INDUCTION OF MAGNETISM BY ELECTRIC CURRENTS. 
289 
Effect of Reversal of Bars two yards long. 
Sum of tangents of deflection. 
Diameter of bar. At 6 inches distance. At 12 inches distance. 
i inch -0450 ‘0088 
i inch -0850 -0300 
finch *5912 -1922 
1 inch 1-3910 -4598 
The magnetism induced in the smaller bars appears to be nearly proportional to 
the square of the diameter, as might have been anticipated. The ratio of the 
attraction at 6 inches to that at 12 inches is 2-98. 
Effect of Reversal of Bars one yard long. 
Sum of tangents of deflection. 
Diameter of bar. At 6 inches distance. At 12 inches distance. 
f inch ...... *0480 *0138 
finch *1260 *0384 
f inch *4926 *1430 
1 inch 1*0380 *3084 
The magnetism induced in the smaller bars of the above set is nearly proportional to 
the square of the diameter; the greater amount of discrepancy arising in all pro- 
bability from the inferior length of the bars compared with those of the last set. 
The ratio of the attractions at the two distances is as 3*39 to unity. 
In the following experiments on the induction of magnetism in the above bars by 
electrical currents, the method employed was, — 1st, to observe the magnetism of a bar 
under the influence of the current ; 2nd, that left permanently developed ; 3rd, to 
observe the magnetism when the current was reversed ; and 4th, the magnetism 
remaining after the current was the second time cut off. The difference between the 
first and third observations gives the entire change in the magnetism of the bar con- 
sequent on the reversal of the current ; the difference between the second and fourth 
gives the entire permanent change, or as I may term it for convenience, the mag- 
netic set. 
The results were obtained by using currents of four degrees of intensity, in the 
first two of which the needle was at 6 inches distance, in the last two at 12 inches. 
The latter results are reduced to the action at 6 inches distance by employing the 
data arrived at from the foregoing experiments. 
