PREFACE. 



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. No 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 cm.) . 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. 



The sensitiveness may be increased upwards a hundred-fold, however (so 

 that io~ 8 ampere per fringe is measurable), by placing an instrument similar 

 to the last on the displacement interferometer adjusted for achromatic fringes. 

 The reading in such a case must be made with a vibration telescope, syn- 

 chronized with the alternating current in the primary and with the objective 

 vibrating normally to the displacement of fringes. The measurement is thus 

 somewhat awkward, and consists in determining the range of fringe ellipses 

 parallel to the direction of the vibration of fringes. To make amends for this, 

 however, both the amplitude and the phase of the induced current are given 



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