CHAPTER III. 



THE ELONGATION DUE TO MAGNETIZATION, MEASURED WITH THE INTER- 



FERENTIAL CONTACT LEVER 



13. Introductory. The small longitudinal displacements due to magneto- 

 striction have been frequently subjected to investigation and an excellent 

 summary is given in Winkelmann's Handbuch, vol. 5, p. 307, et seq., 1908. 

 The measurements of Professor Knott and his students, notably Nagaoka 

 and Honda (Phil. Mag., vol. 37, p. 131, 1894), are particularly complete. In 

 1911,* my son, Mr. Maxwell Barus, and I used these phenomena for the pur- 

 pose of testing a peculiar type of interferences then under discussion. 



The present purpose is similar, being a test of the contact lever described 

 in Chapter I; i.e., to find how small a field can be detected from the elonga- 

 tions of an iron rod within it, and vice versa. Furthermore, to find to what 

 degree the magnetic field may increase, before the subsequent contractions 

 appreciably cease. 



The elongation phenomena are necessarily complicated by the occurrence 

 of hysteresis loops, to which the present paper (in which the measurements 

 are not made by the continuous variation of currents and field, but by suc- 

 cessively making and breaking the circuit) will give no attention. This sub- 

 ject has been adequately explored by Professor Knott f and the authors cited. 

 The chief interest in this paper is rather the continued increase of the contrac- 

 tions due to magnetization, not only after the latter has practically reached 

 a maximum, but in a marked degree, and at an unexpectedly persistent rate 

 relative to the magnetizing field, so far as I have gone (fields up to 800), 

 indefinitely. There is no sure indication of a cessation of the contraction. 

 Hence the magnetic contribution of the present paper is to lie in the treatment 

 in strong fields. 



14. Apparatus. The contact lever shown in figures i and 2 of Chapter I 

 was modified as indicated in figure 29 (plan), where F is the semi-circular 

 fork in a vertical plane, rigidly attached to the bed-plate of the interferometer 

 by a strong clutch (not shown) holding the cylinder g or handle of the fork. 

 The vertical axis a of the contact lever is secured between the screw-pivot b 

 of the fork. The horizontal strip of brass d, rigidly fastened to the middle of 

 the axle a, carried at its end the auxiliary mirror mm' of the quadratic inter- 

 ferometer. For this purpose, a short length, /, at the end of d has been bent 

 upward at right angles to d, so that mm' may be held between plates of brass 

 by the yoke-shaped steel slip c. At the side of the lever is a vertical brass plate 

 inset c, to which a small glass plate n has been fastened with cement. It is 



* Carnegie Inst. Wash., Pub. No. 149. 



t Knott; Phil. Mag. (5), xxxvn, p. 141, 1894. 



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