The solution was found to be to include grading electrodes between the anode and control grid, connected to an external resistor-capacitor divider circuit.[9] Dr. Uno Lamm conducted pioneering work at ASEA in Sweden on this problem throughout the 1930s and 1940s, leading to the first truly practical mercury-arc valve for HVDC transmission, which was put into service on the 20 MW, 100 kV HVDC link from mainland Sweden to the island of Gotland in 1954.