Preformed electrodes are factory produced on high precision automatic machines with controlled feeds and speeds, thus yielding electrodes considerably more uniform with regards to dimensional tolerances and surface finishes than are the laboratory formed electrodes. We felt therefore, that the use of preforms would tend to reduce variables in the analytical gap which might affect spectrographic precision.
Shirley K. Brown, "Precision of Preformed vs Laboratory Prepared Graphite Electrodes," Appl. Spectrosc. 13, 157-157 (1959)