There are actually three types of scales: balances, spring scales, and electronic scales. Each has its advantages and disadvantages.
The balance works on a pivot mechanism, like a seesaw. The item you want to weigh is on one side, and on the other side there are standard weights, which can be free weights or mounted on a sliding scale. You add or remove weight from the standard side until both sides are balanced. Scales in doctors' offices work on this principle.
Balances are the most accurate type of scale, and because they only require gravity to work, they are durable. However, they tend to "bounce" a lot. Any drafts or vibrations in the room will make them jump, making them difficult to read. They are big, bulky, and expensive.
Spring-loaded scales use tension or compression springs attached to a dial indicator. When an item is placed on it, the spring stretches or compresses, which moves the dial to the proper weight.
These scales are much cheaper, smaller, and easier to read than balances, so they've always been the bathroom and kitchen scale of choice. However, they are not as accurate as a balance because the spring is not linear. If you put twice the weight on it, it doesn't necessarily stretch twice. Springs will also eventually wear out.
There is a metal plate under the weighing pan of the electronic scale through which electric current flows. When an item is placed on top, the current fluctuates, which the microprocessor interprets as weight.
A scale is almost as accurate as a balance, yet compact and easy to use like a spring. Their prices have come down to the point where they are suitable for home scales. They do require electricity or batteries but consume very little power.
Unless you need extreme accuracy or can only afford the cheapest models, a digital scale is probably your best bet.
Xiangshan Chint Electric Co., Ltd. is a digital scale supplier from China. The company designs, processes and sells digital luggage scales, mechanical bathroom scale, and other electronic scale products.