Excerpt from Experiments With the Displacement Interferometer
The present volume contains applications of the displacement interferometer to subjects which suggested themselves from time to time. Unfortunately it was not possible, in the laboratory of Brown University, which is situated on a hill in the middle of a large city, to carry out any experiment to its final degree of rigor. Quiet surroundings, a location free from tremor, and irregular temperature variations would have been necessary. But the development of methods of the kind in question was nevertheless quite feasible; and without attempting to push them to a limit, the range of application could be fully investigated.
Among the subjects selected for treatment was the horizontal pendulum. In the first part of Chapter I certain available forms of the pendulum, with and without a float, are considered and tested as to their discrepancies, through long lapses of time, by a reflection method. Among the interesting results obtained is the suggestion of an apparatus capable of measuring changes of elongation to the amount of even less than 4 X 10 of the total length per vanishing interference ring.
In the second part of the chapter the interferometer itself is used, a serviceable method of application worked out, and the range of application studied through many months. With a relatively very wide scope (several seconds of arc) there should be no difficulty, under proper surroundings, of measuring changes of inclination as small as 3 X 10 seconds of arc per interference ring, and it is probable that one could reach smaller angles by modifying parts of the pendulum.
In Chapter II an attempt is made to use this interferential horizontal pendulum for the measurement of the gravitational attraction of two parallel disks. What was obtained, however, was a definite repulsion of the disks, decreasing with their distance apart and appreciable even within 1.5 mm. of this distance. As the method of measurement contemplates the viscosity of the film of air between the disks, and as the effect of any natural charge or potential would be insignificant in comparison with the forces observed, it is probable that the repulsion in question is attributable to the molecular atmospheres by which the disks are surrounded in air, supposing that such atmospheres of gas increase in density as the surface of the disk is approached.
Chapter III is introduced as a severe test on the interference equation employed for the case of path differences resulting when glass columns as much as 10 inches long are inserted in one of the component beams of the displacement interferometer. It appears that the constants of any dispersion formula may be obtained directly from these observations.
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