Shown is a vintage Sangamo type-H horizontal watt hour meter that I just recently cleaned and restored back into working condition. This is basically two separate watt hour meters in the same enclosure. The two discs turn separate from each other, not like a polyphase watt hour meter. The register uses a small differential to keep the registration constant even if one disc is turning faster than the other. Interesting concept. I think Sangamo was the only company to use a differential.