Monday, November 28, 2011
William Gilbert Exemplary Scientist from 400+ Years Ago
Good Science by William Gilbert 1544 to 1603
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!