The English sovereign rocker was the popular counterfeit coin detector of the 19th century. It was attractive in design, easy to use and truly pocket-seized: the construction was rugged, and there was little to go wrong. Balance rockers were used in Europe and the Middle-East for many centuries. The earliest documented reference to a rocker balance in England is in 1778.
Most varieties of the sovereign balance have been catalogued in "SOVEREIGN BALANCES; 1 - Standard Rockers" by Michael A. (1983) The last photo in the listing is an example of a balance scale variety taken from this rare reference. Due to the age of this scale there is no guarantee it will weigh correctly.