476 EEPORT — 1882. 



2. On Atmospheric Electricity.^ By Professor C. Michie Smith, B.Sc. 



F.B.S.E. 



3. On the Alteration in the Dimensions of the Magnetic Metals by the Act of 

 Magnetisation. By Professor W. F. Barrett, F.B.S.E. 



In 1842 Dr. Joule discovered that when a bar of iron was magnetised by an 

 electric current an elongation of the bar took place. In subsequent experiments, 

 published in 1847, Joule found that the elongation amounted to about ijjj^th of 

 the length of the bar for the maximum magnetisation, and that the total elongation 

 was nearly proportional to the square of the actual magnetisation. By placing the 

 bar in a vessel of water stopped with a capillary tube it was found that the volume 

 of the iron did not augment, and hence Joule concluded that the sectional area 

 diminished in proportion to the elongation. Under longitudinal tension magnetisa- 

 tion caused a shortening of the rod when the tension exceeded 600 lbs. for a rod 

 a quarter of an inch square. Soft steel behaved like iron ; but hard steel, under 

 all circumstances, Joule found to shorten slightly when the magnetising current 

 passed. 



In 1873 Professor Mayer repeated Joule's experiments with new and delicate 

 apparatus ; the elongation of the iron he found to amount to ^.-'^ of its length for 

 the maximum magnetization. Mayer also found that soft as well as hard steel 

 contracted under magnetisation. 



In the same year the author made a series of experiments on the other magnetic 

 metals, nickel and cobalt, and found that whilst cobalt lengthened under magneti- 

 sation, nickel appeared to suffer no change.'^ This result is surprising, for nickel 

 more nearly resembles iron and cobalt than steel in magnetic properties, the former 

 having little coercive force and the latter very considerable retentive power. With 

 entirely new apparatus the experiments were repeated and a distinct shortening of 

 the nickel was now found, cobalt elongating, but not so much as iron. The multi- 

 plying apparatus that was found to yield most satisfactoi-y results was a simple 

 form of optical lever, a mirror vertically fixed over the fulcrum of a lever of the 

 first order and reflecting a scale at some distance into an observing telescope. The 

 apparatus will be more fully described in the report that will be presented next 

 year, a committee, with a small money grant, having been appointed at a previous 

 meeting of the Association to investigate this and certain other molecular changes 

 accompanying the magnetisation of iron described by the author at the Bradford 

 meeting of the Association. 



The results so far obtained may be summed iip as follows : — However often the 

 current traverses the helix aromid the bar of cobalt, the elongation is practically 

 the same after the first current, and amounts to about two-thirds of the elongation 

 produced in an iron bar of the same dimensions. In my measurements the elonga- 

 tion of the iron amounted to about ,-;^~, of its length for the maximum magnetisa- 

 tion ; the iron elongated 55 scale-divisions and the cobalt 35. 



With nickel, after the first magnetising current is passed the retraction i» 

 practically the same for succeeding currents ; with the same scale the nickel retracted 

 100 divisions, or about double the elongation of iron. 



Enclosing the bars in a vessel of water with a capillary tube, having a helix 

 encircling the vessel, no motion of the index was noticed on magnetising the cobalt 

 and the iron, but a slight fall of the index, indicating a diminution of volume, was 

 noticed with nickel. The experiments, however, were somewhat rough, and will 

 shortly be repeated with greater care. 



The ' magnetic tick ' is heard loudly with cobalt and nickel, as well as iron, the 

 former giving a very clear metallic click on magnetisation. 



The author is much indebted to the kindness of Messrs. Johnson and Mattliey for 

 the bars of nickel and cobalt (9^ inches long and 1 inch diameter) with which the 



' Published in Proc. Eoy. Soc, Edin. 1881-82. 

 * Phil. Ma^. January, 1874. 



