The size and operational features of modern mercury-arc rectifiers are determined largely by the mercury transfer from the pool cathode, 80–90% of the total valve volume being required to handle the evaporated mercury.1The amount of mass transfer accompanying spot emission is thus of fundamental importance; it is characterized by the amount of mercury removed from the pool per ampere of emission current per second, the accepted value being 5 mg/amp/sec for power rectifiers.2Many attempts to control this mass transfer have failed through the lack of knowledge of the fundamentals of spot emission and their relations with the features which accompany it.The paper describes the development of means (low-heat-resistance anchors) of reproducing the various distinct phases of spot emission, and demonstrates the existence of functional relationships between the specific phase of emission and the accompanying quantities, in particular of mass transfer. It is suggested that the suppression of mass transfer is equivalent to the selection of specific phases of emission, and that this can be achieved by suitable designs of cathode. Finally, the first practical applications of the new principle are described.In view of several references to the performance and peculiarities of the free-spot emission, these are surveyed in the Introduction.