468 



♦ KNOWLEDGE ♦ 



[May 29, 1885. 



ON THE CHANGES PRODUCED BY MAGNETI- 

 SATION IN THE LENGTH OF RODS OF IRON, 

 STEEL, AND NICKEL. 



Bv Shelford Bidwell, M.A., LL.B. 

 Read at the Meeting of the Eoyal Societ<j, April 23, 18S5. 



THE earliest systematic experiments on the effects produced by 

 magnetisation upon the length of iron and steel bars are those 

 of Joule, an account of which is publislied in the Phil. Ma-j. of 

 18i7. Joule's experiments have nianj- times been repeated, and 

 his general results confirmed. In particular, Prof. A. iX. Mayer 

 carried out a series of very careful observations with apparatus of 

 elaborate construction and great delicacy. The conclusions at 

 which he aiTived were in accord with those of Joule, so far as 

 regards iron : in the case of steel there was some apparent dis- 

 crepancy, which, however, might to a great extent be accounted for 

 by differences in the quality of the metal used, and in the manner 

 of conducting the experiments. In 1882 Prof. Barrett published 

 in Nature an account of some experiments which he had made, not 

 only on iron but also on bars of nickel and cobalt, with the view of 

 ascertaining the effect of magnetisation upon their length. 



The knowledge on the subject up to the present time may be 

 summarised as follows ; — 



1. Magnetisation causes in iron bars an elongation, the amount of 

 which varies up to a certain point as the square of the mag- 

 netising force. AVhen the saturation point is approached the 

 elongation is less than this law would require. The effect is greater 

 in proportion to the softness of the metal. 



2. When a rod or wire of iron is stretched by a weight the 

 elongating effect of magnetisation is diminished ; and if the ratio of 

 the weight to the section of the wire exceeds a certain limit, mag- 

 netisation causes retraction instead of elongation. 



3. Soft steel behaves like iron, but the elongation for a given 

 magnetising force is smaller (Joule). Hard steel is slightly elon- 

 gated, both when the magnetising current is made and when it is 

 interrupted, provided that the strength of the successive cm-rents 

 is gradually increased (Joule). The first application of the mag- 

 netising force causes elongation of a steel bar if it is tempered blue, 

 and retraction if it is tempered yellow : subsequent a]iplications of 

 the same external magnetising force cause temporary retrac- 

 tion, whether the temper of the steel is blue or yellow (Mayer). 



4. The length of a nickel bar is diminished by magnetisation, 

 the maximum retraction being twice as great as the maximum 

 elongation of iron (Barrett). 



In order that the results of Joule and Mayer might be com- 

 parable with those obtained by the author, he made au attempt to 

 estimate the magnetising forces with which they worked. From 

 data contained in their paper, it was calculated that the strongest 

 magnetising force used by Joule was about 126 units, while the 

 strongest used by Mayer did not on the highest probable estimate 

 exceed 118 units. In the author's experiments, the magnetising 

 force was carried up to about 312 units. The metal rods, too, were 

 much smaller than any which had been before used for the pm-pose, 

 ranging in diameter from I'-tl) to 0'2omm. Their length was in 

 every case 100 mm., and the apparatus was capable of measuring 

 with tolerable certainty an elongation or retraction equal to a ten- 

 millionth part of this length. 



By using thinner iron rods and greater magnetising forces than 

 those previously employed, the following curious and interesting 

 fact was established. If the magnetisation be carried beyond a 

 certain critical point, the consequent elongation, instead of remain- 

 ing stationary at a maximum, becomes diminished, the diminution 

 increasing vrith the magnetising force. If the force is sufficiently 

 increased, a point is arrived at where the original length of the 

 rod is totally unaffected 'oy magnetisation ; and if the magnetisa- 

 tion be carried still further, the original length of the rod will be 

 reduced. It also appeared that the position of the critical point in 

 steel depended in a very remarkable manner upon the hardness or 

 temper of the metal ; considerable light is thus thrown on the 

 apparently anomalous results obtained by Joule and by Mayer. 

 Further experiments disclosed strong reason for believing that the 

 value of the critical magnetising force in a thin iron rod was 

 greatly reduced by stretching; this would explain the fact that 

 Joalo obtained opposite effects with stretched and unstretched 

 wires. 



By ascertaining the relative values of the temporary moments 

 induced by gradually-increasing external magnetising forces, an 

 attempt was made to connect the point of maximum elongation 

 with a definite phase of the magnetisation of the several rods in 

 which the elongation has been observed. 



Though more experiments must be made before it is possible to 

 generalise from them with perfect safety, the results so far obtained 



by the author indicate the laws given below. -The elongations 

 and magnetisations referred to are temporary only; before the 

 beginning of an experiment, the rod was permanently magnetised by 

 passing through the magnetising coil a current equal to the 

 strongest subsequently used. In iron the greatest elongation 

 due to permanent magnetisation was generally found to be about 

 one-third of the total elongation, while in nickel the permanent 

 retraction amounted only to about one twenty-fifth part of the 

 whole. 



I. Irox. 



1. The length of an iron rod is increased by magnetisation up to 

 a certain critical value of the magnetising force, when a maximum 

 elongation is reached. 



2. If the critical value of the magnetising force is exceeded, the 

 elongation is diminished until with a sufficiently powerful magne- 

 tising force the original length of the rod is unaffected, and if the 

 force is still ftu'ther increased therod undergoes retraction. Shortly 

 after the critical point is passed, the elongation diminishes in pro- 

 portion as the magnetising force increases. The greatest actual 

 retraction hitherto observed was equal to about half the maximum 

 elongation, but there was no indication of a limit, and a stronger 

 magnetising force would have jiroduced ftirther retraction. 



3. The value of the external magnetising force corresponding to 

 maximum elongation is for a given rod, approximately equal to 

 twice its value at the " turning point." 



Definition. — The tm'ning-point in the magnetisation of an iron 

 bar is reached when the temporary moment begins to increase less 

 rapidly than the external magnetising force. 



4. "The external force corresponding to the point of maximum 

 elongation increases (when the quality of the iron is the same) 

 with the diameter of the rod. So also does its value at the turning- 

 point. 



0. The amount of the maximum elongation appears to vary 

 inversely as the square root of the diameter of the rod, when the 

 quality of the iron is the same. 



6. The turning-point, and therefore presumably the point of 

 maximum elongation, occtirs with a smaller magnetising force when 

 the rod is stretched than when it is unstretched. 



II. Steel. 



7. In soft steel magnetisation produces elongation, which, as in 

 the case of iron, increases up to a certain value of the magnetising 

 force, and atterwai'ds diminishes. The maximum elongation is less 

 than in iron, and the rate of diminution after the maximum is 

 passed is also less. 



8. The critical value of the magnetising force for a steel rod 

 diminishes with increasing hardness up to a certain point, cor- 

 responding to a yellow temper ; after which it increases, and with 

 very hard steel becomes very high. There is, therefore, a critical 

 degree of hardness for which the critical magnetising force is a 

 minimum ; in steel of a yellow temper the value of the critical 

 magnetising force is lower than in steel which is either softer or 

 harder. 



9. In soft steel a strong magnetising force subsequently 

 diminished may cause a greater temporary elongation than the 

 diminished force is capable of producing if applied in the first 

 place. 



10. A temporary elongation when once produced in soft steel 

 may be maintained by a magnetising force which is itself too small 

 to originate any perceptible elongation. 



III. Nickel. 



11. Nickel continues to retract with magnetising forces far 

 exceeding those which produce the maximum elongation of iron. 

 The greatest observed retraction of nickel is more than three times 

 the maximum observed elongation of iron, and the hmit has not yet 

 been reached. 



12. A nickel wire stretched by a weight undergoes retraction 

 when magnetised. 



<Buv aSHbist Column. 



By Five of Clubs. 



THIS game illustrates the necessity of playing to the score, and 

 not trying for more tricks than are wanted. Z starts out 

 rightly with the idea of making a great game ; but so soon as his 

 partner has won the third trick he sees that as T-Z are four by 

 honours the odd trick only is wanted to win the game. Moreover, 

 the odd trick is essential to save the game. Now, to make the odd 



