Miller's vacuum balance, 1870.
© Science Museum / Science & Society Picture Library
This balance was used for comparing primary and secondary weights. Weighing is done using a vacuum, and a system of rods and levers, using mercury-filled tubes as air seals, allow the weights to be moved to and from the scale pans of the balance. Ludwig oertling, who made this balance, settled in London in the early 1850s and his firm became the leading manufacturer of balances in Britain.