1. How Mass Is Measured on the Space Station Without Moving Needles
Inside the International Space Station, there are two main devices: a Russian one and an American NASA one. The Russian device uses a spring to create vibration, and calculates mass from the period of that vibration. The NASA device uses a spring to pull, and calculates mass from the acceleration [5].
According to former NASA astronaut M. Fahl, a person sits on a spring-mounted platform and oscillates. The period is about 3 seconds. Mass is proportional to the square of the period. You must keep your body compact and still. The accuracy is about ±1 kg [6].
What is measured here is mass, not what we call weight on Earth. If mass can be measured where gravity is not used, what do tools on Earth compare?
2. How Balances on Earth Compare Mass Using Equilibrium
A balance is a tool with a beam supported in the middle. Pans hang from both ends. You place the object to be measured on one side and weights on the other. When the beam is horizontal, the mass of the weights equals the mass of the object. This is because if the arm lengths are equal, equilibrium occurs only when the masses on both sides are the same [1].
Balances were used in ancient Egypt. The timing varies by source; some cite murals from around 3000 BC, while others mention around 8000 BC. Here, we say "several thousand years BC" to cover the range [1][2].
Weights were originally stones, often shaped like animals. In ancient Egypt, the balance was said to be a tool of the gods for judging good and evil [1].
With this method, gravity affects both sides equally, so equilibrium is less likely to break if the location changes. However, large objects require large balances, which are inconvenient to carry.
3. How Steelyards Use Levers to Measure Mass and Stay Portable
In Roman times, steelyards using levers became widespread. With just one beam and one weight, they could measure up to a certain limit, making them easy to carry [2].
You slide the weight along the beam to find the balance point. That position becomes the scale for weight. This is similar to a seesaw, where a heavier person sits closer to the pivot to balance. This analogy is the author’s, not from the source material.
In Japan, the government approved the manufacture and use of Western-style weighing instruments in 1875 (Meiji 8) [2].
Both balances and steelyards use weights as a standard and look for equilibrium. How does a method that converts small deformations into electricity work?
4. How Strain Gauges in Scales Turn Force into Electric Signals
Inside scales and platform scales, there are parts called load cells. When force is applied, the load cell deforms slightly. Strain gauges attached to it deform too. A strain gauge is a sensor where electrical resistance changes according to the amount of deformation [3].
If voltage is applied to the strain gauge, an output voltage proportional to the change in resistance is produced. This voltage is converted into a weight number for display. This is the basic mechanism. Load cells are used in scales, balances, and platform scales [3].
Balances compared against "physical standards" called weights. Strain gauge types measure by converting deformation into an electrical quantity. Even though both "measure weight," the thinking is different.
Does this method show the same number if the location changes?
5. How High-Precision Electronic Balances Reveal Differences in Gravity
Most electronic balances that measure with a precision of 1/100,000 or better use electromagnetic force restoration. A coil generates electromagnetic force to keep the beam position the same before and after placing an object. The current size becomes the weight value (zero method). Strain gauge and capacitive types are called "deflection methods" because they look at displacement [4].
Structurally, electronic balances measure the gravity acting on the sample as mass. Therefore, if gravitational acceleration changes, the display value changes. Gravitational acceleration varies by latitude and altitude. It is larger in Hokkaido than in Okinawa, and larger at lower altitudes than at higher altitudes [4].
If you measure a 500 g weight at an altitude of 100 m, the display is about 0.0157 g smaller. This is an upper limit estimate calculated using Earth's radius of 6,371,000 m. As a ratio, it is about 0.003% (0.0157 ÷ 500) [4].
Electronic balances with a minimum display of 0.1 mg or less are called analytical balances. Ultra-micro balances with a minimum display of 0.0001 mg (0.1 μg) are also manufactured. They detect changes in temperature, humidity, air pressure, and vibration that humans do not feel [4].
Home scales have much coarser display increments, so you will not notice this difference. For a 60 kg weight, the difference is about 2 g at the same ratio (author's calculation).
6. How to Explore Equilibrium and Force with Kitchen Scales and Rulers
Press gently on a digital kitchen scale with your hand. A number appears. This experiment helps you explore how a load cell works. Do not press hard, as it may damage the scale; press gently and briefly.
Place a pencil on a desk and put a ruler on it in the middle. Place the same number of 10-yen coins on both ends, and it will be horizontal. If you increase the coins on the left, how many do you need on the right to balance? Or, if you arrange 10-yen coins at different distances from the pivot, what happens? This helps you understand the lever principle used in steelyards.
For gravity correction in electronic balances, manufacturer technical documents contain diagrams and formulas. For measurement in space, you might look for explanations from space agencies in Japanese with an adult.