372 RECORD OF SCIENCE FOR 1886. 



WiedemaDii has given a resumed of his researches made several years 

 ago on the rehition between the mechanical and magnetic properties of 

 bodies, especially the torsional effects of magnetism, and has added cer- 

 tain new experiments in confirmation of his theory. (Wied. Ann.,Maryh, 

 1886; Phil. Mag., July, 1886, V, xxii, 50.) Bidwell has replied to 

 Wiedemann, giving some experiments of his own which support Max- 

 well's theory. If a longitudinally magnetized wire be fixed at the 

 south end, and if a battery current be passed through it from south to 

 north, the free north end of the wire, as seen from the fixed end, will be 

 observed to twist in the direction of motion of the hands of a clock. 

 Maxwell explains this by the fact that the wire is magnetized both cir- 

 cularlj' and longitudinally, and that the resulting magnetization is in 

 the direction of a right handed screw round the wire. Since Jule has 

 shown that an iron bar is increased in length and contracted trans- 

 versely when magnetized, a spirally magnetized wire would necessarily 

 twist. Bidwell has confirmed Barrett's observation that nickel contracts 

 in length when magnetized. Hence on Maxwell's theory it should twist 

 oppositely to an iron wire when spirally magnetized. This he finds to 

 be the fact. Moreover, Bid well's experiments on iron show that on 

 increasing the magnetization of iron it ceases to elongate and then 

 actually contracts. And further that if the iron wire is stretched, the 

 contraction takes place with smaller magnetizing forces. Now, on mag- 

 netizing such an iron wire spirally, its free end can be made to twist in 

 either direction by varying the current through the surrounding helix, 

 and when the wire is loaded the reversal of the torsion occurs with 

 smaller currents. (Phil. Mag., Se|)tember, 1886, V, xxii, 251.) 



Sack has experimented to determine the specific induction constants 

 of magnets in magnetic fields of different strengths in order to fix the 

 limits of the effect noticed by Lamont that the change of magnetism is 

 greater when the force acts in opposition to the previous magnetiza- 

 tion than when it strengthens it. He finds that the coefficients of 

 weakening and strengthening are appreciably the same for fields in 

 which the strength does not exceed 1.2. With stronger magnetic fields 

 the magnitude of the constant of weakening which is produced by 

 the magnetic field on closing is greater than the constant of strengthen- 

 ing. (Wied. Ann., 1886, No. 9; Phil. Mag., October, 1886, Y, xxii, 386.) 



Deprez has given the results of his experiments on the magnetic in- 

 tensity of the field in dynamo-electric machines, and has observed that 

 this, intensity decreases much less rapidly than the distance between the 

 l)ole pieces increases ; the intensity of the field in one of his experiments 

 diminishing to only one-half its value, while thedistance separating the 

 pole pieces was increased from 7.5 to 75 millimeters, or tenfold. (C. K., 

 October, 1886. c;iii, 712.) 



Leduc has studied the variation in the resistance of bismuth when 

 placed in a magnetic field, and finds that this resistance increases by 

 more than 15 per cent, in a magnetic field whose intensity is 10,000 



