484 Intelligence and Miscellaneous Articles, 



This relation, in general very complicated, is simplified in a special 

 case, as we will show. 



Let us consider the potential V of a pole P. It is known that 

 this potential has for its value in absolute measure the opening of 

 the cone under which the pole P regards the current. If, then, the 

 circuit B starts from negative infinity to reach the pole and then 

 remove thence to positive infinity, the potential varies by the quan- 

 tity 47T. 



From this it follows that, in the above-indicated conditions of 

 displacement, V 2 — V is a constant quantity, and equal to 47r for 

 all the poles ; consequently it can be put as a factor, which gives 



j: 



i dt= — =5- 4tt2/*= — =p 4?rM, 



designating by M the total magnetism of the arrangement. 



In a long magnet the magnetism can be regarded as collected in 

 the vicinity of the extremities ; therefore if the coil be placed on the 

 middle portion of such a magnet and the latter be afterwards 

 removed to a great distance, the conditions will be sensibly those 

 of the preceding theory. It hence follows that the quantity of the' 

 current can serve for measuring the total magnetism of the half of 

 a bar, provided that the bar be not too short — that is to say, that 

 its polar distance be not less than 8-10 centims. 



We see also that the current is independent of the diameter of 

 the coil, if this diameter is a small fraction of the length of the bar. 

 The last proposition was experimentally verified by Paraday and 

 Lenz, and more recently by M. Gaugain. — Comptes Bendus de VAca- 

 demie des Sciences, vol, lxxx. pp. 653-656. 



ON CAMACHo's NEW ELECTROMAGNET. 



To the Editors of the Philosophical Magazine and Journal, 

 Gentlemen, 

 In the April Number of your Magazine Mr. E. S. Culley states 

 that he had in his possession, in 1852, a magnet similar in principle 

 to that of M. Camacho, which was invented by the late Richard 

 Roberts. I take the following extract from a paper on the Con- 

 struction of Galvanic Magnets, by John B. Zabriskie, M.D., pub- 

 lished in the American Journal of Science and Arts for July 1839. 

 After dwelling upon the difficulty of " saturating with magnetism 

 large masses of iron," he continues, " but if we divide perpendicu- 

 larly each extremity of a large magnet into four equal parts and 

 wind each part separately, there will be no difficulty in completely 

 saturating the whole." This seems to me to be the same idea upon 

 which both Mr. Roberts and M. Camacho proceeded to construct 

 their magnets. 



Tours sincerely, 



John Trowbridge, S.D. Harv. Coll. 

 Cambridge, U. S. 





