114 Mr. S. BidwelL On the Changes produced by 



Fig. 1. 



in the table, and its upper end, which is chisel-shaped, acts at B upon 

 a brass lever, one end of which abuts upon the knife edge, A, and the 

 other upon a short arm, C, fixed to the back of the mirror. Shallow 

 notches in the form of obtuse angles are cut in the lever (which is of 

 square section) at the points where it bears upon the knife-edge 

 fulcrum and the end of the rod. The end of the lever remote from 

 the fulcrum has the form of a chisel, the edge of which is turned 

 upwards and fits into a shallow groove cut transversely in the arm of 

 the mirror. By means of a magic lantern, L, illuminated by a lime- 

 light, the image of a horizontal wire is, after reflection from the 

 mirror, projected upon a distant vertical scale, E. A slight deflec- 

 tion of the mirror causes a considerable movement of the image of the 

 wire upon the scale. The actual dimensions are as follows : — The dis- 

 tance AB = 10 mm., BC = 170 mm., DC = 7 mm., DE = 3200 mm. 

 (10 feet 6 inches). The multiplying power of the arrangement, the 

 beam of light being horizontal, is 3200x17x2/7 = 15,543 times.* 

 The scale is one of Elliott's ordinary galvanometer scales, each divi- 

 sion of which is equal to a fortieth of an inch, or 0'64 mm. There- 

 fore a movement of the focussed wire through one scale division 

 indicates a difference in the length of the rod of 0*64/15,543 = 

 0'000041 mm. The length of magnetic metal in the rods used is 

 100 mm., so that a movement of one division shows a difference of 

 0*00000041 in the length, equal to about a two and a half millionth 

 part. 



Eor projecting the image of the wire upon the scale, a half-plate 

 photographic portrait lens of high quality was used. When the best 

 definition was secured, it was possible after a little practice to read 

 the deflections to a quarter of a scale division with tolerable certainty. 



* The multiplier 2 is used because the angle through which the reflected beam of 

 light is deflected is twice the angle of deflection of the mirror. 



