Barus, C: Displacement Interferometry by the Aid of the Achr

Excerpt from Displacement Interferometry by the Aid of the Achromatic Fringes, Vol. 4

The anomalous behavior Observed in the last report, in treating the elastic deformations Of small bodies on the interferometer, induced me to endeavor to devise a different method for the same purpose. This led to the construc tion of the contact lever, using achromatic fringes described in the first chapter. The instrument at once functioned admirably, when employed either as a surface tester or as a spherometer.

The contact lever is then modified (chapter II) for the interpretation Of the elastic discrepancy specified, and it is shown that both the new and the Old methods lead to trustworthy results, even for material as rigid as brass, if the rods examined are sufficiently slender.

A different kind Of application Of the contact lever is made in Chapter III. The very small elongations with subsequent contractions experienced by iron in magnetic fields are peculiarly interesting, because these phenomena are at their maximum variation after the metal has become magnetically saturated; so that something persists here, Of which the magnetic moment gives but an inadequate account. Hence particular attention is given to the occurrences in strong magnetic fields, though the behavior in very weak fields is also explored. With metals other than iron no effect was Observed.

An instrument which lends itself with equal facility to the measurement Of thermal expansion and to the determination Of elastic moduli is in a measure self-contained for the solution Of many thermodynamic problems. A project of this kind, bearing on the specific heat Of liquids under pressure and tempera ture, is discussed, with the requisite experimental data, in Chapter IV.

Chapters V and VI contain contributions to the electro-dynamometry Of very weak (telephonic) alternating currents. N 0 available effect is Obtained unless the vibrator Of the measuring-instrument is sharply in resonance with the alternation Of current. When it is so, the response is astonishingly large and very definite in amount. In Chapter V the measurement is made by means Of the vibrating telescope, the vibrator of the telephonic system carry ing the Objective. This chapter is merely introductory to the next, and the sensitiveness is not beyond a few micro-amperes per ocular scale-part of reason able value (o.oi Within these limits, however, it may be very serviceable - for instance, in determining the number Of turns in each Of a variety Of secondary coils successively slid over the same long solenoidal primary.

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