English physician, physicist, natural
philosopher and author of De Magnete (On the Magnet and Magnetic
Bodies) and of “On the Great Magnet the Earth.”
Wonderful account of scientific
thinking and experiments from the distant past.
Among his conclusions were that the
Earth's core was made of iron. And he correctly observed and identified as important that when magnets are cut in two,
each forms a new magnet with north and south poles. In physics we say that magnetic monopoles do not exist (under earthly conditions.) An exception might be during the very early stages of the big bang. But I digress.
...but back to Dr. Gilbert in the year 1600.
A few impressive bits from the
quotation below....
Notice Gilbert clearly describes the
distinction between the magnetic north pole and the geodetic or
geographic north pole. This is called the “variation of the
location,” that is, the difference in direction between true north
and magnetic north. A “must know” for navigation by magnetic
compass.
An iron bar suspended from a cross-woven
silk cord becomes an elegant experiment under Gilbert.
QUOTATION from Chapter 12 Book 1 of De
Magnete (with a little editing)
A long piece of Iron, even though not
excited by a loadstone, settles itself toward North and South.
Every good and perfect piece of iron,
if drawn out in length, points North and South, just as the loadstone
or iron rubbed with a magnetical body does; a thing that our famous
philosophers have little understood, who have sweated in vain to set
forth the magnetick virtues and the causes of the friendship of iron
for the stone.
You may experiment with either large or
small iron works, and either in air or in water. A straight piece of
iron six feet long of the thickness of your finger is suspended (in
the way described in the foregoing chapter) in exact equipoise by a
strong and slender silken cord.
But the cord should be cross-woven of
several silk filaments, not twisted simply in one way; and it should
be in a small chamber with all doors and windows closed, that the
wind may not enter, nor the air of the room be in any way disturbed.
For which reason it is not expedient
that the trial should be made on windy days, or while a storm is
brewing.
For thus it freely follows its bent,
and slowly moves until at length, as it rests, it points with its
ends North and South, just as iron touched with a loadstone does in
shadow-clocks, and in compasses, and in the mariners' compass. You
will be able, if curious enough, to balance all at the same time by
fine threads a number of small rods, or iron wires, or long pins with
which women knit stockings; you will see that all of them at the same
time are in accord, unless there be some error in this delicate
operation: for unless you prepare everything fitly and skilfully, the
labor will be void.
Make trial of this thing in water also,
which is done both more certainly and more easily. Let an iron wire
two or three digits long, more or less, be passed through a round
cork, so that it may just float upon water; and as soon as you have
committed it to the waves, it turns upon its own center, and one end
tends to the North, the other to the South; the causes of which you
will afterwards find in the laws of the direction.
This too you should understand, and
hold firmly in memory, that as a strong loadstone, and iron touched
with the same, do not invariably point exactly to the true pole but
to the point of the variation. So does a weaker loadstone, and so
does the iron, which directs itself by its own forces only, not by
those impressed by the stone.
And so every ore of iron, and all
bodies naturally endowed with something of the iron nature, and
prepared, turn to the same point of the horizon, according to the
place of the variation in that particular region (if there be any
variation therein), and there abide and rest.
-William Gilbert De Magnete published
in 1600!
