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



magnetic meridian and pointing it towards the centre of a mag- 

 netic needle. When the rod thus directed gave indications of 

 polarity, its S. end was placed downwards, with the axis of the 

 rod in the line of the dip, and its upper end was struck with 

 a light mallet. The rod was tested until, after one or more 

 operations of this kind, it gave no indications of polarity. But 

 on placing the rod in the helix it, of course, was again magne- 

 tized, but feebly, by the earth's induction. This fact serves to 

 determine the distance at which the magnet, which determined 

 the residual polarity, had to be suspended above the rod. If 

 this magnet is placed too near the rod, then an interaction be- 

 tween it and the soft iron of the rod takes place by the inductive 

 action of the magnet, and the vibrations of the latter are more 

 frequent than when it is alone acted on by the earth; but if it 

 be removed to a certain distance above the rod, then the mag- 

 net sm of the rod acts as a "damper" on the magnet, and its 

 vibrations are slower than when it is only under the earth's in- 

 fluence. There is therefore an intermediate position, at which 

 the magnet vibrates the same, whether the rod remains under it 

 or is taken away. This distance, of course, varies with the rod 

 used ; but on the average it was about 3 inches. 



Thus arranged the rod was allowed to remain until its tem- 

 perature had become constant and the scale-reading in the tele- 

 scope was stationary. 



The interpolar connexions with the battery were made so that 

 the helix, on closing the circuit, would magnetize the rod with 

 the same direction of polarity it already had from the earth's 

 action. The current was now passed from the 25 cells by 

 plunging the amalgamated wire of the open part of the circuit 

 into a cup of mercury ; then the scale-reading was immediately 

 noted ; the circuit was at once broken and another reading ob- 

 tained. The thermometers, which were placed on various parts 

 of the apparatus, and which had been read just before closing 

 the circuit, were now again observed and the room vacated and 

 closed. At intervals of a half hour during the three to six sub- 

 sequent hours the room was entered and readings of scale and 

 thermometers obtained. 



It may here be well to give a general account of the pheno- 

 mena which the rods exhibit when the voltaic circuit is succes- 

 sively closed and opened. When the rod has for the first 

 time the heliacal current passed round it, a sudden elongation 

 takes place ; and this elongation remains steadily of the same 

 amount as long as the circuit is closed and the temperature of 

 the rod remains constant. Now on breaking the circuit the rod 

 retracts, with a less velocity than that with which it first elon- 

 gated ; but the retraction does not equal the elongation. The 



