not a constant, but depends on the degree to which the iron is magnetised. In such iron as is used in the construction of dynamo magnets it is fairly large at moderate induction, but becomes very much reduced at high induction. With an induction of about 14,000 it may be as much as 1,500, whilst with an induction of 20,000 it may be as low as 30 or even less. It is convenient to represent the magnetic quality of any brand of iron by a so-called "magnetisation curve," where on the horizontal axis are plotted the ampere-turns required by each centimetre of iron path, and on the vertical the corresponding inductions. By using such curves the above formula can be simplified as follows—
The values of x are taken from the magnetisation curve and correspond to the different values of the induction which is found by dividing the flux by the cross section of the iron. By assuming different values for the total flux and calculating the ampere-turns for each case, we get a series of co-ordinate values of ni and Φ, which, plotted in a curve, characterise the machine as regards its magnetisation. Such a curve is therefore
