Evening star., 1940-03-12
Of Stors, Men
And Atoms
Notebook Of Science Progress
In Field, Laboratory
And Study
By Thomas r. Henry.

And Study
By Thomas r. Henry.
Equipping ships with electric girdles,
| such as that which protected the Queen
Elizabeth against magnetic mines, prom
| ises to be an efficient but extraordinarily
| costly measure
For a relatively small ship, 1t is esti-
mated by experts here, the expense would
| run somewhere between $4,000 and
$50,000 and involve the use OF an enor-
| nous amount OF copper.
The general principle-although not
| the refinements which the British may
have introduced-are well known to ex-
| perts on the earth's magnetism and also
| to the United States Navy. The depart
ment of terrestrial magnetism OF the
Carnegie Institution of Washington has
| long been the world headquarters for the
study of magnetic phenomena and in
| keeping up to date the magnetic map
of the earth. During the last war it was
engaged largely in studying the possible
military and naval applications of mag
netism and much that has developed in
the present war was already known to
its experts 20 years ago
| Both German and British develop
ments OF the magnetic mine and of the
electric girdle, however, were entirely
independent of any American work. The
| general principles are explained by ex-
perts of the terrestrial magnetism labora-
tories.
Anything made Of steel possesses, to
some extent, magnetic field For such
! bulk as the hull of ship this is con-
siderable, and depends to large extent
on the position in which the craft lay
| in the yards while it was under construc-
| ton. The force is greatest if the ship
| lay end-to-end in line between the two
magnetic poles, and least If 1t lay at
right angles to this line.
The magnetic attraction of such a hull
| Is great only relatively, when compared
to the magnetic field of the earth in
| specific place Actually it probably
would not draw to itself iron filings an
inch away. The attraction decreases
very rapidly with distance This de-
crease is proportional to the cube of the
| distance.
The really significant factor about the
Shlps magnetism Is the enormous dis
| tortlon 1t may cause in the magnetic
field OF the earth in its immediate
vicinity. The intensity of this field NAV
be increased over limited distance and
in certain direction as much as 200
to 300 per cent
| The earth Is a weak magnet It's field
has two components-the vertical and
horizontal Assume a magnetized needle
mounted on universal axis so that it
could turn freely in any direction and
in all dimension. When one is on the
magnetic equator such a needle will be
| absolutely horizontal between the north
and south magnetic poles. The mag
netlc equator is an irregular belt around
the world in about the position of the
terrestrial equator. For long distances,
In TACT, the two are Identical but in
some places the former lies far from the
latter. The Carnegie Institutions mag
netic observatory in Peru, for example, is
almost exactly on the magnetic equator
but II degrees south of the terrestrial
| equator.
| AT the North and South Poles. on the
other hand, the position of the needle
will be absolutely vertical. Between the
magnetic equator and either of the poles
such a needle will take nearly all pos
Slole positions between horizontal and
! vertical, depending on the latitude. This
would be the case, at least. under ideal
conditions. 1t is hard to find such con-
dltions because of the disturbing in
fuence OF magnetic bodies.
This degree of dip depends upon the
pattern OF the earth's magnetic field in
a particular place. Such a needle can
be weighted so as to take any desired
angle between the horizontal and verti-
| cal. 1t will remain motionless so long
as the magnetic field Of the earth re-
mains unchanged at that spot.
The presence of any steel ship alters
the whole magnetic geography of its
immediate environment. A needle
weighted for one magnetic field takes
quite different position when this field
| Is suddenly changed.
This, experts believe, is the basic prin-
SipAo VI lile HldSNeLlC mine. 1t is the
disturbance of the balance adjusted to
the earth's field which makes the elec-
trical contact and sets off the explosive.
AS matter of fact the principle was
known to the American Navy during the
World War and some magnetic mines
! actually were constructed.
Essentially the same idea of making
use OF the vertical dip of the needle has
had a wide application in prospecting
instruments used to locate ore deposits.
Enormous refinements and improve
ments, it is probable, have been devised
since. These would come under the
head of military secrets.
The possibility of the electric girdle
defense, experts say, would be fairly
obvious to any one very familiar with
the workings of magnetism.
The purpose is not to "demagnetize"
the ship. 1t is to so alter the magnetism
| OF the craft that it will produce a Diff
ferent distortion Of the earths field.
This results in the mine being set of be-
| fore the ship reaches it, at distance
sufficient to reduce the effect to a bad