The steelyard is a weighing device which has been used since Roman times. It is particularly suitable for weighing objects which can be suspended from a hook, e.g., sacks of farm produce and car-cases of meat. It consists of a graduated steel rod pivoted near one end and balanced by a heavy piece of metal B (Fig. 6.12). The object to be weighed is suspended from a hook. The moment of the object about the pivot is balanced by that of a small rider weight which slides along the arm of the steelyard. This arm is calibrated to read the mass being measured directly in appropriate units.
Some types of platform weighing machines employ the steelyard principle. In these, the object is placed on a metal platform and its weight is transmitted to a balancing arm by a system of levers (see also Fig. 6.13 opposite).