Book Description

Excerpt from Interferometer Experiments in Acoustics and Gravitation In Chapter V, on the direct interferometry measurements of the compression of a sound-wave, much of my work has been superfluous, as it was anticipated in an admirable paper by Raps, using the Jamin interferometer. I have therefore given only as much as is necessary for the coordination of the other chapters. My method, however, is, I think, superior, owing to its much greater flexibility and the ease with which fringes in any orientation may be produced and shortened to a string of silvery beads. The simple organ-pipe blower or adjustable embouchure much used in the chapter will, I think, be found serviceable for many purposes, both of research and instruction. As the telephone is an indispensable convenience throughout these chapters, it was thought necessary to begin an interferometer investigation on the vibrations of the plate of that remarkable instrument. What comes out definitely in the research, Chapter V, is the readiness of the plate to quiver in overtones. A small mirror at the center is not therefore displaced, as a rule, translationally, but rather rotationally, giving rise to very complicated wave-forms, difficult to analyze. In corroboration of this, it was found (in Chapters IV and V, for instance) that a telephone current may often be commutated. In the endeavor to place the Foucault mirror on the interferometer I have thus far, for incidental reasons, failed of achievement; but as different apparatus useful in experiments of the present kind were tried out in the course of the work, I have given a brief account of it in Chapter VII. In Chapters VIII and IX, in deference to the wishes of Dr. R.S. Woodward, I have begun a search for methods of measuring the acceleration of gravity other than those classically in use. Such an inquiry necessarily consists in referring gravitational forces to forces generated in other mechanisms. An interferometer torsion-balance is first tested, but the results are found to contain relatively large and uncontrollable temperature coefficients, both of rigidity and viscosity, even if the ordinary effects of viscosity can be allowed for. The other (pneumatic) method for g, in which gravitational pull is referred to the pressure of a gas, has at the outset much to recommend it, for it admits of rough handling in spite of the otherwise surprising precision of results. The two errors which offer a serious menace to the accurate hydraulic weighing of the Cartesian diver, viz, the diffusion and solution discrepancies, though at first approach apparently insuperable, may not remain so indefinitely. At least, in experiments on the diffusion and convection of gases in narrow tubes, made in the lapse of years, coefficients of a negligibly small order of value were obtained. Though the work is very laborious, I think it will be worth while to carry it further. The remainder of the volume is largely concerned with work (Chapters XI and Xii) bearing on the constant of gravitation. The object of these experiments was at the outset a mere endeavor to read the deflections of the gravitation needle by displacement interferometry. The plan succeeded at once, almost beyond my expectations; but on computing the Newtonian constant it came out actually several times too large. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com