Omitting correction terms depending on the temperature and on the inductive effect of the earth's magnetism on the moment of the deflecting magnet, if 0 is the angle which the axis of the deflected magnet makes with the meridian when the centre of the deflecting magnet is at a distance r, then **zM** sin B=I+P+y2 &c., in which P and Q are constants depending on the dimensions and magnetic states of the two magnets.

For a curve of the order the expression **Zm**(m - I) - 6 - K is termed the " deficiency " (as to this more hereafter); the equation (to) expresses therefore that the curve and its reciprocal have each of them the same deficiency.