very rapidly diminishes according to a certain mathematical relation, so that at the points A and B it is already inappreciable. The question we have to answer is: What energy is required to carry unit pole from a point infinitely distant on the right through the coil to a point at infinite distance on the
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left? The mathematical investigation of this problem is best made by means of the calculus, but it would go beyond the scope of this book to give it in detail. For our purpose it must suffice to note the result. It is this: The energy required to carry unit of magnetic matter once through the coil is exactly the same as that required to carry it once round an infinitely long wire in which the same volume of current flows as in the coil. Thus
