554 



NA TURE 



[October 6, 1892 



The lecturer proceeded to show in action a novel apparatus 

 lie had devised to exhibit the magnetizing process in actual iron, 

 and to test the magnetic qualities of metals. This magnetic 

 curve tracer (Fig. i) consists of two wires — A A and BB — 

 tightly stretched in two narrow gaps in the magnets DD and C 

 ■respectively. The magnet C consists of a piece of slotted iron 

 tube, which is kept constantly magnetized. Consequently, 

 when a variable current passes along the wire BB that wire 

 sags out or in, giving azimuthal movement to a mirror E. 

 The variable current which passes through the wire 

 B serves to magnetize the electromagnet DD, which 

 consists of two bars of the iron to be tested, sunk into 

 fixed pole pieces and united at the back end by a short 

 yoke-piece of soft iron. When the magnetism of DD varies it 

 causes the wire AA, which carries a constant current, to sag up 

 and down, and this gives movement in altitude to the mirror E. 

 The mirror is pivoted on a single needle point, and has freedom 

 to respond to the motion of both the stretched wires AA and 

 BB. Since its azimuth movement is proportional to the magne- 

 tizing current, and its altitude movement is proportional to the 

 mj^netism acquired by DD, the mirror causes a spot of light 

 reflected from it to trace out the ordinary magnetization curve, 

 showing the relation of magnetism to magnetizing force. By 

 making the variable current change continuously from a positive 

 to an equal negative value and back again, a complete cycle of 

 magnetization was performed in the bars DD, and in this way 

 the magnetic characteristics of the bars could be completely 

 determined in a few seconds. The lecturer proceeded to test in 

 succession a pair of wrought-iron bars, then a pair of hard steel 

 bars, and finally a pair of cast-iron bars, causing the cyclic curve 

 for each material to be automatically drawn on the screen, on 

 a very large scale, to exhibit the features of difference. The 

 mirror and other moving parts of the apparatus were so dead 

 beat that it was possible to go through a cycle ten or even 

 twenty times a second without experiencing inconvenience from 

 the effects of inertia. In that case, however, the iron must be 

 laminated to avoid sluggishness in the magnetizing process itself. 

 Using an instrument with a magnet consisting of a split ring of 

 iron wire, a process of periodic reversal was performed at a 

 speed sufficient to make the curve traced out by the light-spot 

 become a continuously luminous line (Fig. 2), and the process 



Fig. 2.— Photograph of Magnetiiation Curve traced by Prof. Ewing's 

 Magnetic Tester. 



of demagnetizing by reversals was illustrated by making this 

 curve gradually contract itself to zero by slowly reducing the 

 strength of the current while the rapid periodic reversals were 

 continued. The effect was also shown of superposing one 



NO. 1 197, VOL. 46] 



periodic alternation upon another, by which loops resembling 

 those of Fig. 3 were drawn. The lecturer pointed out that these 

 experiments went some way towards answering the question 

 whether the magnetizing process went on in the same way, and 

 involved the same dissipation of energy through hysteresis, at 



Fig. 3.— Photograph of Magnetization Ci 



ith Loops. 



high speeds as at low speeds. He concluded by expressing the 

 hope that this apparatus would prove of some service to the 

 builders of dynamos and transformers by giving them a novel 

 means of testing the magnetic properties of their iron with great 

 completeness and in a manner sufficiently simple for workshop 



BOTANICAL PAPERS AT THE BRITISH 

 ASSOCIATION. 



T N our account of the proceedings of Section D of the British 

 ■•■ Association (Nature, August 25, p. 403), we promised to 

 refer on a later occasion to some botanical papers which could 

 not then be noticed. The following are abstracts of several of 

 the more important of these papers : — 



" Observations on Secondary Tissues in Monocotyledons," by 

 Dr. Scott and Mr. Brebner. (i) The secondary tracheides in 

 Dracaena and Yucca develop simply by the enormous growth in 

 length of single cells, the nuclei remaining undivided, and not by 

 cell fusion, as many authors suppose. (2) The cambium in the 

 roots of many species of Dracaena does not appear in the peri- 

 cycle, but in the cortex outside the endodermis. The secondary 

 growth starts from the insertion of a rootlet, the cambium being 

 pericyclic near, and cortical at a greater distance from, the 

 rootlet. (3) Description of secondary thickening in Iridacese. 



" On the Simplest Form of Moss," by Professor Goebel. The 

 author stated that previous researches had led him to the con- 

 clusion that mosses and ferns did not stand in direct genetic 

 relationship with each other, but that they are descended from 

 simple alga-like forms ; in fact the mosses pass through a de- 

 velopmental stage so alga- like in appearance that it was formerly 

 described as an algal genus Proto7iema, If the sexual organs 

 of the moss arose not on the stem but on the protonema, we 

 should have the sexual generation agreeing perfectly with the 

 filainentous algae. The leaves of the moss would then arise 

 originally as protective organs for the antheridia and archegonia. 

 This, up to the present, hypothetical form, actually occurs in 

 Buxbaumia. In this moss the antheridia occur at the end of a 

 protonema-branch, surrounded by a mussel-shaped envelope. 

 The female plant is more highly organised, but is still much 

 simpler than in other mosses. These and other observations lead 

 Prof. Goebel to the conclusion that Buxbaumia is a very 

 ancient form which stands in the closest relation to the lower 

 algse. 



