How Do Scales Work?

For those struggling with a diet or an exercise plan, the bathroom scale is the finicky harbinger of either great news or horrible news, the latter leading to more exercise and a stricter diet. A different kind of scale is important in laboratories, where scientists can use scales to determine the mass of various things in order to carry out experiments with precise measurements. In both cases, scales measure the force that an object exerts on the scale and computes it into a weight.

Two types of bathrooms scales exist. The traditional bathroom scale tells you your weight by using a dial with possible weights printed on it, and when you step on the scale, the dial spins to the correct weight. Digital bathroom scales show you your weight on a digital display. Both scales use similar mechanisms to weigh you, relying on a system of brackets and levers.

When you step on a scale, your weight is distributed evenly throughout a system of brackets and levers. On the end of the scale where your weight is placed, there is a lever that transmits your weight to the other end of the scale. However, instead of transmitting the entirety of your weight, it only transfers it in a ratio. That is so that the spring on the other side of the scale does not have to support hundreds of pounds of weight.

In traditional bathroom scales, the spring is what moves the dial to your correct weight. Since the ratio of the conversion of your actual weight to the weight on the spring is constant, the spring is set to twist the dial to a certain point depending on the amount of weight that gets on the scale. As the spring gets smaller, so does the ratio of your actual weight to the weight that the spring feels.

Digital bathroom scales use a beam instead of a spring. These beams are connected to the levers on the end of the scale that you stand on and "feel" the weight the same way that the springs do in traditional bathroom scales. However, the amount that the beam moves is what allows the scale to compute your weight, as the output signal of the beam is turned into the digital readout of your weight.

Bathroom scales are surprisingly complex on the inside but rather simple to use, since all you have to do is stand on it. More complex digital scales are being built every day, including new scales that use electrical impulses to measure body fat, but the general principle remains the same.

Resources

• Find out more about how scales work from the Physics forums.
• Read reviews about different types of bathroom scales.