SPECIAL EXPERIMENT ON ETHERIAL VISCOSITY
The balance of evidence at this stage seems to incline in the sense
that there is no ether drift, that the ether near the earth is
stagnant, that the earth carries all or the greater part of the
neighbouring ether with it,--a view which, if true, must singularly
complicate the theory of ordinary astronomical aberration: as is
explained at the beginning of the last chapter.
But now put the question another way. _Can_ matter carry neighbouring
ether with it when it moves? Abandon the earth altogether; its motion
is very quick, but too uncontrollable, and it always gives negative
results. Take a lump of matter that you can deal with, and see if it
pulls any ether along.
That is the experiment which I set myself to perform, and which in the
course of the years 1891-97 I performed. It may be thus described in
essence:--
Take a steel disk, or rather a couple of large steel disks a yard in
diameter clamped together with a space between. Mount the system on a
vertical axis, and spin it like a teetotum as fast as it will stand
without flying to pieces. Then take a parallel beam of light, split it
into two by a semi-transparent mirror, M, a piece of glass silvered so
thinly that it lets half the light through and reflects the other
half, somewhat as in Fig. 7; and send the two halves of this split
beam round and round in opposite directions in the space between the
disks. They may thus travel a distance of 20 or 30 or 40 feet.
Ultimately they are allowed to meet and enter a telescope. If they
have gone quite identical distances they need not interfere, but
usually the distances will differ by a hundred-thousandth of an inch
or so, which is quite enough to bring about interference.
The mirrors which reflect the light round and round between the disks
are shown in Fig. 11. If they form an accurate square the last two
images will coincide, but if the mirrors are the least inclined to one
another at any unaliquot part of 360° the last image splits into two,
as in the kaleidoscope is well known, and the interference bands may
be regarded as resulting from those two sources. The central white
band bisects normally the distance between them, and their amount of
separation determines the width of the bands. There are many
interesting optical details here, but I shall not go into them.
[Illustration: FIG. 11. Diagrammatic Plan of Optical Frame for Ether
Machine; with Steel Disks, one yard in diameter, inside the frame. The
actual apparatus is shown in Figs. 13 and 14 and Fig. 12.
M is a semi-transparent mirror, reflecting half an incident
beam and transmitting the other half. The two half-beams each
go three times round the square contour, in opposite
directions, and then reunite. It is an extension of the idea of
Fig. 7.]
The thing to observe is whether the motion of the disks is able to
replace a bright band by a dark one, or vice versa. If it does, it
means that one of the half-beams, viz. that which is travelling in the
same direction as the disks, is helped on a trifle, equivalent to a
shortening of journey by some quarter millionth of an inch or so in
the whole length of 30 feet; while the other half-beam, viz. that
travelling against the motion of the disks, is retarded, or its path
virtually lengthened, by the same amount.
If this acceleration and retardation actually occurs, waves which did
not interfere on meeting before the disks moved, will interfere now;
for one will arrive at the common goal half a length behind the other.
Now a gradual change of bright space to dark, and vice versa, shows
itself, to an observer looking at the bands, as a gradual change of
position of the bright stripes, or a shift of the bands. A shift of
the bands, and especially of the middle white band, which is much more
stable than the others, is what we look for. The middle band is, or
should be, free from the "concertina"-like motion which is liable to
infect the others.
At first I saw plenty of shift. In the first experiment the bands
sailed across the field as the disks got up speed until the crosswire
had traversed a band and a half. The conditions were such that had the
ether whirled at the full speed of the disks I should have seen a
shift of three bands. It looked very much as if the light was helped
along at half the speed of the moving matter, just as it is inside
water.
On stopping the disks the bands returned to their old position. On
starting them again in the opposite direction, the bands ought to have
shifted the other way too, if the effect was genuine; but they did
not; they went the same way as before.
The shift was therefore wholly spurious; it was caused by the
centrifugal force of the blast of air thrown off from the moving
disks. The mirrors and frame had to be protected from this. Many other
small changes had to be made, and gradually the spurious shifts have
been reduced and reduced, largely by the skill and patience of my
assistant, Mr. Benjamin Davies, until presently there was barely a
trace of them.
But the experiment is not an easy one. Not only does the blast exert
pressure, but at high speeds the churning of the air makes it quite
hot. Moreover, the tremor of the whirling machine, in which from four
to nine horse-power is sometimes being expended, is but too liable to
communicate itself to the optical part of the apparatus. Of course
elaborate precautions are taken against this. Although the two parts,
the mechanical and the optical, are so close together, their supports
are entirely independent. But they have to rest on the same earth, and
hence communicated tremors are not absent. They are the cause of most
of the slight residual trouble.
The whole experiment is described in fairly full detail in the
_Philosophical Transactions of the Royal Society_ for 1893 and 1897.
And there also are described some further modifications whereby the
whirling disks are electrified--likewise without optical effect, and
are also magnetised; or rather a great iron mass, strongly magnetised
by a current, is used to replace the steel disks.
The effect was always zero, however, when spurious results were
eliminated; and it is clear that at no practicable speed does either
electrification or magnetisation confer upon matter any appreciable
viscous grip upon the ether. Atoms _must_ be able to throw it into
vibration, if they are oscillating or revolving at sufficient speed;
otherwise they would not emit light or any kind of radiation; but in
no case do they appear to drag it along, or to meet with resistance in
any uniform motion through it. Only their acceleration is effectual.
In the light of Larmor's electron theory, we know now that
acceleration of atoms, or rather of a charge upon an atom, necessarily
generates radiation, proportional in amount to the _square_ of the
acceleration--whether that be tangential or normal. There is no
theoretical reason for assuming any influence on uniform velocity. And
even the influence on acceleration is exceedingly small under ordinary
circumstances. Only during the violence of collision are ether waves
freely excited. The present experiment, however, has nothing to do
with acceleration: it is a test of viscosity. An acceleration term
exists in motion through even a perfect fluid.
[Illustration: FIG. 12. General view of whirling part of Ether
machine, with pair of steel disks, and motor. _To face page 72._]
[Illustration: FIG. 13. General view of optical framework--sustaining
mirrors, telescope, and collimator--to surround the disks of the Ether
machine. Compare fig. 11. _To face page 73_.]
The conclusion at which I arrived in 1892 and 1893 is thus expressed
(p. 777 of vol. 184 _Philosophical Transactions of the Royal
Society_): "I feel confident either that the ether between the
disks is quite unaffected by their motion, or, if affected at all, by
something less than the thousandth part. At the same time, so far as
rigorous proof is concerned, I should prefer to assert that _the
velocity of light between two steel plates moving together in their
own plane an inch apart is not increased or diminished by so much as
the 1/200th part of their velocity_."
That was the conclusion in 1893; but since then observations have been
continued, and it is now quite safe to change the 1/200th into
1/1000th. The spin was sometimes continued for three hours to see if
an effect developed with time; and many other precautions were taken,
as briefly narrated in the _Philosophical Transactions_ for 1897.
The following illustrations give an idea of the apparatus employed.
Fig. 12 shows a photograph of the whirling machine before being bolted
down to its stone pier; with the pair of disks at top ready to be
whirled by an armature on the shaft, which is supplied with a current
sometimes of nine horse-power. The armature winding was of low
resistance, and was specially braced, so as to give high speed without
flying out, and without generating too much back-E M F. The
ampere-meter and volt-meter and the carbon rheostat (in armature
circuit), for regulating the speed, are plainly seen. The smooth
pulley on the shaft is for applying a brake. The small disk above it
is perforated to act as a siren for estimation of speed; but other
arrangements for this purpose were subsequently added. The two large
disks at top were of the best circular-saw steel; they are somewhat
thicker at middle than at edge, and are strongly bolted up between
iron cheeks, which are attached to the shaft. The lower end of the
shaft is a step-bearing of hardened steel in a vessel of oil. The
upper collar is elastic, so as to allow for a steadying teetotum
action at high speeds.
Fig. 13 is a photograph of the optical square, which was ultimately to
be placed in position surrounding the disks. The slit and collimator
are shown; the micrometer end of the observing telescope is out of the
picture.
The mirrors on the sides of the square are accurately plane; they are
adjustable on geometric principles, and are pressed against their
bearings by strong spiral springs. They were made by Hilger.
A drawing of the arrangement is given in Fig. 14, and here the double
micrometer eye-piece is visible.
In Fig. 15 the whole apparatus is shown mounted. The whirling machine
strongly bolted down to a stone pier independent of the floor; the
optical frame independently supported by a gallows frame from other
piers. The centrifugal mercury speed-indicator is visible in front,
and Mr. Davies is regulating the speed. At the back is seen a
boiler-plate screen for the observer with his eye at the telescope.
(See Frontispiece.)
[Illustration: _Lodge_
_Phil. Trans. 1893. A. Plate 31._
The mirror frame is pivoted on a vertical axis here Horizontal
axis of frame Mode of mounting the semi-transparent mirror M so
as to give altitude and azimuth movement to the reflected beam
Details of brass plate supporting fourth mirror front, side and
back views Back view shows the three slots in which the ends of
the supporting screws rest giving a fine adjustment, the plate
being supported by three rigid pushes and three elastic pulls.
FIG. 14 Plan of optical frame with steel disk in position, and
glazed drum to isolate them from the frame. G represents one of
the panes of optical glass. Supports of telescope and collimator
also shewn, and part of the fixing of the four mirrors 1.2.3.4.,
three of them let into recesses in the wooden frame, each mirror
held by a brass plate supported by three finely cut screws
against which it is pressed by the spring-bolts shewn M is the
semi-transparent mirror West, Newman lith FIG. 14.
_To face page 74._]
The expense of the apparatus was borne by my friend the late George
Holt, shipowner, of Liverpool.
Fig. 16 exhibits something like the appearance seen in the eye-piece,
with the interference bands on each side of the middle band, and with
the micrometer wires set in position--each moved by an independent
micrometer head. The straight vertical wire was usually set in the
centre of the middle white band, and the =X= wire on the yellow of the
first coloured band on one side or the other.
The method of observation now consists in setting a wire of the
micrometer accurately in the centre of the middle band, while another
wire is usually set on the first band to the left. Then the micrometer
heads are read, and the setting repeated once or twice to see how
closely and dependably they can be set in the same position. Then we
begin to spin the disks, and when they are going at some high speed,
measured by a siren note and in other ways, the micrometer wires are
reset and read--reset several times and read each time. Then the disks
are stopped and more readings are taken. Then their motion is
reversed, the wires set and read again; and finally the motion is once
more stopped and another set of readings taken. By this means the
absolute shift of middle band, and its relative interpretation in
terms of wave-length, are simultaneously obtained; for the distance
from the one wire to the other, which is often two revolutions of a
micrometer head, represents a whole wave-length shift.
In the best experiments I do still often see something like a fiftieth
of a band shift; but it is caused by residual spurious causes, for it
repeats itself with sufficient accuracy in the same direction when the
disks are spun the other way round.
Of real reversible shift, due to motion of the ether, I see nothing. I
do not believe the ether moves. It does not move at a five-hundredth
part of the speed of the steel disks. Further experience confirms and
strengthens this estimate, and my conclusion is that such things as
circular saws, flywheels, railway trains, and all ordinary masses of
matter do not appreciably carry the ether with them. Their motion does
not seem to disturb it in the least.
The presumption is that the same is true for the earth; but the earth
is a big body,--it is conceivable that so great a mass may be able to
act when a small mass would fail. I would not like to be too sure
about the earth--at least not on a strictly experimental basis. What I
do feel sure of is that if moving matter disturbs ether in its
neighbourhood at all, it does so by some minute action, comparable in
amount perhaps to gravitation, and possibly by means of the same
property as that to which gravitation is due--not by anything that
can fairly be likened to etherial viscosity. So far as experiment has
gone, our conclusion is that the viscosity or fluid friction of the
ether is zero. And that is an entirely reasonable conclusion.
[Illustration: FIG. 16. Approximate appearance of the interference
bands and micrometer wires as seen in the eye-piece of the telescope
of the Ether machine.]
[Illustration: FIG. 18. Appearance of the interference bands in the
channel of the iron spheroid. They were reflected in the upper iron as
shown.]
_To face page 76._
Oblate spheroid for Whirling Machine.
[Illustration: FIG. 17. Section of oblate spheroid of soft iron for
whirling machine, showing arrangement for winding central core with
wire so as to be able to magnetise it strongly while spinning inside
the optical frame.]
_To face page 77._
MAGNETISATION.
For testing the effect of magnetism, an oblate spheroid was made of
specially selected soft iron, 3 feet in diameter, weighing nearly a
ton. Its section is shown in Fig. 17. It had an annular channel or
groove, half an inch wide and 1 foot deep, round the bottom of which
was wound a kilometre of insulated wire to a depth of 4½ inches;
the terminals of which were brought out to sliding contacts on the
shaft, so that the whole could be very highly magnetised while it was
spinning. Everything was arranged so as to be symmetrical about the
central axis.
To the coil of wire, whose resistance was 30 ohms, 110 volts was
ordinarily, and 220 volts exceptionally, applied. The magnetic field
with 110 volts was about 1800 c.g.s., on the average, all over the
main region through which the beam of light circulated.
This light-bearing space, or gap in the magnetic circuit, was only
half an inch wide; and accordingly in the eye-piece the iron surfaces
could be seen, above and below, as well as the interference bands in
the luminous gap. The whole appearance is depicted in Fig. 18.
ELECTRIFICATION.
For the electrification experiment, a third and insulated disk was
clamped between the two steel disks and kept electrified to sparking
tension. The arrangement is shown diagrammatically on a smaller scale
in Fig. 19.
[Illustration: FIG. 19. Arrangement for electrifying a third or middle
steel disk to sparking potential while spinning.]
The electrification test was exceptionally easy to apply, by
connecting the insulated charging pin to a Voss machine in action:
because when the disks were spinning and the bands in good condition,
the electrification could be instantaneously applied, taken off,
reversed, or whatever was desired; and the effect of the sudden
lowering of potential by sparks passing between the revolving plates
could be exactly looked for.
The conclusion of my second _Philosophical Transactions_ paper--that
of 1897--is that _neither an electric nor a magnetic transverse field
confers viscosity upon the ether, nor enables moving matter to grip
and move it rotationally_.
QUESTION OF A POSSIBLE LONGITUDINAL MAGNETIC DRIFT.
Later I tried a longitudinal magnetic field also; arranging a series
of four large electric bobbins or long coils along the sides of a
square inscribed at 45° in the optical square, Figs. 11 and 13; so
that the light went along their axes.
The details of this experiment have been only partially recorded, but
the salient points are to be found stated in the _Philosophical
Magazine_ for April, 1907, pages 495-500.
The result was again negative; that is to say, a magnetic field causes
no perceptible acceleration in a beam of light sent along the lines of
force. The extra velocity that could have been observed would have
been 1/9th of a millimetre per second, or 16 miles per hour, for each
C.G.S. unit of field intensity.
Another mode of expressing the result is that the difference of
magnetic potential applied, namely, a drop of two million C.G.S. units
of magnetic potential, does not hurry light along it by so much as
1/50th part of a wave-length.
There may be reasons for supposing that some much slower drift or
conveyance than this is really caused in the ether by a magnetic
field; but if so, the ether must be regarded as so excessively dense
that the amount of such a drift for any practicable magnetic field
seems almost hopelessly beyond experimental means of detection.