Despite the uninteresting name, these rectifiers are one of the most beautiful gadgets ever made. As a result they are often seen if films and are quintessential elements of mad scientist labs, yet few people could name them or know what they were used for. Invented at the beginning of the 20th Century, Mercury Arc Rectifiers were like spectacularly large versions of the Tubes in pre-transistor era electronics. They were used to convert electricity from alternating to direct current.