It’s a measuring device, not a scale. So why do we call these contraptions a balance?
Like with many things in aerospace, it’s because that’s exactly what the first one was.
Back when the Wright brothers started their campaign of wind tunnel tests to collect their own data set for wing shapes, they made a genuinely brilliant invention. If they were going to test wing shapes, they’d need something to measure the physics they were after.
This device held a miniature wing shape and four small metal plates. These plates were allowed to move independent of the wing shape, so when the wind in the tunnel was turned on, they would move to literally balance the lift of the wing shape against the pressure on the metal plates.
The movement of the device to balance the plates against the wing shape would rotate an attached pointer to a position on a circular scale. Measuring the angle indicated would provide the coefficient of lift for that wing shape.
They also had a second similar device called a “drift balance” that would measure the ratio of lift to drag. With these two balances the brothers tested over 200 wing shapes, creating the foundation of their aerodynamic understanding.
And it was this understanding that led to the first successful flight of a heavier-than-air, powered vehicle.
Nowadays our wind tunnel balances don’t really balance anything, and we don’t need to read angles to pull out the measurements.
Instead our balances use strain gauges to quantify and capture the effects of air pressure on a model. These strain gauges output electrical voltage readings to amplifiers, and then to computers, which turn the electrical signals into the numbers and data points we obsess over.
But the end result is more or less the same: data, in coefficient form, that tells us what we want to know about any given aircraft (or other vehicle) when subjected to air moving at a defined speed.
If you want to see some neat photos and more technical detail about the lift and drift balances, I’d highly recommend checking out this website. The site is a bit dated, but provides more info than even the Smithsonian!