How do scales work? There are four different types of scales and they work in different ways. They are balances, steelyard balances, spring scales, and digital scales.
Balance scales are the easiest to explain and they were the first scales used to measure the weights of objects. The first balance scales were used in the Fourth Dynasty of Egypt, 2600 BC. They work on the principle of balance. If you have a horizontal bar with a pivot in the middle and put two things of the same weight on each side at equal distances, it will balance. These scales would have trays on each side. On one side would go the thing to be measured and on the other side you could use a variety of pre-weighed metal weights. Balances are not completely accurate, but they are fine for general weighing.
Steelyard balances have been around for a long time as well. A steelyard balance works on the principle of balance as well, but the method is slightly different. It consists of a long bar with a pivot very close to the weighing end. A movable weight is suspended on the other end. Unlike the regular balance, there is only one weight used on a steelyard balance. The object to be weighed is placed in the tray and the counterweight is moved along the bar until the bar balances. The bar usually has weight markings on it and these are worked out using the law of the lever. The weight of the object being weighed multiplied by the length of the bar to the pivot is equal to the weight of the counterweight multiplied by the distance of the counterweight from the pivot. Steelyard balances can be used for any weight, so long as the bar can support the thing being measured and the counterweight. They were very common in agricultural areas and could be used to measure carts.
The first spring scale was invented in Britain in 1770 by Richard Salter. Spring scales are very common in kitchens, even today. They have a pan for the thing being weighed that sits above the scales. When the item is placed in the pan, it descends, pushing a needle around a dial of calibrated weights. They work using the principle of Hooke’s law, named for Robert Hooke. His law says that the force exerted on a spring is directly proportional to the distance it moves as a result of this force. If you double the weight on the spring, the spring will stretch twice as far. You can see this if you have ever used those scales that you can buy to weigh your suitcase. In kitchen spring scales, a spring is attached inside the device. When something is placed in the weighing pan, it pushes down, stretching the spring. As the spring stretches, it operates a rack and pinion gear, turning the needle.
The digital set of scales you use in your kitchen when you are cooking were invented in about 1980. They measure the weight of objects using electrical resistance. Inside the set of scales is a device called a foil strain gauge. This is a zigzagged piece of wire with a foil backing. The foil strain gauge is attached to the plate that is on the top of the set of scales. When the device is on, an electric current is passed through the wire and the electrical resistance of that wire is known. When something is placed on the top of the scales, the plate will move down. As the plate moves, it stretches the foil strain gauge, and the wire becomes thinner. The thinner wire has a higher resistance. When the scales are made, the wire’s resistance at 0g and at 1kg is tested. The microchip embedded in the scales can work out how much the resistance of the wire has increased, and it can calculate the weight of the object you are weighing using that information. That information is then displayed as a number in grams. Temperature and the accuracy of the initial setting of your kitchen scales can affect their accuracy, and what you are weighing might not be exactly as heavy as your scales display. This is unavoidable.
So, the different types of scales work in different ways. Digital scales are probably the most accurate. However, they cannot measure weights below 1g. There are scales called ultra-microbalances that can weigh weights of 0.1 µg (0.0000001g). It uses the changes in the resonance of quartz crystal to measure objects. Even 0.1 µg of weight can change the resonance of the crystal and the scales can calculate this. They are incredibly accurate. And this is what I learned today.
Sources
https://www.adamequipment.com/aeblog/how-do-balances-and-scales-work
https://electronics.howstuffworks.com/gadgets/fitness/digital-scale.htm
https://www.marsden-weighing.co.uk/blog/how-weighing-scales-work
https://www.explainthatstuff.com/weights_and_balances.html
https://en.wikipedia.org/wiki/Steelyard_balance
https://en.wikipedia.org/wiki/Spring_scale
https://en.wikipedia.org/wiki/Microbalance