.516 PROF. JACOBI ON THE APPLICATION OF ELECTRO-MAGNETISM 
105—120,” that this author established a remarkable law, viz. that the 
magnetizing power is in the direct ratio of the perimeter of the electro- 
motor, and that the surface has scarcely any effect in increasing this 
power. I did not delay making some experiments in order to confirm 
this law, which appeared to me of great importance for the ceconomical 
effects of the magnetic machine. Two plates of zine and copper, 36 
inches long and 7 inches wide, were coiled into a helix, and separated 
from each other to a distance of one fourth of an inch by small pieces 
of wood in the manner of the calorimotor; the whole was plunged 
into water acidulated with ten per cent. of sulphuric acid. The mean 
weight which the bar was capable of supporting, whilst this pair of 
252 square inches was employed, was taken from five experiments and 
amounted to 297-12 lbs. 
From the same piece of copper and zine I also cut two plates, 
96 inches long and half an inch wide. These plates were coiled in the 
same manner and separated to an equal distance. The mean value of 
the magnetic force, when this electromotor of 48 square inches was 
employed, was also drawn from five observations and amounted to 
133°79 lbs. By employing a liquid much more acidulated the weight 
might be increased to 180749 lbs. 
These two experiments cannot be classed with the others, as the cir- 
cumstances attending them differed. But the perimeter of the first 
electromotive helix being 86 inches long, and that of the second 193 
inches, it does not appear that the law of M. dal Negro is confirmed 
by these two experiments. There are many empirical formule in 
physics incapable of being carried out to the extremes, but they ought at 
least to be sufficiently general not to fail on the slightest attempt to ex- 
tend their limits. Besides, I have taken the pains to calculate the ex- 
periments of M. dal Negro from the formula of M.Ohm. The fourth 
column of the following table indicates the results according to the 
41°55 x 
ascertained formula A = ——— 
144+ x 
, in which a represents the surface 
of the pair of plates. 
Force 
Surface. Perimeter. 
a observed. calculated. 
6 square inches 14 inches 13°85 kilogr. 12:22 kilogr. 
12 16 18-2 - 18°89 - 
18 18 22°8 23°08 
20 24-6 25°97 
22 25°8 28°07 
24 29°6 29°68 
26 30°3 30°94 
28 32°8 32 
30 33 32°8 
32 35°6 Jor 
