278 PHYSICS. 



seemed conclusive that the copper wire really was itself a magnet in 

 this case. — {Nature^ xxi, 145, December, 1879.) 



Lemstrom has described to the Physical Society of St. Petersburg 

 an experiment of great apparent significance. He finds that a ring of 

 insulating material, when rotating about its axis of symmetry with a 

 high velocity, acts like a galvanic circuit and produces a magnetic field 

 in the space within it. He ex^^lains it by supposing that the ether in the 

 insulator, being dragged along by the ring, produces, vortical motion of 

 the ether in the central space, which vortical motion be conceives to be 

 the essential condition of a magnetic field. On this experiment the 

 author founds his theory of terrestrial magnetism. Since rotating an 

 iron bar within an insulating medium ought to magnetize the bar, the 

 earth being a magnetic body rotating in an insulating medium ought to 

 be magnetized by its rotation.— (JV^atwre, xxii, 89, May, 1880.) 



Piazzoli has studied the influence of magnetism on the tenacity of 

 iron. Wires were hung between two hooks and ruptured by pouring 

 water into a vessel suspended from them. These wires were 'dlj"'^ long, 

 and were inclosed in a helix of four layers of wire. The current 

 was made to traverse these layers all in one direction, or two in one 

 direction and two in the opposite, tlie effect of heating being thus elimi- 

 nated. The wires annealed in charcoal broke at 12(30 to 1300 with, and 

 1213 to 1270 without magnetization. Wires annealed in carbonous oxide 

 1732.4 to 1742.7 with, and 1703.02 to 1719.87 without the current. 

 Those annealed in hydrogen, 1289.5 to 1310.1 with, and 1263 to 1299.7 

 without the magnetism. The values are about from 1 to 3 per cent, 

 greater for the magnetized than for the unmagnetized wires, showing 

 that the tenacity of iron increases on magnetization. — {Nature, xxii, 89, 

 May, 1880.) 



Eighi has sought to harmonize the discordant results as to the changes 

 in the length of iron bars when magnetized. By means of a mirror 

 method which magnified the elongation 8,CC'0 times, he ascertained tha 

 (1) magnetism produces an increase in iron and steel in the dimension 

 of the direction of magnetization; (2) a part of this increase remains 

 after the current ceases, being more or less according to the coercive 

 force; (3) when the current is not very strong, the elongations are pro- 

 portional to the square of the current strength ; (4) after a strong current 

 has been sent through the spiral, a weak current sent in the opposite 

 direction produces a shortening, though the bar, even when demagnetized 

 by it, remains longer than normal ; (5) during reversal of polarity the 

 length becomes momentarily less, the eflect being oscillatory; (G) a bar 

 shortens at the instant of closing the circuit, when the current traverses it 

 directly; (7) it elongates on opening, but by a less amount; (8) on rever- 

 sal, the bar elongates and oscillates ; (9) the contraction is greater if 

 the bar has been longitudinally magnetized previously; and (10) some 

 bars show a tendency to take spiral magnetization, *. e., to rotate the 

 magnetic axes of their molecules in the direction of the turns of the 

 helix.— {Mature, xxii, 543, October, 1880.) 



