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When I went shopping as a child in the 1940s and 50s, some high street shops were still using scales that relied on balancing goods against standard weights. Greengrocers in particular were still using their old-style scales for heavy goods like potatoes.
The Imperial System of Weights
16 drams = 1 ounce (oz)
16 ounces (oz) = 1 pound (lb)
14 pounds = 1 stone (not abbreviated)
112 pounds = 1 hundredweight (cwt)
20 hundredweights = 1 ton
2240 pounds = 1 ton
However a different type of scale was creeping into shops, relying on compressing a spring:
With these scales, as goods were placed in the scale-pan, the spring compressed and swung a pointer round to show the weight. The photo below is from the customer's side of the shop counter.
The goods being weighed no longer had to weigh simple multiples of standard weights, as was the case for pricing with the older balancing scales. However, pricing didn't seem to concern shopkeepers at all. They just announced the price as if it were obvious. This worried my mother. She said that she had always been regarded as excelling in mental arithmetic at school, but there was no way that she could come up with prices so quickly. How could this be?
I was able to explain how shopkeepers could come up with prices so quickly when weighing out odd weights because I had worked on sales jobs in school holidays. To understand what was happening, one had to view the scales from the shopkeeper's side of the counter. Then all became clear, because the customer's and the shopkeeper's sides were very different.
On the customer's side, all that was visible was a plain background, a pointer and a scale of weight. So all the customer saw was the weight of a purchase as indicated by where the pointer stopped its swing.
The shopkeeper's side was very different:
At first glance the background is a mass of numbers. Closer inspection shows that these numbers are arranged in arcs where the numbers on any one arc show how how price increases with weight for a given price per pound. The lowest arc is for the cheapest price per pound and the top arc is for the most expensive.
The pointer carries its own scale in prices per pound, again with the cheapest at the bottom and the most expensive at the top.
When the goods to be priced are placed on the scale-pan, the pointer swings round and stops at a position indicating the weight of the goods. All that the shopkeeper then has to do is to run his or her eye up the pointer to find the price per pound at which the goods are being sold, and then note the corresponding price on the background. underneath. No mental arithmetic is involved at all.