In conclusion, I am requested by Count Th. du Moncel to explain that the experiments undertaken by him in order to determine the ratios of magnetic moment for the standard electro-magnet have not been numerous and varied enough for them to be taken as presenting great exactitude. Further experiments in this direction would therefore be of advantage, and it must also be borne in mind that owing to the very varying nature of the iron employed in electro-magnets we can never be certain that the real results correspond with the theoretical. What, however, may be taken as certain is, that electro-magnets constructed and used in accordance with his formulas are employed in the most advantageous manner possible. 8 AND 9, HOLBORN VIADUCT, C. J. WHARTON. January 1883. CONDITIONS OF MAXIMUM FOR ELECTRO-MAGNETS ON A SIMPLE 2. Conditions of maximum relating to the proportion which should exist between the depth of the mag- 2 ELEMENTS OF CONSTRUCTION FOR ELECTRO-MAGNETS. INTRODUCTION. It is generally complained, and not without reason, that the manner in which the question of electromagnets is ordinarily treated by scientists is so undecided, and so little practical in the deductions which are given, that it is impossible for a constructor or an inventor to profit by them. It is certain that mathematicians look upon the questions involved from too great a height to occupy themselves with the details of application; and it must also be admitted that magnetic theories are so vague that it is difficult to translate into mathematical symbols many of the laws which have been pointed out, and of which some are still contested by punctilious spirits. We, who have made many experiments with electromagnets, are less sceptical, for although we may not have been able to verify with extreme exactness the laws propounded by Lenz, Jacobi, Dub, and Muller, we have found results approaching near enough to B the truth to be accepted as data for the construction of electro-magnets. The laws of Ohm for electric currents are in the same predicament, for it is difficult to bring into formulas which represent them a crowd of secondary influences which derange more or less the effects enunciated. But these laws are useful guides, and serve as premises for correct deductions, and this is the essential point. For scientific data to be of any real use in their application they must be cleared of all the hypotheses of high science, and of terms which many electricians cannot understand; and further, we must start from experiences obtained under ordinary conditions of application. It is certain that if, to appreciate a magnetic force, we are obliged to base our calculations, through complicated formulas, on the oscillations of a magnetised needle or on the currents induced by this force, constructors would say that this means nothing to their mind, and that, in fact, they know nothing of magnetic force but that which corresponds with some weight lifted or supported; in short, their desire being to have the greatest possible power under the given conditions, the rest is of no consequence to them. It is, then, under these conditions that we must state the question to obtain deductions applicable in practice. Now, I have always admitted that the known laws of electro-magnets were sufficient to satisfy constructors in this respect, and it is this which has led me to publish my various pamphlets on the best methods of constructing electro-magnets. Wishing to be |