Scales are commonly used in households, scientific laboratories, businesses, and industry to measure the weight or mass of an object or substance. The earliest weighing mechanism is the equal-arm balance. An equal-arm balance consists of a bar with two pans hanging from each end and a support (called a fulcrum) at the centre of the bar upon which the bar can balance. The Egyptians used a balance of this type about 2500 BC. To use an equal-arm balance, an object of unknown weight is placed in one of the pans, and objects of known weight are added to the other pan until the bar holding the pans is balanced; then the weight of the unknown load is the same as the known weight in the other pan.
© Islington Artefact Library
Other mechanical scales in use today include pendulum scales and spring scales. In a pendulum scale, a platform is connected to a weighted pendulum. When an object is placed on the platform, the weighted pendulum swings out to the side to balance the load and a needle attached to the pendulum indicates the weight. In a spring scale, a platform is connected to a spring, which either stretches or compresses to balance a load placed on the platform. A needle whose position depends on the extent to which the spring is stretched or compressed indicates the weight of the load. Many bathroom scales are spring scales. Electronic scales, developed during the 20th century, use electricity to measure loads. Electronic scales are faster, and generally more accurate than their mechanical counterparts. In addition, electronic scales can be incorporated into computer systems, which for most applications makes electronic scales more useful and efficient than mechanical scales.
The weighing scales in the picture were part of a range of equipment used for cooking in Victorian kitchens. The Egyptians and Babylonians developed standard weights for everyday needs, such as buying and selling. They crafted their weights into shapes to represent gods, animals and everyday objects. Imperial units of weight are stones, pounds and ounces. In Roman times there were 12 ounces to a pound, but over the next few hundred years this became 16 ounces. There are then 14 pounds in one stone. The stone dates from a time when people used real stones as weights. The word pound comes from a Roman phrase libra pondo which means one pound by weight. The shortened form of libra is lb, which is the symbol we use for pounds today. This also explains why the symbol for the birth sign Libra is a balance. Imperial measures have now made way for the more commonly used metric system. This was introduced during the French Revolution in the 1790s by a committee of scientists and politicians. This new system uses easy numbers such as tens, hundreds and thousands, which makes long complicated calculations much easier. The metric system uses weights called kilograms and grams. There are 1000 grams in a kilogram. Some microbalances are so accurate that they can weigh a very tiny fraction of the weight of the ink used to make a full stop.