Edison developed the first meter that measured the amount of electricity instead of how long the circuit was energized. This meter was connected across a shunt in the load and consisted of several jars with zinc plates and a chemical solution. One set of jars was intended for the main reading while the second set was operated off a smaller shunt and was intended for comparision purposes (a primitive check on the meter's accuracy). The monthly reading was made by removing the plates from the jars and weighing them with a laboratory balance. The change in the plates' weight between readings was a measure of electricity consumption. This meter was very inconvenient to use, and in a couple cases mishandling of the plates resulted in large billing errors. Also, as there was no ready way to indicate the usage to the customer, this also made it hard for them to trust its accuracy. These disadvantages and the fact that this meter itself had high internal losses made it unpopular enough that these meters were rapidly replaced by more reliable meters in the late 1880's, including the Thomson Recording Wattmeter.
Courtesy: David Dahle www.watthourmeters.com