105—120," that this author established a remarkable law, viz. that the magnetizing power is in the direct ratio of the perimeter of the electromotor, 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 œconomical effects of the magnetic machine. Two plates of zinc 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 zinc 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 180·49 lbs.
These two experiments cannot be classed with the others, as the circumstances attending them differed. But the perimeter of the first electromotive helix being 86 inches long, and that of the second 193 nches, it does not appear that the law of M. dal Negro is confirmed by these two experiments. There are many empirical formulæ 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 extend their limits. Besides, I have taken the pains to calculate the experiments of M. dal Negro from the formula of M. Ohm. The fourth column of the following table indicates the results according to the ascertained formula , in which represents the surface of the pair of plates.
| Surface. | Perimeter. | Force | ||||||
| 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 | " |
| 24 | " | " | 20 | " | 24·6 | " | 25·97 | " |
| 30 | " | " | 22 | " | 25·8 | " | 28·07 | " |
| 36 | " | " | 24 | " | 29·6 | " | 29·68 | " |
| 42 | " | " | 26 | " | 30·3 | " | 30·94 | " |
| 48 | " | " | 28 | " | 32·8 | " | 32 | " |
| 54 | " | " | 30 | " | 33 | " | 32·8 | " |
| 60 | " | " | 32 | " | 35·6 | " | 33·51 | " |
