THE AMAZING FEAT OF WEIGHING THE EARTH

Greymouth Evening Star, 15 September 1928, Page 9

THE AMAZING FEAT OF WEIGHING THE EARTH

A Scale Three Feet High

CiEN CE is now engaged in th® heaviest job that can be undertaken — that of weighing the earth with a new scale that is only three feet tall! . The result is difficult to convey except in mighty figures. One investigator attempts to. simplify calculations by explaining that if it were possible for a man to construct a globe 800 ft in diameter, and to place upon any one point of its surface an atom 1-4,380 th of an inch in diameter and l-120th part of hn inch in height, it would correctly denote the proportion man bears to the earth on which he stands. The leader of this fascinating feat that so grips the imagination is Dr. Paul R. Heyl, a physicist of the United States Bureau of Standards in Washington, D.C.. who is now stationed in a laboratory cave 35ft underground. Dr. Heyl is equipped with a wonderful new apparatus invented and perfected more than a year ago.

Dr. Heyl’s task will take several months, but in the end he expects to arrive at a measurement which will stand as the last word in accuracy for years. ,

That man, a mere speck standing on a globe 8,000 miles in diameter, should be able to weigh that globe, seems almost beyond belief. Yet, actually, the way in which Dr. Heyl does it is fairly simple and understandable. How he merely applies the law of gravitation, discovered. by Isaac Newton more than two centuries ago, is set forth in the “Popular Science Monthly” by Edwin Ketchum.

This law states that every particle of matter in the universe is attracted toward every other particle with a force varying directly as the product of their masses and inversely as the square of the distance between them. In other words, the force of gravitation acting between two bodies depends on two factors —their distance apart and their mass or weight. It must follow, then, that if the value of this force is known, the mass of on.e of the bodies, and the distance between the two, then the mass of the other body can be calculated. This is exactly how Dr. Heyl calculates the mass of the earth. He asks the question: “How much mass must the earth have to exert the attraction

it 'does upon a body at its surface. 4,000 miles distant from its centre?" And he iAids the answer to this question by first measuring the value of the force of attraction—a value which is known as the “constant gravitation.” This force of gravitation keeps the planets on' their place in the heavens, and it is the most vital, yet most mysx terious, force in the universe. It holds the stars in their courses. Without it, everything on the earth would fly helter-skelter through space. Q It is the only known force which is absolutely unchanging, and which cannot be halted or obstructed. * Electricity can be blocked by insulation, light can be excluded by any opaque substance; but nothing has ever been found, that will shut out gravitation. Dr. Heyl tackles the problem first by placing, side by side, two small objects which can be handled readily, and measuring the gravitational at traction bet ween them. Then he applies the result proportionately to larger bodies, such as the earth. To obtain the greatest, accuracy, Dr. Heyl dpes his “weighing” in a small . room beneath the earth’s surface. This not only assures the constant tempera ture necessary for the use of delicate instruments, but it prevents the upsetting of his calculation by the gravitational attraction of moving objects such a's automobiles and people.

If you should visit this underground laboratory, you would be surprised to find that his earth-weighing “scale” is an apparatus only about three feet tall. It consists of an instrument known as a “torsion balance,” which measures the gravitational attraction betweep little glass balls weighing about two ounces each and steely cylinders weighing 1401 b. / In a vacuum within an iron case hangs a light aluminium rod, suspended in horizontal position from au ; exceedingly fine tungsten wire; and at each end of the rod is a glass ball. This pendulum arrangement is made to swing back and forth, twisting and untwisting the wire. On opposite sides of the iron case, and as close as possible to the balls, hang the two cylinders. The attraction between cylinders and balls affects the swinging time of the pendulum. Thus, when the cylinders; are shifted to positions somewhdt farther from the balls, the gravitational attraction is correspondingly lessened, and the time of swing is changed accordingly. By measuring this difference in time with’ a moving beam of light on a scale, Dr. Heyl calculates the actual value of the attraction.

Thus with this tiny measurement completed, weighing the earth becomes simply a problem, in proportion.