Shown is a vintage Duncan Direct Current Model E Watthour Meter. I removed the case to expose the interior. This design intrigues me and I'm trying to figure out how these types of meters operate. I cleaned it up and decided to wire it up to AC on a 2 1/2 amp load. Surprisingly, it runs well on AC. Similar to design of the General Electric C-6 DC watthour meter. Update: I read an article pertaining to this particular meter in a 1924 Electrical Meterman's Handbook and even though they were designed for DC use, they could also be used on AC lighting circuits successfully.