Theory of Magnetism. 247 
apart fell nearly at the same current-strength of about 1 am- 
pere. ‘The increases themselves cannot be in proportion to 
the distances apart of the parallel faces, since the side faces 
of the pole-pieces must take so much the more part in the 
magnetic action the further the parallel faces were separated, 
and consequently so much the greater must be the magnetic 
resistance of the interval. That a maximum increase takes 
place with all distances apart, and that with increasing current- 
strength this uniformly decreases to a small amount, is a 
consequence of the large moment of the electromagnet itself, 
which, in consequence of the small magnetic resistance of the 
entire circuit, with strong currents approaches its maximum 
of magnetization. The experiments certainly seem to indicate 
an increase in the resistance and magnetic disturbance of the 
nonmagnetic material for very high magnetic moments, but 
not decisively. For such experiments we must employ electro- 
magnets of small length and so large section that the resistance 
of the iron to magnetic disturbance may remain very small in 
comparison with that of the surrounding medium. 
The experiments show, however, that a maximum of mag- 
netization does not occur sooner with atmospheric air than 
with iron. It follows from this that the strength of a mag- 
netic field is limited only by the maximum of magnetism in 
the iron, and that the magnetic conductivity of nonmagnetic 
material may be taken as constant. Since this value for iron 
is very variable and the law is not yet known of its dependence 
upon the magnetic moment of the mass of iron, there is 
no possibility of comparing the magnetic conductivity of the 
two in general. Since, however, in the construction of mag- 
netic machines it is always precisely that magnetic condition 
of the iron for which the increase of magnetism with increas- 
ing current-strength is a maximum which is of the most im- 
portance, we must make this point the starting-point of the 
inquiry. 
To investigate this point I had two square iron plates made, 
A millim. thick and 80 millim. square, which could be screwed 
on sideways to the prismatic movable pole-pieces of the pre- 
viously described horseshoe-magnets. At a distance of 5 
millim. from each other, with a current-strength of 0:1 ampere 
the plates gave the same increase of magnetism as a piece of 
sheet-iron of 1 square millim. section which, after screwing 
off the plates, was made to connect the pole-pieces through a 
distance of 5 millim. 
With feeble magnetizing-forces the iron was the more 
_ powerful, with stronger currents the plates. This shows a 
magnetic conductivity for iron in its condition of greatest 
