SCIENTIFIC TESTING AND MEASURING
Science, whether it be of the pure variety, that which is pursued for
its own sake--for the mere greed for knowledge--or applied science, the
purpose of which is to assist manufacture, is based entirely upon
accurate testing and measuring. It is only by discovering and
investigating small differences in size, weight or strength that some of
the most important facts can be brought to light. There are some
problems, too, that defy theory, since they are too complicated; they
involve too many theories all at once, and such can only be solved by
accurate tests. And all these necessitate the use of very ingenious and
often costly devices.
Electrical measuring instruments were of sufficient importance and
interest to warrant a chapter of their own, but there are many others of
great value, and not without interest to the general reader.
For example, some years ago there was a collision in the Solent, just
off Cowes, between the cruiser _Hawke_ and the giant liner _Olympic_.
The cause of this was a subject of dispute and of litigation; the
theorists theorised; some reached the conclusion that the _Hawke_ was to
blame, and others the _Olympic_; and where doctors disagree who shall
decide? It was wisely decreed that tests should be made to settle the
question.
The main point was this. The officers of the _Hawke_, by far the smaller
vessel, averred that they were drawn out of their course by suction
caused by the movement of so large a ship as the _Olympic_ in the
comparatively narrow and shallow waters of the Solent; in other words,
that the _Olympic_ in moving through the water caused a swirling,
eddying motion in the water, tending to draw a lighter vessel towards
itself. And that is just one of those problems with which theory is
unable to deal. So it was transferred to the National Physical
Laboratory at Teddington, near London, for investigation by experiment.
At this institution, which is a semi-national one, there is a tank
constructed for purposes such as this. The word tank leads us to
underestimate its size somewhat, for it is 494 feet long and 30 feet
wide. It is solidly constructed of concrete, with a miniature set of
docks at one end, and a sloping beach at the other.
On either side are rails upon which run trollys which support the ends
of a bridge which spans the whole. This bridge can be propelled along,
by means of electric motors operating the wheels of the trollys, from
one end of the tank to the other, at any desired speed, within, of
course, reasonable limits, and from it may be towed any model which it
is desired to test.
The models used are usually made of wax, by means of a machine specially
designed for the purpose. It should be explained that the plans of a
ship consist of a series of curves, each of which represents the contour
of the vessel at one particular height. For example, if you can imagine
a ship cut horizontally into slices of uniform thickness, then each
slice could be shown on the drawing (the "shear plan," as it is termed)
by a curved line. Near the keel the lines would, of course, be almost
straight, but they would bulge more and more as they occur higher up.
And what this machine is required to do is to make, quickly and
economically, a wax model which shall be an exact reproduction, on a
small scale, of the vessel under discussion. It may be--it most often
is--a ship as yet unbuilt, the behaviour of which it is desired to test.
Or it may be an existing vessel, as it was in the case mentioned just
now. However that may be, the model is made from the drawings.
A block of wax rests upon a table, while the drawing is spread upon a
board near by. A pointer is moved by hand along one of the lines, and
its movement is repeated by a rapidly revolving cutter which cuts away
the wax to a similar curve. By suitable adjustments the cutter can be
made to magnify or reduce the size, so as to produce any desired scale.
Thus every line is gone over and a similar curve cut in the wax at the
correct height. Of course this only produces a lump of wax shaped _in
steps_, as it were, but it is then quite easy to trim it down by hand,
so as to produce a smooth model of the ship, perfectly accurate in its
shape, and a copy on a small scale of the vessel portrayed on the
drawing.
It can also be hollowed out, ballasted with weights inside, and so made
to sink to any desired level, thereby representing the vessel when fully
loaded, half loaded and so on. All sorts of unequal loading can be
produced if needed, indeed every condition of the real ship can be
imitated in the model.
It can then be towed to and fro in the tank by the travelling carriage
described above. The speed of towing can be varied by changing the speed
of the motors which drive it. The force needed to pull the model through
the water is measured by means of a dynamometer which registers the pull
on the towing apparatus.
A matter very often needing investigation is the shape and size of the
wave thrown up by the bow of the vessel, and of that left behind her,
known as the "bow wave" and the "stern wave" respectively. These waves
represent wasted energy, for they are no use and are produced actually
by the power of the engines of the ship as they drive her along. The
ideal ship would cause no waves, but since that is a degree of
perfection impossible even to hope for, the shipbuilder has to content
himself by so designing his ships that these waves shall be as small as
possible.
The waves are recorded photographically, in some cases by the
kinematograph.
Some of the large shipbuilders have their own tanks, and so have the
naval authorities of the great naval Powers. The one at Teddington was
established through the munificence of a famous British shipbuilder, Mr
Yarrow, who not only defrayed the cost of construction, but gave an
endowment to assist in its upkeep. It is intended to serve the needs of
the smaller builders who have not tanks of their own, and also for the
investigation of matters of general interest to shipbuilders, and for
such tests as that relating to the _Hawke_ and _Olympic_. In this
last-named case, of course, two models were made, one to represent each
ship, and they were towed along in such a way as to imitate very closely
the movements of the ships at the time when they collided. It was as the
result of these tests that the _Olympic_ was ordered to pay damages to
the Admiralty, it being held that she was the cause of the accident.
A very interesting investigation of this kind was recently carried out
in the tank at the United States Navy Yard. The port of New York
consists very largely of jetties projecting out from the banks of the
river. With the growth of the Atlantic liner the old jetties had become
too short, and questions arose as to the elongation of them. If it were
done, how would it effect the current in the river, and the handling of
shipping generally? If, on the other hand, it were not done, what would
be the effect of the ships lying with their ends projecting out into the
stream unprotected by a jetty.
To determine these points the experimental tank was converted into a
model of the New York Harbour, or at all events of that part in
connection with which these questions arose.
A false floor was put in, so as to make the depth exactly right in
proportion to the width. Little model jetties were arranged to represent
exactly the real ones, while against them were moored model vessels, so
that the effect upon them could be observed as the model of the large
vessel was towed past.
In addition to this, special appliances were arranged for finding out
what the disturbance might be which the movement of a giant liner
produces under the surface as well as above it. For this purpose buoyant
balls were employed, moored at various distances below the surface, from
which thin rods projected upwards, the movement of which rendered
visible the movements of the submerged balls and therefore the effects
of the under-water currents.
All these things had to be observed at one and the same time--the moving
model itself, the models alongside the jetties, the commotion on the
surface, the swayings to and fro of the rods attached to the submerged
floats--all, or most of which, at all events, it was impossible to make
self-recording. Yet, seeing that it was of the utmost importance that
the relations between all these things should be observed, and recorded
from time to time as the model was towed along, it is evident that
something must be done, and a cunning use of the kinematograph solved
the problem quite easily. At various points commanding a good view of
the model harbour and its shipping these machines were placed, and so
several series of photographs were obtained, by the study of which all
the different movements could be seen and compared. A large dial too was
rigged up upon the travelling carriage by which the model was towed, a
finger on which denoted the distance which the carriage had travelled at
any moment. This large dial came into each photograph, of course, and so
each picture bore upon itself a clear record of that particular moment
in the voyage of the model to which it referred.
Thus we see an instance of how the very latest and most up-to-date
methods of amusement are sometimes applied to serve very practical
purposes.
Akin to the experiments upon ships are aerial experiments to determine
matters connected with the navigation of the air. At Barrow-in-Furness
the great firm of Vickers, shipbuilders and armament manufacturers, and
latterly builders of aerial craft for the British Admiralty, have
erected a machine for testing the efficiency of aerial propellers and
other things of a kindred nature. Upon the top of a tall tower there is
pivoted a long arm of light iron framework. To the end of this a
propeller can be fixed, so that as the arm revolves there is produced
almost exactly the same conditions as those which prevail when a
propeller drives an aeroplane or steerable balloon.
By means of suitable mechanism the propeller can be turned at any
desired speed, with the result that it drives the arm round and round
upon its pivot on the top of the tower. The force which the propeller
thus exerts can easily be measured, and so can be determined such
questions as the most efficient speed for each type of propeller, the
power which any particular one can develop, the best form for each
particular need, and so on.
Materials, too, require the most careful testing, in order that they may
be put to the best possible use in modern machinery and structures. For
example, anyone can measure the strength of a spring, but what do we
know as to its lasting power? Springs often have to form part of a
machine in which they are stretched and compressed millions of times,
and the question arises as to what is the best shape and material for
the purpose. It may be that the spring which works best a few times will
be the first to become "weary," for with repeated strain such things as
steel get tired, just as the human frame does. Now that is a matter
which will yield to no calculation, the only way to determine it is
actual test. So a mechanism has to be employed which will extend and
compress the spring over and over again, just as it will be in actual
use, with a counter of the nature of a cyclometer to count how many
times it has been subjected to this distortion. Then the apparatus is
set going and left to itself for hours, or even for days, during which
time it may work the spring millions of times. This may go on until it
breaks, or else it may be done a prearranged number of times, and then
the spring taken out and tested by other means to see how its strength
has been affected.
Metal bars are often subjected to sudden blows, light in themselves but
oft repeated. The point to be determined then is how many times the blow
may fall before permanent injury is done to the bar. To investigate such
matters we have the "repeated-impact" machine. The bar is held in a
suitable holder, under a hammer which gives it a blow, the force of
which can be easily regulated, at regular intervals, the number of blows
being counted by a suitable recording mechanism. Ultimately the bar
breaks, under a blow the like of which it can endure singly without any
apparent strain at all. The machine, by the way, can be caused to turn
the bar round to some degree after each blow, so that it is struck from
all directions in succession.
The microscope, too, has established its place in the testing
laboratory. It is a very valuable adjunct to chemical and mechanical
tests.
Suppose, for example, that a bar of steel is being investigated; it can
be put into a machine and pulled until it breaks in two. The machine
registers the amount of the pull which was applied. Or a small piece can
be put under a press and compressed to any desired degree. It can also
be tested by impact or even pulled apart by a sudden blow, as described
in _Mechanical Inventions of To-day_. The bar can be supported by its
ends and loaded or pulled down in the centre, so that its power of
resisting bending can be determined. It can be judged, too, from its
chemical composition. Steel, in particular, depends for its properties
very largely upon its chemical composition. The difference between
cast-iron, wrought-iron and steel, also the differences between the
innumerable varieties of steel, are due almost entirely to the admixture
of a certain percentage of carbon with the metal. This can be
ascertained by chemical analysis. This form of inquiry has the advantage
over the more purely mechanical methods in that the latter, for the most
part, have to be applied to the bar as a whole, whereas the quality may
vary in different parts, the surface in particular being liable to
differ from the interior. In such cases, one analysis can be made of a
piece cut from the surface and another of a piece from the centre.
And it is here, too, that microscopical analysis comes in. For this
purpose a piece is sawn off the bar, and the end ground perfectly
smooth. This is then washed in a suitable chemical, such as a mild acid,
which acts differently upon the different materials of which the "metal"
is built up, thereby rendering them visible one from another. A
photograph taken through a microscope then shows the structure of the
metal; how the different constituents are built together.
This is known as metallographic testing, and its advantage as compared
with chemical analysis is that the latter shows, as we might say, what
are the bricks of which the thing is built, while the former shows how
the bricks are arranged. Indeed it is hardly correct to speak of the
advantage or superiority of one over the other, since each is the
complement of the other, supplying the information which the other fails
to give.
And there are other mechanical tests which have not yet been mentioned.
There are machines which twist a bar so as to discover its power to
resist torsion, there are others which apply a downward pressure on one
part of the bar and an upward one on an adjacent part, so as to show its
capabilities in withstanding shearing strain.
Moreover, many of these tests are nowadays, in a well-equipped
testing-house, carried out in conjunction with the use of heat. It
stands to reason that a part of a machine which will have to work under
considerable heat may have to be of different material from a part which
works under a normal temperature. In some cases the bar is surrounded by
a spiral wire through which electric current is passing, and by the
regulation of this current any desired temperature can be set up in the
bar. Or it may be placed in a bath of hot oil in such a way that the bar
shall be raised to any temperature required, without interfering with
the machinery which exerts the tension or pressure, or whatever it be.
Years ago such elaborate tests as these were never thought of. There are
certain well-known figures, to be found in all engineering text-books,
which give what stresses different materials ought to be able to stand,
and these were, and are still, to a large extent, relied upon, it being
taken for granted that the material used will be up to the average
standard. In large and important works, however, the testing has been
developed upon scientific lines, so that it is known from actual
experiment what each particular thing is capable of. This not only means
security but economy, for it is sometimes found that a substance is
stronger than it is thought to be, and so things made of it can be
designed to give the requisite strength lighter and cheaper than they
would have been otherwise.
Some of the machines employed are of enormous strength, capable of
exerting a pull or a compression of, it may be, 100 tons or more. They
are often made, too, with self-recording appliances, whereby the course
of the test is set down automatically upon a chart. For example, when a
bar is being tested for tension, it is desirable to know not only the
actual pull under which it came in two, but the behaviour of the test
piece during the period before that. It begins to stretch as soon as the
tension is applied, theoretically at all events, and if the metal were
perfectly ductile it would stretch continuously as the load increases,
until at last the breaking stress is reached. But in actual practice it
probably stretches somewhat by fits and starts, and a record of that
fact will be of great value in estimating the strength of the material
in actual work. For such, an automatically made record, which can be
studied at leisure, is of the utmost importance.
But perhaps the finest instance of scientific methods in manufacture is
to be found in the methods by which standard parts of machines are
measured, so as to ensure that they shall be interchangeable.
It may surprise the casual reader to be told that an absolutely exact
measurement is an impossibility. It is safe to say that out of a
million similar articles--articles made with the intention that they
shall be exactly alike--there are no two which are, in fact, absolutely
similar. They may be made with the same machines and the same tools,
handled by the same man, but machines and tools wear or get out of
adjustment, while man's liability to err is proverbial. Astronomers are
the greatest experts in the art of measurement, and they recognise the
possibility, nay, the probability, of error so frankly as to make every
measurement several times over; if it be an important one they make it,
if possible, a great many times over, and then take the average of the
results. By this means they eliminate, to a certain extent at any rate,
the error which cannot be avoided. That process is to allow for errors
on the part of their instruments, for the most part. To deal with
personal errors another method is used as well, for it is known that
some observers have a natural tendency to err on one side more or less,
while others tend to make mistakes in some degree on the other side.
This tendency to err is known as the "personal equation" of the
observer, and there are machines and tests by which the personal
equation of each man can be determined, or perhaps it would be more
correct to say estimated, so that in all observations made by him the
proper allowance can be added or deducted.
But of course it would be extremely difficult to apply such methods in a
workshop. It would never do to have to measure everything several times
over, hoping that the average would come out in such manner as to
indicate that the thing being measured was the size required. Instead,
therefore, of wasting time seeking an accuracy which is known to be
unattainable, the manufacturing engineer adopts a scientific system of
measurement wherein a certain amount of inaccuracy is determined upon as
permissible, and then simple appliances are used to see that it does, in
fact, fall within those limits. For instance, a round bar is to be made,
say, an inch in diameter. Now we know from what has just been said that,
when made, we have no means of telling whether the bar is really and
truly an inch in diameter or not. We consider, then, what it is for, and
decide, say, that it will be near enough so long as we are sure that it
is not larger than one inch plus one thousandth, nor less than one inch
minus one thousandth. So long as it does not exceed or fall short of its
reputed size by more than one thousandth of an inch, then we know that
it will answer its purpose.
Now, having come to that decision, we can build up a system upon which
any intelligent workman can proceed, with the result that all the inch
bars which he makes will be the same size within the limits of 1/1000
over or under, so that the greatest possible difference between any two
will be 1/500.
This system involves the use of two gauges for every size. The man
employed upon making one-inch bars has a plate with a hole in it
1-1/1000 inches in diameter and another hole 999/1000 of an inch in
diameter. One of these is the "go in" gauge; the other is the "not go
in." So that all he has to do, in order to be quite sure that his work
is right, is to see that it can be poked through one of these holes, but
not through the other. No trouble at all, it will be observed, adjusting
fine measuring appliances, simply a plate with two holes in it, and the
workman can be sure that he is turning out articles every one of which
is practically correct, with no variation beyond a slight inequality too
small to matter.
And probably at some other part of the factory there is a man making
articles each of which has a hole in it, into which this bar must fit.
How does he manage? He is provided with a gauge somewhat the shape of a
dumb-bell, one end of which is slightly larger than the other. One is
the "go in" end, the other the "not go in" end. If the hole which he
makes will permit the former to enter, but will refuse admittance to the
latter, then he knows that that hole is sufficiently near its reputed
size to answer its purpose.
[Illustration: _By permission of The Mining Engineering Co., Sheffield_
A MINERS' RESCUE TEAM
These men are equipped with breathing apparatus which enables them to
pass safely through the deadly fumes after an explosion, to rescue their
unfortunate comrades]
In the instances mentioned, a thousandth of an inch either way has been
mentioned as the limit of inaccuracy, or the "tolerance," as it is
sometimes termed, but often the limits are much narrower than that. The
gauges themselves are a case in point, for they must be true within,
say, a ten-thousandth, or even less. And they too are checked by master
gauges of a finer degree of accuracy still, being made by the most
laborious methods, and checked over and over again, so as to reach the
utmost limits in the way of correctness.
So this methodical "scientific" system of "limit gauges" is based upon
the principle of having one gauge limiting the error one way and another
defining it in the other. Anything simpler or more effective it would be
impossible to conceive. It is due very largely to this system that many
manufactured articles are now so much cheaper than they used to be. For
it enables each individual part to be made wholesale on a large scale,
by machines specially adapted to the work, operated by men specially
trained to work them, with the practical certainty that these parts when
assembled together will fit each other.
In conclusion, there is another very interesting instrument which was
first made for a purely utilitarian use--namely, the investigation of
the methods of making coloured glass--but which has since been applied
to some interesting problems in pure science. It is called the
"ultra-microscope."
It must first be pointed out that there is a limit to the power of the
ordinary microscope, beyond which the skill of the optician cannot go.
He is baffled at that point not because of any lack of ability on his
own part, but because of the nature of light itself. An opaque object,
unless it be self-luminous, which few things are, can only be seen by
reflected light. Generally speaking, we see things because they reflect
in some degree the light which falls upon them. But light consists of
waves, and when we reach an object so minute that its diameter is about
half the wave-length of light, then we cannot see it because it is
unable to reflect the light on account of its smallness. We can see
this any day by the seaside, or by a river or large pond. There it is
evident that the waves and ripples are reflected by such things as large
stones, wood posts or anything of any size which come in their way; but
when a wave encounters an object much smaller than itself it simply
swallows it up, as it were, flows all over it or around it, without
being in any way reflected by it. And it is just the same with the waves
of light; they are unaffected by obstacles below a certain size, and so
are not reflected by them. For this reason things smaller than about a
seven-thousandth of a millimetre cannot possibly be seen by a microscope
in the ordinary way.
But if an object can be made self-luminous, then it can be seen,
whatever its size, if the magnifying power of the microscope be great
enough. So this ultra-microscope, as it is called, is really an ordinary
microscope of the highest power possible, with an added apparatus for
making the tiny particles which are being sought for self-luminous. This
is done by directing upon them a pencil of light of exceeding intensity.
Generated by powerful arc lamps, the light is concentrated by a system
of lenses until it is of an almost incredible brightness, after which it
falls upon the object.
Now at first sight this seems to be no different from the usual
procedure with a microscope, and there appears to be no reason why it
should be more successful, but the explanation is this: light is a form
of energy, and the waves of this very intense beam, falling upon the
object, throw it into a state of violent agitation, by virtue of which
it shines, not with reflected light, but with light of its own. It is
not that the waves are reflected, but that they so shake up the particle
that it gives off light waves itself. And thus it comes within the range
of human vision.
In this way, not only have the very small particles of colouring matter
in glass been seen individually, but it is thought that the actual
molecules of matter have been seen, or if not the molecules
individually, little groups of molecules, dancing and capering about,
just as scientific people for years have believed them to be doing,
although they could not see them. So here we have an instance in which
manufacture has aided science--an inversion of the usual order of
things.