/jie 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 oeconomical 

 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 Avater 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 18049 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 formulae 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 a; 



ascertained formula A ^ 

 of the pair of plates. 



14*4 + a; 



, in which x represents the surface 



