358 Effects of Magnetization in changing the Dimensions of Iron. 



brass rods firmly cemented into a block of stone. Between 

 these rods, securely fixed in the stone, were placed the rods of 

 iron whose elongation he desired to measure. On the vertical 

 rods slid a transverse bar of brass carrying " a vertical rod of 

 brass which moves freely and accurately in a long brass collar. 

 The lower end of the brass rod rests upon the upper flat surface 

 of the iron bar. To the top of the brass rod is attached a point 

 of steel ; and this point passes against a plate of agate, near a 

 pivot which forms the fulcrum of a lever. The distant end of 

 the lever is connected by a very fine wire, with an axis on which 

 is fixed a small circular mirror. If the steel point be pushed up 

 against the agate plate, the end of the lever is raised ; the axis 

 is thereby caused to turn, and the mirror rotates." The angular 

 deflections of the mirror he determined by the method of Pog- 

 gendorff — that is, by viewing in a telescope the divisions of a 

 fixed scale reflected from the mirror. 



Dr. Tyndall gives the following account of his experience with 

 this apparatus :— " Biot found it impossible to work at his expe- 

 riments on sound during the day in Paris ; he was obliged to 

 wait for the stillness of night. I found it almost equally diffi- 

 cult to make accurate experiments, requiring the telescope and 

 scale, with the instrument just described, in London. Take a 

 single experiment in illustration. The mirror was fixed so as to 

 cause the cross hair of the telescope to cut the number 727 on 

 the scale : a cab passed while I was observing; the mirror qui- 

 vered, obliterating the distinctness of the figure, and the scale 

 slid apparently through the field of view and became stationary 

 at 694. I went upstairs for a book ; a cab passed, and on my 

 return I found the cross hair at 686. A heavy waggon then 

 passed, and shook the scale down to 420. Several carriages 

 passed subsequently ; the figure on the scale was afterwards 350. 

 In fact so sensitive is the instrument, that long before the sound 

 of a cab is heard its approach is heralded by the quivering of 

 the figures on the scale. 



" Various alterations which were suggested by the experiments 

 were carried out by Mr. Becker, and the longer I worked with 

 it the more mastery I obtained over it ; but I did not work with 

 it sufficiently long to perfect its arrangement. Some of the 

 results, however, may be stated here : — 



Figure of scale. 

 Bar unmagnetized . . . 577 

 Bar magnetized . . . . 470 

 Bar unmagnetized . . . 517 



" Here the magnetization of the bar produced an elongation 

 expressed by 107 divisions of the scale, while the interruption 



