Theory of Magnetism. 243 
the resistance to the magnetic disturbance of the surrounding 
space been increased by the shortening of the magnet, but the 
magnetizing force has also been decreased, since the coils of 
wire exerted some magnetic force upon the portions cut off. 
I endeavoured to determine the length for which the mag- 
netism of the open magnet would, with a constant current, 
become doubled, by lengthening the poles with pieces of iron 
of the same diameter and 10 millim. in height. This con- 
dition, according to the following Table, was exceeded when 
five pieces had been added to the poles, corresponding to an 
increase in length of the original magnet by something more 
than half. 
Tasye II. 
Current 0°1 ampere. 
Magnetism. Increase. 
Without addition ......... 1950 if 
1 piece on each side......... 2430 480 
2 pieces on each side ...... 2805 465 
3 a5 Ses ea rCOre 3330 435 
4 i rte ere 3750 420 
5 5 PA MY asissets 4125 375 
We see from the increase in magnetism for each lengthening 
of the magnet by 10 millim. that there is a considerable falling 
off in this increase with the number of pieces added. This is, 
partly at any rate, a consequence of the stronger direct action 
of the coils upon those of the added pieces which were nearer 
to the coils, which also explains the too rapid doubling of the 
magnetism with increased length. Nevertheless these expe- 
riments render it in the highest degree probable that the 
magnetism produced by a magnetizing force in an open mag- 
net is a function of its surface. This was further confirmed 
by the observation that the magnetism was increased just as 
much by placing on the magnet-legs pieces of thin-walled 
iron tube as by using solid cylinders of equal diameter. 
Closing the tubes by means of iron covers made no perceptible 
difference if there was no increase in length produced by 
doing so. 
To determine the resistance opposed to the production of 
magnetism by the nonmagnetic surrounding space, it was 
necessary to compare the resistance offered to magnetic dis- 
turbance by a space filled with air, or empty, with that of 
iron. This ratio cannot be constant, since the specific 
