When this condition is satisfied y is a function of x which has a differential coefficient.
It appears that we can always find the area A if we know a function F(x) which has ƒ(x) as its differential coefficient.
It depends also on the rate at which currents rise and fall, and this is indicated by the differential coefficient dC / dt.
Thus he differentiated with respect to x and equated the differential coefficient to zero.
The only way of 544 finding out whether this condition is satisfied or not is to attempt to form the differential coefficient.