190 Prof. A. M. Mayer on the Magnetic Elongations 



cupied in gradually decreasing the interposed resistance until it 

 was entirely out of the circuit ; yet during this very slow increase 

 of the current-strength the scale slowly and smoothly moved 

 upwards in its readings, and when all the interposed resistance 

 had been passed over the elongation again equalled 1-3 division. 



The establishment of the above fact was of considerable im- 

 portance ; for it rendered applicable the following simple and 

 precise method of measuring the change in dimensions of bodies 

 on their magnetization. Two iron, steel, or bismuth bars are 

 placed parallel to each other, in Vs, with their similar ends 

 strongly pressed against a firm support, so that, if the rods 

 change in length on magnetization, their free ends will move. 

 Now imagine a lever so arranged that one end of it carries a 

 plano-convex lens and the other end a micrometer-screw. The 

 convex side of the lens is opposite a plane glass which terminates 

 the end of one of the rods, while the point of the micrometer- 

 screw touches the end of the other rod, against which it is pressed 

 by a spring. A piece of plane glass, inclined, placed in front 

 of the lens sends the light from a sodium flame down to the 

 lens and plane glass behind it; and by means of a microscope 

 we can look through the inclined glass on to the lens, and thus 

 accurately view and measure the Newton's rings which we shall 

 now observe. If round the rods we now pass a voltaic current 

 of gradually increasing intensity, we shall see the rings gradually 

 displaced ; and from the amount and direction of this displace- 

 ment, together with the knowledge of the wave-lengths of the 

 rays of sodium-light, we can accurately determine the amount 

 and direction of the motion of the ends of the rods. If, how- 

 ever, the current had passed at once with its full intensity, 

 then would have followed a sudden displacement of the rings, 

 but the amount and direction of this displacement it would 

 have been impossible to determine. By making the arm of 

 the lever which carries the convex lens longer than the arm 

 which carries the screw, we can increase the delicacy of the ap- 

 paratus ; for it is understood that, as the rods move in the same 

 direction, the rod carrying the plane glass moves towards the 

 lens, while at the same time the other rod, through the interven- 

 tion of the lever, pushes the lens towards the plane glass. 



The examination of the experiments of the first and second 

 series, contained under the heading " on gradually increasing 

 the resistance," makes known a remarkable phenomenon. In 

 these experiments the current with its maximum strength was 

 first passed through the helix, and then it was gradually brought 

 down to its minimum strength by sliding the copper battery- 

 wire over the fine wire of German silver until the whole length 

 of the latter was brought into the circuit. At the moment of 



