English: Weighing a prototype kilogram with a precision Ruprecht balance, around 1900. The balance is manipulated from a distance by the man on the right using long extension controls, to avoid his body temperature upsetting the delicate thermal equilibrium of the balance, causing convection currents which would bias the measurement. The operator looks through a telescope to read the balance's scale. The prototype kilogram, removed from its storage bell jar on the stool at right, is being compared to the set of working standard kilogram weights in the case on the stool at left, being examined by the other two men.