490 



its spiral ; 2nd. the attraction still subsisting after the circuit was 

 broken ; 3rd. the attraction of the other pole of the needle on the 

 reversal of the current ; and 4th. the attraction remaining after 

 this reverse current was cut off. The sum of the 1st and 3rd 

 observations gives the total change in the magnetism of a bar by 

 the reversal of the current. The sum of the 2nd and 4th gives the 

 total permanent change of magnetism, or the magnetic set. 



The experiments were made with iron bars of the several dia- 

 meters, ^, -^, -i-, i, y, and one inch, the length being in each 

 case one yard ; and also with iron bars i, --, y and one inch diame- 

 ter, of the length of two yards. In all the bars of small diameter 

 up to i of an inch, the magnetic set obtained by the use of feeble 

 currents was found to be proportional to the square of the current 

 employed in producing them. This law was found to subsist 

 through a long series of electric intensities ; but when the current 

 was increased to a certain amount, the set, as observed in the bars 

 of ^ and p^ of an inch diameter, increased in a much higher ratio, 

 so as to vary, in some instances, with the 4th and 6th powers of 

 the current. The point at which this phenomenon takes place is 

 called the magnetic breaking point. A further increase of the cur- 

 rent was attended with a rapid decrease of this ratio as the satura- 

 tion of the bar was approached. 



The total change of magnetic condition by reversal of the cur- 

 rent, minus the magnetic set, is found to be nearly proportional to 

 the intensity of the current. 



Results of exactly similar character were obtained by the use of 

 an electro-magnet, consisting of a bar of hard steel of an inch in 

 diameter and 7f inches long. 



In conclusion, the author points out the striking and instructive 

 analogy which exists between the above phenomena and those of 

 the set of materials as exhibited by Professor Hodgkinson, who, in 

 his admirable researches, has proved that the set, or permanent 

 change of figure, in any beam is proportional to the square of the 

 pressure to which it has been exposed. 



Communications were read also from the ASTRONOMER ROYAL 

 and Mr. MACQUORN RANKINE*. 



* Notices of these will appear in the next Number. 



