Application to Weighing.
The interferometer has much refined the indications of the balance. In a noteworthy experiment Professor Michelson found the amount of attraction which a sphere of lead exerted on a small sphere hung on an arm of a delicate balance. The amount of this attraction when two such spheres touch is proportional to the diameter of the large sphere, which in this case was about eight inches. The attraction on the small ball on the end of the balance was thus the same fraction of its weight as the diameter of the large ball was of the diameter of the earth,—something like one twenty-millionth. So the force to be measured was one twenty-millionth of the weight of this small ball. In the interferometer the approach of the small ball to the large one produced a displacement of seven whole fringes.
In order that this instrument may yield the best results, great care must be exercised in its construction. The runways of the frame are straightened with exactitude by a method due to Mr. F. L. O. Wadsworth. The optical surfaces of the planes and mirrors in the original designs were from the master hand of Mr. John A. Brashear of Allegheny, Pennsylvania. Each mirror is free from any irregularity greater than 1⁄880,000 inch, and the opposite faces of the mirrors must be parallel within one second of arc, or 1⁄1,296,000 part of a circle.[27]
[27] Interferometers in a variety of designs are manufactured by William Gaertner & Co., 5347 Lake Avenue, Chicago.