526 PROF. JACOBI ON THE APPLICATION OF ELECTRO-MAGNETISM 
speed which we had reason to suppose must be infinitely accelerated. 
This speed had never surpassed 120—130 revolutions in a minute, on 
employing a pile of four pairs of plates two feet square. We must not 
lightly abandon conclusions founded on the nature of things, and those 
to which I refer are drawn solely from the integral f* Mds, expressing 
the magnetic attraction and supposed to be independent of the speed. 
Besides, it rests upon the legitimate supposition that the electro-mag- 
netic excitation of the soft iron operates instantaneously. If this were 
not the case, my apparatus would have shown that magnetism and 
electricity ought to be attributed to the motion of material parti- 
cles, or to oscillations much more perceptible than are those of the pro- 
pagation of sound. In short no one can deny that it is the nature of 
a force not to require time to act, and that, if its different effects were 
not instantly perceptible, it would then be some molecular motion, 
under the influence of mechanical laws, which takes place. 
18. 
At the end of my first note I said, that in using thermo-electric piles 
for the movement of machines, there was reason to fear the magneto- 
electric currents developed by magnetism in motion. The reaction 
which thence arises would be almost entirely destroyed in the hydro- 
electric pile, the liquid conductors offering too much resistance to the 
passage of these currents. These considerations were founded upon 
detached experiments. On employing a thermo-electric pile, the de- 
viation of the needle was affected by a magnet which had been placed 
in a helix forming part of the circuit; this was not the case with a 
voltaic pair of plates of small dimension. The deviation of the gal- 
vanometer, extremely sensitive as it is, was not altered by it. This 
did not surprise me, since the conducting power of liquids is much 
below that of metals; but in making experiments on the magnetic 
force of a bar of soft iron, I have sometimes found considerable dif- 
ferences for which I could not account. I was curious to know if 
these differences proceeded from the weakening of the electric current 
produced by a pair of plates with a surface of half a square foot, or 
from the nature of the iron. I therefore inserted in the circuit a gal- 
vanometer at some little distance, that it might not be affected imme- 
diately by the magnetism of the bar. I was much astonished to see 
the needle recede upon my applying the armature, and advance as soon 
as it was taken off; for it was the first time that I had recognised the 
double office of the connecting wire, viz. that of conducting the voltaic 
current, and of representing at the same time a common wire sub- 
jected to the influence of a magnet in motion. The helix producing a 
magnet by the voltaic current is at the same time a magneto-electric 
