determining the slope p, relative to the change in Ri as measured by step wherein {HEAD ~ p1} is compared to {TAIL ~ a reference value} po 1 and if p1 <po 1, decreasing the distance between the anode and cathode and as soon as the internal resistance Ri exceeds Ro+r, increasing the rate of introduction of alumina for a time T to a rapid rate CR which is from 20 to 100% greater than the normal rate of consumption CN 
determining the slope p , relative to the change in Ri as measured by step wherein { HEAD ~ p1 } is compared to { TAIL ~ a reference value } po 1 and if p1 < po 1 , decreasing the distance between the anode and cathode and as soon as the internal resistance Ri exceeds Ro+r , increasing the rate of introduction of alumina for a time T to a rapid rate CR which is from 20 to 100 % greater than the normal rate of consumption CN
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH