

352 Dr. L. T. More on the Changes in Length 



steel support for the fulcrum of the lever. The side view 



of the support was of this shape i J, the two projections 



supporting the knife-edge, and the hody of the lever passing 

 between them. To keep the lever horizontal and to register 

 changes of length, another knife-edge with its blade upwards 

 was embedded in the lever. This knife-edge pressed against 

 the piece mn which was screwed to the wire at m. Now if 

 the points m and a were separated or brought closer together, 

 the knife-edges would no longer remain in a horizontal line 

 and the lever would tilt. The end p of the lever, and the 

 surface d of the arm bed were made planes to support a small 

 brass table, having on its upper face a vertical plane mirror, 

 and for legs three short bits of needles, two of which stood on 

 p and one on d. By this arrangement the change in the 

 length of m a already magnified by the lever was much more 

 apparent when the tilting of the mirror-table was read by 

 means of a telescope and scale. 



The most important dimensions are : — 



Length of wire stretched, ma . . . . 69*9 cm. 



Ratio of lever arms 119/4*65 



Distance between needle-points . . . 2*3 mm. 



„ to telescope and scale . . 1660*0 „ 



The multiplying power of the apparatus was therefore 



119 1660 _ n , n 

 itfk X ~^To X 2 = 36941. 

 4*65 2*3 



And as the scale was graduated to millimetres, one scale- 

 division represented an actual change of length of 2*7 x 10~ 5 

 millimetres. 



The coil to magnetize the wire stood on the stone slab, and 

 was long enough to reach just below the arm ce, so that the 

 part of the wire ma experimented on was in a practically 

 uniform field. 



The principal dimensions of this coil were as follows : — 



Length of coil 83*7 cm. 



External diameter 6*3 „ 



Internal „ 3*6 „ 



Number of wire 18 



Number of turns 3045 



„ layers 7 



„ turns per centim. . . 36*4 



Resistance of coil 10*53 ohms. 



Strength of field per ampere . . 45*74 C.G.S. 



Maximum field 260*00 C.G.S. 



