This is a usage counter that works by moving mercury in a capillary tube via electrochemistry. As electricity flows through the device, mercury atoms are transferred across an electrolyte gap in the capillary. The position of the gap changes as mercury is transferred, and shows how much total charge has been passed through the device.
I realize that the divider network looks more like 20:1. This might kill my theory that the mercury ions are in the 2+ state. I'm really not sure.