Balance scales for weighing, early 20th Century and before
How balance scales work
Scales based on balance have been used for comparing the weights of two objects for thousands of years.
A balance works on the simple principle of balancing whatever is to be weighed on one side of an arm against one or more known weights on the other side.
Using the little man sketch as an example, at the start of weighing, the balance arm is horizontal, but when he steps on, his weight forces his side down. Known weights are put on the other side of the arm until the arm is horizontal again. Then the known weights are the same weight as the man.
Balance scales were the most widely used scales the early 1900s, scales based on springs were in general use in shops by the 1950s.
Setting up balance scales
It was necessary for shopkeepers to set up their scales to be set to balance properly, so that the shop was not seen to be selling under-weight. Equally shopkeepers would not want to be giving away more than they were charging for. So there was an arrangement on all scales whereby a small weight could be moved slightly one way or the other along the arms of the scales to balance up each side. Unfortunately it tends to be rather difficult to see in the photos below.
Accuracy of balance scales
In practice, balance scales were often too rough and ready for perfect balance, or maybe the shopkeepers had to work too fast to bother. Either way, it was normal practice for them to give customers the benefit of any doubt by putting just too much on the customer's scale pan and giving the customer time to register that this was happening. Presumably there was a skill in making the 'just' as small as possible.
There were all sorts of different styles of balance scales, each one designed for specific purposes, with their pans specially shaped for efficient use. Some examples are shown in the following photographs.
Most of the above photos show the weights that were used with the weights - generally round brass or iron ones that stacked for tidy storage.
There were also heavy duty weights that incorporated handles for lifting, and there were extremely delicate weights which were used by pharmacists for making up prescriptions. The delicate weights were stored in air-tight boxes to avoid contamination from the air. They were shaped with 'necks and heads' so that they could easily be grasped with the tweezers that were stored with them to prevent greasy fingers from altering their weights. The following photographs were taken at Milestones Museum in Basingstoke.
Disadvantages: shopkeepers need to work out prices
With balance scales, shopkeepers needed to work out prices in their heads (which they did not have to do with spring balances) but the calculations were not particularly arduous because it was normal practice to sell in simple fractions or simple multiples of a pound weight (or whatever was the appropriate unit of weight for the goods).