S C I K C 



581 



Set 



'J. KJCJH rimentx OH the Mapnrtnm l AVr/ II,,l /ron, an,! 

 on the Kfl'ccts oj licit on Magnets. 



The object of Mr. Barlow's experiment* was to de- 

 termine the relative magnetic power of different kinds 

 of iron and steel m deflecting a magnetised needle 

 from its natural direction. The following were the 

 results which he obtained : 



Scace, wholly iwlfpcndi-ntofthe thickness of the mcUl. 



* Ui I', .luuinul "/'.SVa//.i, vul. II. 



Kx peri- 

 menu on 



tin- 11 lag - 



aetitni of 



red hot 

 iron, and 

 on the ef- 

 r heat 



, 



As the hardest iron and steel were thus proved to 

 have the least power over the needle, Mr. Barlow next 

 tried to determine their relative powers when heated 

 in a furnace, and while each of the different speci- 

 mens were rendered soft. The results which he ob- 

 tained on this point were not so uniform as the pre- 

 ceding, but they were very remarkable. The malle- 

 uhlc iron tvhick had by for the greatest power when cold, 

 had the least of any when heated, and the cast iron, 

 which had the least power when cold, had the greatest 

 power when hot, the increase of power in the latter case 

 being nearly as 3 to 1. 



When the iron passed from the state of while heal, 

 where every kind of magnetic action disappears, to the 

 state of blood red heat, where the magnetic action is 

 strongly developed, there was an intermediate action ; 

 when the iron passed through the shades of bright red 

 and red, which attracted the needle the contrary way 

 to what it did when cold, or at the blood red heat ; 

 that is, if the iron and compass are so placed that the 

 north end of the needle is attracted towards the iron 

 when cold, the south end will be attracted when the 

 iron is red hot, and vice versa ; but as the red changes 

 to the darkest shades of blood red, the usual power of 

 the iron commences, and the needle is attracted the 

 contrary way. In addition to this, the negative action 

 is lets in thoxe positions where the natural cold attraction 

 is the greatest, and greatest where the latter is the least, 

 and greatest of all in that position where the cold at- 

 traction is zero ; that is, in the plane of no attraction, 

 provided the needle is sufficiently near the bar. The 

 bars which Mr. Barlow employed, were 25 inches 

 long and 1^ inch square, and were inclined in the di- 

 rection of the dipping needle, the distances varying 

 from 5 to 9 inches ; but the nearer to the bar the more 

 obvious were the effects. The quantity of negative 

 attraction sometimes exceeded 50. 



Mr. Christie of Woolwich has extended his inquir- 

 ies to the action of heat on magnets, and in a very able 

 paper on the subject, which is printed in the Philoso- 

 phical Transactions for 1824, he has given the follow- 

 ing general results. 



1. That from 3 of Fahrenheit, and even much 

 lower, up to 127% the intensity of the magnets de- 

 creased as the temperature increased. 



2. With a certain increase of temperature, the de- 

 crease in the strength of the magnets is not the same 

 at all temperatures, but increases as the temperature 

 increases. 



S. From a temperature of about 80, the intensity 



decreases very rapidly the temperature in 



tint i' up to UM* tosaperatuie the Uifirmce f the 



cUcMOtenta re Marly constant, beyond that 



ture the difference* of the decrement* aUo inorMa*. 



4. Ueyond tlie temperature of 1OO'- a portion of the 

 power of the taagiel > permanently destroyed. 



5. When any change ' temperature takes place in 

 a magnet, the great*** portion of the effect on the 

 strength of the magnet is produced tnatuntaneoualjr, 

 winch show* that the magnetic power reside* in or 

 very near ike surface. 



'H produced OB unnolarieed iron by 

 changes of temperature are directly the reverse of 

 those produced on a magnet, an increase of tempera- 

 ture causing an increase in the magnetic power of the 

 iron, the limit* between which Mr. Christie observed 

 to beSO'aud 100. 



3. On the effecti of rotation on Magnets and other boditi. 



One of the most curioua discoveries that has been On the ti - 

 recently made iu magnetism, relates to the influence of fecu of ra- 

 rotation on the magnetic forces. This curious property 

 was discovered by Mr. Christie, who found that a plat* 

 of iron made to revolve round u axis passing through 

 its centre, acquires, during its rapid rotation, and 

 possesses, while at perfect rest, a power of producing 

 a deviation in the magnetic needle. The extent of the 

 deviation during rotation, was to the extent after rota- 

 tion as 3 to 2, and in the same direction. Mr. Christie 

 considers the effects which he observed as nearly 

 independent of the velocity of rotation ; a single revo- 

 lution of the plate, or even less, being sufficient to pro- 

 duce the whole effect. Mr. Christie supposes that all 

 the phenomena which he observed may be explained 

 on the supposition that the mass of revolving iron acts 

 from its centre, and that the rotation polarises it in a 

 direction at right angles to the dip. 



This curious subject was taken up by Mr. Barlow, 

 who fixed a 13 inch mortar-shell to the mandril of a 

 powerful turning lathe, wrought by a steam engine, 

 and caused it to perform 840 revolutions in a minute. 

 The magnetic needle deviated several degrees from the 

 magnetic meridian, and remained stationary during 

 the motion of the shell. When the motion of the shell 

 was inverted, an equal and opposite deviation of the 

 needle took place, but the needle always remained 

 stationary during the motion of the shell. When the 

 action of the earth on the needle was neutralised, so 

 that the needle obeyed no other force but that of the 

 magnetism of the revolving ball , and when the needle 

 was made a tangent to the ball, its nurlh eml was at- 

 tracted to the ball when the motion of the ball was towards 

 the needle, and repelled when the motion of the ball 

 was from the needle. No effect was observed in the 

 two extremities of the axis, but the deviation was a 

 maximum, and towards the centre of the ball, in two 

 opposite points, at right angles to the axis. From these 

 facts, and from the non-coincidence of the magnetic 

 axis with the earth's axis, Mr. Barlow is disposed to 

 think that the earth's magnetism is of the induced 

 kind. 



Long after Mr. Christie had discovered the effect 

 of rotation in developing magnetism in iron disks,* 

 M. Arago discovered that plates of copper and other 

 substances, put into rapid rotation beneath a magnet- 

 ised needle, caused it to deviate from its direction, and 

 finally dragged it round with them. This experiment 



* Mr. Christie's discovery was, we believe, made so long ago as 1921 or 1882, M. Arago's in 1824, or the beginning of 1325. 



