Evening star., 1928-12-23
BY DIR. PAUL r. HEYL,
Bureau Of Standards.

1
Have you ever wondered, when you
have seen men excavating the cellar Of
building and taking away the earth
| q ton at time, how many tons there
might be in the whole earth? Scientific
men are the greatest wonderers to be
found anywhere, and they have not
only asked themselves this question
(there was no one else to ask, but they
have devised experiments to give the
answer.
1t Is, Of course, very large figure
about six thousand million million mil
| lion tonS-Six followed by 81 .ciphers.
| The number Is really known a little
more accurately than that. The first
three figures are known, If any one
| should need them, and experiments
are now in progress at the Bureau of
Standards which, If all goes well, will
give the fourth figure.
But why should any one need to
know this number at all? OF what
possible use can It be to anybody?
There are two answers to this ques-
ton. .t o..
Interest to Astronomers.
| In the first place, knowledge Of the
mass of the earth Is of interest to as
| tronomers, for 1t Is a starting point
from which are obtained the masses OF
the moon and of the sun and of the
other planets Of the solar system. Our
earth, therefore, is a natural standard
of reference with which other quanti-
| ties are compared, and 1t is the busi-
| ness of the Bureau of Standards to de-
| termine such fundamental reference
quantities as accurately as possible.
! In the second place, knowledge OF
| the earth's mass enables us to learn
something about the interior of the
| earth, that region about which so much
| imaginative literature has been writ
ten and which no one can ever hope to
| investigate directly. From the earth's
mass we can calculate its density, that
is, the number of times it is heavier
than an equal globe Of water; and this
leads us to a rather remarkable con-
clusion.
The various rock materials which
| make up the outer layer of the earth,
which are accessible to test and meas
ure, turn out to have an average
/ density between two and three times
that of water, while, curiously enough,
| the figure for the earth as a whole
comes out about twice this value. The
inference Is obvious-the core of the
earth must be composed of something
much heavier than the outer crust.
What can it be?
Core Ball Of Iron.
] For this question, too, it Is possible
to find an answer. Several lines OF ar
gument, including the speed Of travel
of earthquake waves, the phenomena
of terrestrial magnetism and some 9TH
ers, have led those who have given
most attention to the subject to the
strange conclusion that the core Of the
earth is a great ball of ironl
Occasionally there fall to the earth
bodies called meteoriteS, which are
often composed of metallic iron, some
times with stony admixture. These
strange bodies seem to be floating about
in space, and occasionally one Of them
comes near enough to the earth to be
drawn in by It's attraction. The idea
Is that the earth Is like great me-
teorite, or the result Of the accretion
or massing together Of many such par
ticles, in which process the heavier iron
portions have gone to the center by
virtue of their own weight or attraction
for each other. while the lighter stony
portions formed the surface layers. The
meteorites that fall to earth may thus
be regarded as chips left over or un-
used portions Of material.
So much comes out of an apparently
dry and abstuse determination Of a
numerical constant of nature.
How Is the earth weighed? Like
many other scientific measurements,
this operation Is carried out by round
about process.
The first thing to do Is to set up a
| miniature system representing the earth
| and body near its surface, and deter
mine by experiment the actual force of
attraction between these bodies. This
force depends on several things In the
first place, on the masses Of the bodies,
greater bodies attracting each other
with more force than smaller ones; and
second, on the distance between their
centers, bodies attracting more strongly
the closer they are together. All these
quantities are measured in the minia-
ture system: the masses of the two bod
les, their distance from each other and
the force of attraction between them.
Calculate Mass Of Earth.
In the actual case of the earth and
| a body on its surface, we can measure
all but one of the quantities involved.
We can weigh the small body and thus
determine the force with which the
earth attracts 1t. We can determine
by the same operation the mass of the
small body. and we know the distance
from its center to the center of the
earth. What We do not know is the mass
OF the earth. But by applying the re-
suits of our miniature experiment in
which every quantity was known, and
solving problem in proportion, we can
calculate the mass Of the earth.
The most difficult part of this expert
ment Is to determine the force of at
traction between the miniature earth
and the small body attracted by it.
Even with mass of steel weighing 14O
pounds (Which Is used in the arrange
ment employed in the experiment now
In progress at the Bureau of Stand
ards) and ball of gold weighing an
ounce and a half, the force is measur-
able only in millionthS of a grain, in
metric units about thousandth of a
milligram.
To determine this very small force
with the necessary precision, an ar
rangement called a torSion pendulum
is used. A light rod of aluminium about
inches long is hung by very fine
filament Of tungsten at its center. This
| f1lament is such as Is used in incan-
descent lamps. The rod hangs in a
horizontal position and carries at each
| end gold ball. The rod swings very
slowly back and forth In small arc,
with q time of swing of about half an
hour. So delicate is this arrangement
that very small force applied to the
balls will perceptibly alter its time Of
swing.
Experiment In a Case.
The swinging system is inclosed in
an alright case from which the air Is
exhausted This eliminates trouble
from air currents which might disturb
the swings and also allows the system
to swing for long time before coming
to rest
Outside this case the two large steel
masses are placed, at first as close as
possible to the gold balls inside. In