What instrument measures mass?

Traditionally a balance was used to measure mass. Objects of known or accepted mass was balanced against another object of unknown mass. When the balance was level the two pans had the same mass. The term balance or scales is still used even though there is no balance used. The scales were the pans that the objects sat on.

Now electronic balances are used that gauge the mass of the objects. While you can measure mass with a spring loaded scale, it is less accurate than a balance as it relies on a spring loaded to a specific acceleration of gravity for any given mass. A change in altitude will change the results given on the scale, due to the actual change in gravitational force (it is small at only about 0.031% error for every kilometer increase in altitude)*. A balance would experience no such change as the mass of an unknown quantity is compared to the mass of a known quantity, thus negating any effects of gravity.

Another instrument that measures mass is an inertial balance. An inertial balance doesn't require gravity to work, so it can be used in space. It measures the mass of an object by attaching it to a spring and seeing how it affects the spring's period of oscillation.

Very tiny masses can be measured directly (using a "massometer"). But we normally use scales, which measure weight, which is directly proportional to mass (a property of matter equal to its resistance to a change in speed or direction of travel). The mass of an object is the same everywhere in the universe. Its weight, however, changes depending upon its location: a bowling ball has greater weight on the surface of the Earth than it does on the Moon. Great confusion arises when people interchange units of weight with units of mass. Scientists, particularly physicists, are very careful about not using the terms interchangeably, but regular folks have no choice for practical applications. For example, when you weigh a regulation ten-pin bowling ball, the scale will tell you it "weighs" between 4 and 7 (3.63 to 7.27 actually) kilograms, even though the kilogram is, technically speaking, a unit of mass, not weight.

You can also measure the mass of an object using a scale, as long as you factor in the gravitational constant (G). For instance, in an environment with only 1/2 the gravity of That on earth, you would have to double the weight displayed on the scale to determine the actual mass. As an example, 10Kg of lead in a 0.5G environment would only "weigh" 5Kg on a scale...half as much as on earth, even though its mass is unchanged.