It is these
detectors that I wish specially to bring to your notice.
Prof. Minchin, whose long and patient work in connection with
photo-electricity is now becoming known, and who has devised
an instrument more sensitive to radiation than even Boys’
radiomicrometer, in that it responds to the radiation of a star while
the radiomicrometer does not, found some years ago that some of his
light-excitable cells lost their sensitiveness capriciously on tapping,
and later he found that they frequently regained it again while Mr.
Gregory’s Hertz-wave experiments were going on in the same room.
These “impulsion cells,” as he terms them, are troublesome things for
ordinary persons to make and work with--at least I have never presumed
to try--but in Mr. Minchin’s hands they are surprisingly sensitive to
electric waves.[11]
The sensitiveness of selenium to light is known to everyone, and Mr.
Shelford Bidwell has made experiments on the variations of conductivity
exhibited by a mixture of sulphur and carbon.
Nearly four years ago M. Edouard Branly found that a burnished
coat of porphyrised copper spread on glass or ebonite, diminished
its resistance enormously, from some millions to some hundreds of
ohms, when it was exposed to the neighbourhood, even the distant
neighbourhood, of Leyden jar or coil sparks. He likewise found that a
tube of metallic filings behaved similarly, and that both recovered
their original resistance on shaking or tapping. Mr. Croft exhibited
this fact recently at the Physical Society. M. Branly also made pastes
and solid rods of filings, in Canada balsam and in sulphur, and found
them likewise sensitive.[12]
[11] _Phil. Mag._, Vol. XXXI., p. 223.
[12] E. Branly, _Comptes Rendus_, Vol. CXI., p. 785; and Vol. CXII., p.
90.
With me the matter arose somewhat differently, as an outcome of the
air gap detector employed with an electroscope by Boltzmann (Fig. 16).
For I had observed in 1889 that two knobs sufficiently close together,
far too close to stand any voltage such as an electroscope can show,
could, when a spark passed between them, actually cohere; conducting an
ordinary bell-ringing current if a single voltaic cell was in circuit;
and, if there were no such cell, exhibiting an electromotive force
of their own sufficient to disturb a low resistance galvanometer
vigorously, and sometimes requiring a faintly perceptible amount of
force to detach them. The experiment was described to the Institution
of Electrical Engineers in 1890,[13] and Prof. Hughes said he had
observed the same thing.
[13] _Journal_ Institution of Electrical Engineers, 1890, Vol. XIX.,
pp. 352-4; or “Lightning Conductors and Lightning Guards,” pp. 382-4.
[Illustration: FIG. 16A.--Receiver in Syntonic Jar Experiment, with
Knob Coherer and Tapper-back (_cf._ Fig. 4).]
The experiment of the syntonic Leyden jars can be conveniently shown
with the double-knob or 1889 coherer. The pair of knobs are arranged
to connect the coatings of the receiving jar (a large condenser being
interposed to prevent their completing a purely metallic circuit),
and in circuit with them is a battery and a bell. Every time the
receiving jar responds syntonically to the electric vibration of the
other jar, the knobs cohere (if properly adjusted) and the bell rings.
If the bell is free in air it continues ringing until the knobs are
gently tapped asunder; but if the bell stands on the same table as the
knobs, especially if it rests one foot on the actual stand, then its
first stroke taps them back instantly and automatically, and so every
discharge of the sending jar is signalled by a single stroke of the
bell. Here we have in essence a system of very distinctly syntonic
telegraphy, for the jars and their circuits must be accurately tuned
together, if there is to be any response. A very little error in
tuning, easily made by altering the position of the slider (Fig. 4),
will make them quite unresponsive, unless the distance between them is
reduced.
[Illustration: FIG. 17.--Early Form of Coherer, consisting of a spiral
of thin iron wire mounted on an adjustable spindle and an aluminium
plate. When the lever is moved clockwise the tip of the iron wire
presses gently against the aluminium plate, whose end is bent at right
angles and passed through into the hollow circular wooden box, of which
the upper figure shows the top and general appearance, and the lower
figure shows the inside.]
At the maximum distance of response the tuning required is excessively
sharp. But, certainly, for these closed and durably-vibrating circuits,
the distance of response is small, as has been said before. Fig. 16A
shows the syntonic Leyden jar experiment arranged with the double knob
coherer, instead of with the spark gap of Fig. 4.
_Coherer in open, responding to Feeble Stimuli:--Small Sphere,
Gas-lighter, Distant Sphere, Electrophorus._
Well, this arrangement, which I call a coherer, is the most
astonishingly sensitive detector of Hertz waves. It differs from an
actual air gap in that the insulating film is not really insulating;
the film breaks down not only much more easily, but also in a less
discontinuous and more permanent manner, than an air gap. Branly’s
tube of filings, a series of bad contacts, clearly works on the same
plan; and though a tube of filings is by no means so sensitive, yet it
is in many respects easier to work with, and except for very feeble
stimuli, is more metrical. If the filings used are coarse, say turnings
or borings, the tube approximates to a single coherer; if they are
fine, it has a larger range of sensibility. In every case what these
receivers feel are sudden jerks of current; smooth sinuous vibrations
are ineffective. They seem to me to respond best to waves a few inches
long, but doubtless that is determined chiefly by the dimensions of
some conductor with which they happen to be associated. (Figs. 17 and
18.)
[Illustration: FIG. 18.--Early Form of Iron Borings Tube. One-half
natural size, with solid brass cylinder terminals in each end of the
tube, making contact with the borings.]
_Experiment showing Filings Tube responding to Sphere, to
Electrophorus, and to a Quasi-“Spark” from the Discharge of an ordinary
Gold-leaf Electroscope._
I picture the action as follows: Suppose two fairly clean pieces of
metal in light contact--say two pieces of brass or of iron--connected
to a single voltaic cell; a film of what may be called oxide intervenes
between the surfaces so that only an insignificant current is allowed
to pass, because a volt or two is insufficient to break down the
insulating film, except perhaps at one or two atoms.[14] If the film
is not permitted to conduct at all, it is not very sensitive; the most
sensitive condition is attained when an infinitesimal current passes,
strong enough just to show on a moderate galvanometer.
[14] See _Phil. Mag._, Jan., 1894, p. 94, where this explanation
(whether true or not) was first given, and where the author first
published his fuller experience of coherer behaviour.
Now let the slightest surging occur, say by reason of a sphere being
charged and discharged at a distance of forty yards; the film at
once breaks down, perhaps not completely--that is a question of
intensity--but permanently. As I imagine, more molecules get within
each other’s range, incipient cohesion sets in and the momentary
electric quiver acts somewhat like a flux. It is a singular variety of
electric welding. A stronger stimulus enables more molecules to hold
on, the process is surprisingly metrical; and as far as I roughly know
at present, the change of resistance is proportional to the energy of
the electric radiation, from a source of given frequency.
It is to be specially noted that a battery current is not needed to
_effect_ the cohesion, only to demonstrate it. The battery can be
applied after the spark has occurred, and the resistance will be found
changed as much as if the battery had been on all the time.
The incipient cohesion electrically caused can be mechanically
destroyed. Sound vibrations, or any other feeble mechanical
disturbances, such as scratches or taps, are well adapted to restore
the contact to its original high resistance sensitive condition.
The more feeble the electrical disturbance the slighter is the
corresponding mechanical stimulus needed for restoration. When working
with the radiating sphere (Fig. 19) at a distance of forty yards out
of window, I could not for this reason shout to my assistant to cause
him to press the key of the coil and make a spark, but I showed him
a duster instead, this being a silent signal which had no disturbing
effect on the coherer or tube of filings. I mention 40 yards, because
that was one of the first outdoor experiments; but I should think that
something more like half-a-mile was nearer the limit of sensitiveness
for this particular apparatus as then arranged. However, this is a
rash statement not at present verified.[15] At 40 or 60 yards the
exciting spark could be distinctly heard, and it was interesting to
watch the spot of light begin its long excursion and actually travel a
distance of 2 in. or 3 in. before the sound arrived. This experiment
proved definitely enough that the efficient cause travelled quicker
than sound, and disposed completely of any sceptical doubts as to sound
waves being, perhaps, the real cause of the phenomenon. Signals were
obtained across the full width of the college quadrangle, and later,
with larger apparatus, between the college tower and another high
building half-a-mile away.
[15] This statement has been absurdly misunderstood, as if it was a
prediction of what would always be the limit of sensitiveness for any
apparatus and any sized sender. Nothing of the kind was in my mind.
Such predictions are always preposterous, and I am not obliged to those
who imagined that I had been guilty of one of them.--O. J. L., 1899.
[Illustration: FIG. 19.--Radiator used in the library of the Royal
Institution, exciting the Coherer (Fig. 17) on the lecture table in the
Theatre. I also used a radiator with two or with three large spheres
between two knobs, and described it in _Nature_, Vol. 41, p. 462, 1890.
This is the radiator which Prof. Righi has improved and made in a
compact form with oil between the two middle spheres.]
Invariably, when the receiver is in good condition, sound or other
mechanical disturbance acts one way, viz., in the direction of
increasing resistance, while electrical radiation or jerks act the
other way, decreasing it. While getting the receiver into condition,
or when it is getting out of order, vibrations and sometimes electric
discharges act irregularly; and an occasional good shaking does the
filings good. I have taken rough measurements of the resistance by the
simple process of restoring the original galvanometer deflection by
adding or removing resistance coils. A half-inch tube, 8 in. long, of
selected iron turnings (Fig. 18) had a resistance of 2,500 ohms in the
sensitive state. A feeble stimulus, caused by a distant electrophorus
spark, brought it down 400 ohms. A rather stronger one reduced it by
500 and 600, while a trace of spark given to a point of the circuit
itself ran it down 1,400 ohms.
This is only to give an idea of the quantities. I have not yet done any
seriously metrical experiments.
_Added later._--My assistant, Mr. E. E. Robinson, early noticed that
when a telephone was used as receiver, say with a single-point coherer
(_see_ illustration on opposite page), which is a very sensitive
arrangement, every disturbance of the coherer due to received waves is
accompanied by a crackle or tick in the telephone, without any tapping
back being necessary. This is, indeed, the easiest mode of receiving
signals, and we often practised it. If a suitable, well-damped
galvanometer, such as a Thomson marine speaking-galvanometer, is
included also in the circuit (a more sensitive one is sometimes
necessary--and we frequently used a D’Arsonval--but it must be well
damped), the meaning of these ticks is recognised; each represents a
minute change in the resistance of the coherer--not at all the full
change usually employed, but little subsidiary changes, sometimes up
and sometimes down, barely sufficient to affect a galvanometer, but
quite adequate (being so sudden) to disturb a telephone. This method
of receiving, which at first is very sensitive, after a time becomes
less so; the point shows signs of fatigue, probably due to too perfect
cohesion having been gradually established, and a mechanical tap back
is desirable to restore it to its original condition.
If all the signals received were precisely of the same strength, I
doubt if these superposed crimples of resistance would occur; but
signals depending on quality of sending spark never are of the same
strength, and accordingly the sudden slight variations of resistance
do occur. Usually an ordinary high resistance telephone was employed,
and it was joined to the coherer circuit through one of the usual small
transformers--a plan which has many obvious advantages.
[Illustration: Simplest Receiving Arrangement: a Telephone in Circuit
with Single-point Coherer without Tapper-back. B a needle resting
against a watch-spring A adjusted by screw C.]
[Illustration: Syntonic Sender and Receiver used in the experiments
plotted on page 28. The switch enables the coherer K to be connected
either to the tuned resonator M L N or to the detecting circuit E F.
Weak impulses are felt when the switch is C E, D F; strong impulses
when the switch is C A, D B; provided the coil L is similar to the coil
of the radiator above. The impulses are plotted in the diagram Fig.
19A.]
[Illustration: _October 27, 1897._
FIG. 19A.--Current through Coherer after successive Electrical Stimuli,
without any mechanical tapping back. The sudden rises are obtained when
the circuits are syntonised. Weaker stimuli cause the descents.]
The fluctuations of resistance of a coherer dependent on various
strengths of stimulus are instructively shown in some metrical
experiments made by Mr. Robinson, and a plotting of which I showed
to the Physical Society of London in 1897. This plotting is here
reproduced, and it shows the singular fact that, whereas a stronger
electrical stimulus usually decreases the resistance, as is natural,
a weaker subsequent stimulus usually increases it again: so that
alternately strong and weak stimuli send the curve zigzagging up and
down, until it gets into a condition demanding rejuvenation by a
mechanical tap back.
Sometimes a decidedly strong electrical stimulus knocks down the
conductivity of the coherer as if it had been tapped back. This is
almost certainly due to a burning of the delicate contacts--a blowing
of a fuse as it were,--and the effect of this electrical burn back is
quite different from the effect of a mechanical tap back, inasmuch as
it leaves the coherer insensitive. A shaking up is necessary to restore
it.
I now call your attention to the Table on next page of various kinds of
detector for electric radiation distributed in groups.
Selenium is inserted in this table in the microphone column, because
it is a substance which in certain states is well known to behave to
visible light as these other microphonic detectors behave to Hertz
waves. It is inserted with a query, to indicate that its position in
the table is not _certainly_ known. It may possibly belong to some
other column.
_Electrical Theory of Vision._
And I want to suggest that quite possibly the sensitiveness of the
eye is of the coherer kind. As I am not a physiologist, I cannot be
seriously blamed for making wild and hazardous speculations in that
region. I therefore wish to guess that some part of the retina is
an electrical organ, say like that of some fishes, maintaining an
electromotive force which is prevented from stimulating the nerves
solely by an intervening layer of badly conducting material, or of
conducting powder with gaps in it; but that when light falls upon the
retina these gaps become more or less conducting, and the nerves are
stimulated. I do not feel clear which part is taken by the rods and
cones, and which part by the pigment cells; I must not try to make the
hypothesis too definite at present, though I hope it is obvious what I
intend to suggest.
If I had to make a demonstration model of the eye on these lines,
I should arrange a little battery to excite a frog’s nerve-muscle
preparation through a circuit completed all except a layer of filings
or a single bad contact. Such an arrangement would respond to Hertz
waves. Or, if I wanted actual light to act, instead of grosser waves, I
would use a layer of selenium.
But the bad contact and the Hertz waves are the most instructive,
because we do not at present really know what the selenium is doing,
any more than what the retina is doing.
And observe that (to my surprise, I confess) the rough outline of a
theory of vision thus suggested is in accordance with some of the
principal views of the physiologist Hering. The sensation of light is
due to the electrical stimulus; the sensation of black is due to the
mechanical or tapping back stimulus. Darkness is physiologically not
the mere cessation of light. Both are positive sensations, and both
stimuli are necessary; for until the filings are tapped back vision is
persistent. In the eye model the period of mechanical tremor should
be, say, ⅒th second, so as to give the right amount of persistence of
impression.
----------------------------------------------------------------------
DETECTORS OF RADIATION.
--------+---------+------------+-------------+-----------+------------
Physio- |Chemical.| Thermal. | Electrical. |Mechanical.|Microphonic.
logical.| | | | |
--------+---------+------------+-------------+-----------+------------
| | | | |Selenium.(?)
| | | | |
Eye. |Photo- |Thermopile. | Spark. | Electro- | Impulsion
| graphic | | (Hertz.) | meter. | Cell.
| Plate. | | |(Blyth and | (Minchin.)
| | | | Bjerknes.)|
| | | | |
˟Frog’s |Explosive| Bolometer. | Telephone; | Suspended | Filings.
Leg | Gases. |(Rubens and | Air-gap | Wires. | (Branly.)
(Hertz | | Ritter.) | and Arc. |(Hertz and |
and | | | (Lodge.) | Boys.) |
Ritter.)| | | | |
| | | | |
|Photo- | Expanding |Vacuum Tube. | | Coherer.
| electric| Wire. | (Dragoumis.)| |Hughes and
| Cell. |(Gregory.) |Galvanometer.| | Lodge.
| | Thermal |(Fitzgerald.)| |
| | Junction. |Air-gap and | |
| |(Klemencic.)|Electroscope.| |
| | | (Boltzmann.)| |
| | |Trigger Tube.| |
| | |(Warburg and | |
| | | Zehnder.) | |
--------+---------+------------+-------------+-----------+------------
˟ The cross against the frog’s leg indicates that it does not appear
really to respond to radiation, unless stimulated in some secondary
manner. The names against the other things are unimportant, but
suggest the persons who applied the detector to electric radiation.
The interrogation mark against Selenium indicates that its position
in the microphonic column may be doubtful.
No doubt in the eye the tapping back is done automatically by the
tissues, so that it is always ready for a new impression, until
fatigued. And by mounting an electric bell or other vibrator on the
same board as a tube of filings, it is possible to arrange so that
a feeble electric stimulus shall produce a feeble steady effect, a
stronger stimulus a stronger effect, and so on; the tremor asserting
its predominance, and bringing the spot back, whenever the electric
stimulus ceases.
An electric bell thus close to the tube is, indeed, not the best
vibrator; clockwork might do better, because the bell contains in
itself a jerky current, which produces one effect, and a mechanical
vibration, which produces an opposite effect, hence the spot of light
can hardly keep still. By lessening the vibration--say, by detaching
the bell from actual contact with the board, the electric jerks of
the intermittent current drive the spot violently up the scale;
mechanical tremor brings it down again. It must be clearly understood
that electric jerks, due to the make-and-break of an ordinary current,
are quite adequate to electrically stimulate a coherer in their
neighbourhood. It is constantly to be noticed that a coherer responds
best to excessively short sparks of a certain sharp quality.
You observe that the eye on this hypothesis is, in electrometer
language, heterostatic. The energy of vision is supplied by the
organism; the light only pulls a trigger. Whereas the organ of hearing
is idiostatic. I might draw further analogies between this arrangement
and the eye, _e.g._, about the effect of blows or disorder causing
irregular conduction and stimulation, of the galvanometer in the one
instrument, of the brain cells in the other.
A handy portable exciter of electric waves is one of the ordinary hand
electric gas-lighters, containing a small revolving doubler--_i.e._, an
inductive or replenishing machine. A coherer can feel a gas-lighter
across a lecture theatre. Minchin often used them for stimulating his
impulsion cells. I find that when held near they act a little even
when no ordinary spark occurs, plainly because of the little incipient
sparks at the brushes or tinfoil contacts inside. A Voss machine acts
similarly, giving a small deflection while working up before it sparks:
indeed, these small sparks are often more effective than bigger ones.
_Demonstration of Ordinary Holtz Machine Sparks not exciting Tube:
except by help of a polished knob._
And notice here that our model eye has a well-defined range of vision.
It cannot see waves too long for it. The powerful disturbance caused
by the violent flashes of a Holtz or Wimshurst or Voss machine it is
blind to. The loud sparks have no effect on it. They are like infra-red
radiation to the eye. If the knobs of the machine are well polished
the coherer begins to respond again, evidently by reason of some high
harmonics, due to vibrations in the terminal rods; and these are the
vibrations to which it responds when excited simply by an induction
coil. The coil should have knobs instead of points. Sparks from
points or dirty knobs hardly excite the coherer at all. But hold a
well-polished sphere or third knob between even the dirty knobs of a
Voss machine, and the coherer responds at once to the surgings got up
in that clean sphere.
Feeble short sparks again are often more powerful exciters than are
strong long ones. I suppose because they are more sudden. This is
instructively shown with an electrophorous lid. Spark it to a knuckle,
and it does very little. Spark it to a clean knob held in the hand and
it works well. But now spark it to an _insulated_ sphere, there is
some effect. Discharge the sphere, and take a second spark, without
recharging the lid; do this several times; and at last, when the spark
is inaudible, invisible, and otherwise imperceptible, the coherer some
yards away responds more violently than ever, and the spot of light
rushes from the scale.
If a coherer be attached by a side wire to the gas pipes, and an
electrophorous spark be given to either the gas pipes or the water
pipes or even to the hot-water system in any other room of the
building, the coherer responds. It is surprising how far these impulses
can be felt along an ordinary uninsulated wire or other conductor.
In fact, when thus connected to gas pipes one day when I tried it, the
spot of light could hardly keep still five seconds. Whether there was
a distant thunderstorm, or whether it was only picking up telegraphic
jerks, I do not know. The jerk of turning on or off an extra Swan lamp
can affect it when sensitive. I hope to try for long-wave radiation
from the sun, filtering out the ordinary well-known waves by a
blackboard or other sufficiently opaque substance.
[I did not succeed in this, for a sensitive coherer
in an outside shed unprotected by the thick walls
of a substantial building cannot be kept quiet for
long. I found its spot of light liable to frequent
weak and occasionally violent excursions, and I could
not trace any of these to the influence of the sun.
There were evidently too many terrestrial sources
of disturbance in a city like Liverpool to make the
experiment feasible. I don’t know that it might not
possibly be successful in some isolated country
place; but clearly the arrangement must be highly
sensitive in order to succeed.]
We can easily see the detector respond to a distant source of radiation
now, viz., to a 5 in. sphere placed in the library between secondary
coil knobs; separated from the receiver, therefore, by several walls
and some heavily gilded paper, as well as by 20 or 30 yards of space
(Fig. 19.)
[Illustration: FIG. 19B.--A Portable Detector, B the Collecting Wire.]
Also I exhibit (Fig. 19B) a small complete detector made by my
assistant, Mr. Davies, which is quite portable and easily set up. The
essentials (battery, galvanometer, and coherer) are all in a copper
cylinder, A, three inches by two. A bit of wire, B, a few inches long,
pegged into it, helps it to collect waves. It is just conceivable that
at some distant date, say by dint of inserting gold wires or powder
in the retina, we may be enabled to see waves which at present we are
blind to.
Observe how simple the production and detection of Hertz waves are
now. An electrophorus or a frictional machine serves to excite them; a
voltaic cell, a rough galvanometer, and a bad contact serves to detect
them. Indeed, they might have been observed at the beginning of the
century, before galvanometers were known: a frog’s leg or an iodide of
starch paper would do almost as well.
A bad contact was at one time regarded as a simple nuisance, because
of the singularly uncertain and capricious character of the current
transmitted by it. Hughes observed its sensitiveness to sound waves,
and it became the microphone. Now it turns out to be sensitive
to electric waves, if it be made of any oxidisable medal (not of
carbon),[16] and we have an instrument which might be called a
micro-something, but which, as it appears to act by cohesion, I at
present call a coherer. Perhaps some of the capriciousness of an
anathematised bad contact was sometimes due to the fact that it was
responding to stray electric radiation. (See Appendix III., pp. 109 and
111.)
[16] FitzGerald tells me that he has succeeded with carbon also. My
experience is that the less oxidisable the metal, the more sensitive
and also the more troublesome is the detector. Mr. Robinson has now
made me a hydrogen vacuum tube of brass filings, which beats the
coherer for sensitiveness. July, 1894.
The breaking down of cohesion by mechanical tremor is an ancient
process, observed on a large scale by engineers in railway axles and
girders; indeed, the cutting of small girders by persistent blows of
hammer and chisel reminded me the other day of the tapping back of our
cohering surfaces after they have been exposed to the uniting effect of
an electric jerk.
_Receiver in Metallic Enclosure._
If a coherer is shut up in a complete metallic enclosure, waves cannot
get at it, but if wires are led from it to an outside ordinary
galvanometer, it remains nearly as sensitive as it was before (nearly,
not quite), for the circuit picks up the waves and they run along the
insulated wires into the closed box. To screen it effectively, it is
necessary to enclose battery and galvanometer and every bit of wire
connection; the only thing that may be left outside is the needle of
the galvanometer. Accordingly, here we have a compact arrangement of
battery and galvanometer coil and coherer, all shut up in a copper box
(Fig. 19C). The galvanometer coil is fixed against the side of the box
at such height that it can act conveniently on an outside suspended
compass needle. The slow magnetic action of the current in the coil
has no difficulty in getting through copper, as everyone knows: only
a perfect conductor could screen off that; but the Hertz waves are
effectively kept out by the sheet copper.
[Illustration: FIG. 19C.--Protected Detector. A is an occasional wire
passing through shuttered aperture. E is a lead tube enclosing leading
wires, as in Fig. 21.]
It must be said, however, that the box must be exceedingly well closed
for the screening to be perfect. The very narrowest chink permits their
entrance, and at one time I thought I should have to solder a lid on
before they could be kept entirely out. Clamping a copper lid on to a
flange in six places was not enough. But by the use of pads of tinfoil
and tight clamping, chinks can be avoided, and the inside of the box
becomes then electrically dark.
If even an inch of the circuit protrudes, it at once becomes slightly
sensitive again; and if a mere single wire protrudes through the box,
not connected to anything at either end, provided it is insulated where
it passes through, the waves will utilise it as a speaking-tube, and
run blithely in. And this happens whether the wire be connected to
anything inside or not, though it acts more strongly when connected.
In careful experiments, where the galvanometer is protected in one
copper box and the coherer in another, the wires connecting the two
must be encased in a metal tube (Figs. 19C and 21), and this tube must
be well connected with the metal of both enclosures, if nothing is to
get in but what is wanted.
[Illustration: FIG. 20.--Spherical Radiator for emitting a Horizontal
Beam, arranged inside a Copper Hat, fixed against the outside of a
metal-lined Box, which contains induction coil and battery and key.
One-eighth natural size. The wires pass into the box through glass
tubes not shown.]
Similarly when definite radiation is desired, it is well to put the
radiator in a copper hat open in only one direction (Fig. 20), and in
order to guard against reflected and collateral surgings running along
the wires which pass outside to the exciting coil and battery, as they
are liable to do, I am accustomed to put all the sending apparatus in
a packing case lined with tinfoil, to the outside of which the sending
hat (Fig. 20) is fixed, and to pull the key of the primary exciting
circuit by a string from outside, so that not even key connections
shall protrude, else exact optical experiments are impossible.
[Illustration: FIG. 21.--General arrangement of experiments with
the Copper “Hat,” showing Metal Box on a Stool, standing outside
the Theatre. The Box is not exactly represented, but inside it the
Radiators were fixed with a graduated series of apertures; the Copper
Hat containing the Coherer is seen on the Table with the Metal Box
on the left of the Table containing Battery and Galvanometer Coil
connected to it by a compo pipe conveying the wires, as in Fig. 19C;
the Lamp and Scale barely indicated at one side of the Table; a
Paraffin Prism; and a Polarising Grid of copper wires stretched on a
frame. (This figure is from a thumbnail sketch by Mr. A. P. Trotter,
taken at the Lecture in 1894.)]
Even then, with the lid of the hat well clamped on, something gets
out, but it is not enough to cause serious disturbance of qualitative
results. The sender must evidently be thought of as emitting a
momentary blaze of light which escapes through every chink. Or, indeed,
since the waves are some inches long, the difficulty of keeping them
out of an enclosure may be likened to the difficulty of excluding
sound; though the difficulty is not quite so great as that, since a
reasonable thickness of metal is really opaque. I fancied once or twice
I detected a trace of transparency in such metal sheets as ordinary
tinplate, but unnoticed chinks elsewhere may have deceived me. It is a
thing easy to make sure of as soon as I have more time. (Tinplate is
quite opaque. Lead paper lets a little through.)
One thing in this connection is noticeable, and that is how little
radiation gets either in or out of a small _round_ hole. A narrow long
chink in the receiver box lets in a lot; a round hole the size of a
shilling lets in hardly any, unless indeed a bit of insulated wire
protrudes through it like a collecting ear trumpet, as at A, Fig. 19C.
It may be asked how the waves get out of the metal tube of an electric
gas-lighter. But they do not; they get out through the handle, which
being of ebonite is transparent. Wrap up the handle in tinfoil, and a
gas-lighter is powerless.