﻿484 Mr. F. L. 0. Wadsworth on a JS T eio Method of 



which it was first used. It consists simply of two pieces of 

 round iron |- inch diameter, each bent into the shape of a very 

 shallow U, with legs about 1 inch long and back or yoke 

 2J inches long (this being the distance between the centres 

 of the two members of the astatic system), wound with about 

 350 turns of No. 18 magnet-wire. With a current of 

 5 amperes in the magnetizing coils, and a separation between 

 the poles of the two halves of 3 millim., the density in the 

 iron cores amounts to about 5000 O.Gr.S. 



The system to be magnetized is laid on its back on the edge 

 of a narrow strip of wood b mounted on a board B (figs. 1 & 2, 

 Plate XIII.). This strip is a very little wider than the length 

 of the needles of the system, and of such a height that the top 

 edge is just below the centre of the poles of the electromagnet, 

 the two yokes of which rest on the supporting base. Thin 

 strips of glass or mica are cemented on each side of the 

 central strip opposite the poles of the magnet, to prevent the 

 latter from touching the needles. The coils of the two yokes 

 are so connected that the whole forms a single magnetic 

 circuit with the two air-gaps m, n, in each of which lies one 

 of the members of the magnetic system, so that when the 

 current is turned on by means of a key, K, they are strongly 

 and equally magnetized in opposite directions, as required. 

 In order to secure the maximum permanent magnetization, 

 the strength of the field should be suddenly increased to a 

 maximum and then gradual 1 y decreased, several times in 

 succession, either by varying the strength of the current in 

 the coils by means of a rheostat, or, better, by varying the 

 distance between the two poles *, tapping the support with a 

 light hammer or block of wood when the magnetic induction 

 is at a maximum, in order to assist in "setting" the mole- 

 cular structure. The magnetization being finished, the system 

 is lifted carefully from its support, hung by means of its 

 attached fibre in a glass tube large enough to allow it to 

 swing freely, and tested for astaticism. 



* In no case should the magnetizing current be broken suddenly when 

 the poles are close to the needles, or the strength of the latter may be 

 very considerably weakened, if not destroyed, by the sudden recoil of 

 the field. See Auerbach, Wied. Ann. xiv. (1881), and Fromme, Wied. 

 Ann. v. (1878) :— " Auerbach lays down as a general principle that 

 when the variation of the magnetizing force is slow and continuous the 

 velocity of the transition does not influence the final magnetization ; but 

 sudden transition causes the final magnetization to be less or greater than 

 that obtained by gradual transition, according as the passage is from a 

 greater to a less or from a less to a greater force." (Encyc. Brit. art. 

 " Magnetism," p. 260.) 



In this connexion see also paper by Lord Rayleigh, Phil. Mag. [4] 

 vols, xxxviii. & xxxix. (1869, 1870). 



