Length by Magnetization in Nickel and Iron, 133 



Zinc rods 5 millim. thick were carefully turned on a lathe, and 

 cut into proper lengths. 



The extremities of the zinc rod (z in fig. 1) are concave, 

 so that the convex ends of the brass rods (a, w') come into 

 contact at the axial points of the rod. To the ends of the 

 wire (w) are brazed two short brass rods (w', w'\ about 

 1 centim. long and 5 millim. thick, with their ends made 

 convex. [In brazing the wire care was taken that the axis of 

 the wire coincided with the axes of the rods.] 



A brass rod (b), 5 millim. thick, is placed in contact 

 with w'. At the end of the rod a plane glass plate is 

 attached, so that the steel point p% of the lever comes in 

 contact with it. At the other extremity of the row of rods 

 is a stop. It consists of a triangular prism of brass to 

 which a brass rod (d), 5 millim. thick, is attached. The 

 prism fits in the V-groove, and is fixed tightly by means of 

 a clamping-screw, c. To adjust the length (b c) it is pro- 

 vided with a slit g. The screw (c) can be placed at any 

 part of the slit, and the position of the movable system 

 b, w', iv, w f , z, can be so adjusted that the plane of the lever is 

 perpendicular to the axis of the system. The slight push 

 exerted by the springs (s 1? s 2 ) on the movable system prevents 

 the play of different parts among each other, and was sufficient 

 to overcome friction during contraction. 



Perhaps the following short explanation of figs. 1 and 2 

 will make the various parts of the apparatus clear : — 1 1} l 2 , l 3 , 

 levelling-screws ; a, lever with three steel points (dots in the 

 figure) ; p, pillar with vertical groove ; s 1? s 2 , brass springs 

 attached to the lever for adjustment ; n u n 2i slide arrange- 

 ment for adjusting the strength of the springs ; ki k 2 , k 3 & 4 , 

 clamping-screws for n u ,n 2 respectively ; b, brass rod with 

 plane glass ; w\ iv', brass rods attached to the extremities of 

 the wire w ; iv, wire whose change of length is to be mea- 

 sured ; z, compensating zinc rod; d, brass rod attached to 

 the stop ; g y slit in the stop ; c, clamping-screw. 



The rod was placed inside a solenoid 30 centim. long, which 

 lay in a horizontal position magnetic east and west. It had a 

 resistance of 063 ohm, and gave a field of 37*97 C.G.S. units 

 for a current of one ampere. The internal diameter of the 

 solenoid was 3'0 centim., and no part of the measuring appa- 

 ratus came in contact with it. Care was taken to place the 

 apparatus along the axis of the solenoid, and so to place the 

 wire that its middle point coincided with that of the solenoid. 

 These precautions were always necessary, especially when the 

 wire was thick. 



