10,000, and each square centimetre of polar surface contains something like 400 to 800 units of magnetic matter.
We now return to the consideration of Biot-Savart's law. The definition given by them is as follows: The force exerted on unit magnetic mass at a given point (or, as we may also say, the induction at that point), due to an element of the conductor, is given by the expression: product of the length of the element as seen from that point, the strength of the current; and this divided by the square of the distance. Stated in this form the law sounds rather complicated, but it becomes simple enough if we apply it to some special cases. Take, for instance, a circular conductor. The induction in its centre, produced by an element of one centimetre length of the conductor, would be simply the product of the current multiplied with the visible length (in this case also a centimetre), and this, divided by the square of the radius R. Since in the whole circle there are 2π R such elements, the induction, due to the whole of the conductor carrying a current J, is
