Scientists use a variety of tools to measure mass. The most common tool is a balance that compares an object with known masses. It does not depend on gravity and can be used anywhere in the universe.
Many of us weigh ourselves on a scale that obtains our weight. But there is a more accurate way to find an object’s mass, by measuring its resistance to acceleration.
When talking about objects in everyday use, it is not uncommon to refer to their weight. However, this is not a correct term. Weight depends on the object’s location, while mass is independent of location.
For example, an object’s weight on Earth is equal to its mass multiplied by the strength of the gravitational pull in that particular location. The same object, however, would not weigh the same on the moon or in the vacuum of space.
For this reason, some physics textbooks define weight as the vector quantity a body experiences due to gravity (W = mg), where m is an object’s mass and g is its gravitational acceleration. Others, like the International System of Units, define it as a scalar quantity (W = F), where F is the force exerted by an object on that mass and is expressed in units of newton, the SI base unit for force. In some places, such as the United States, the name weight continues to be used where mass is meant, even though it is a measurement of force and not mass.
The acceleration of gravity, usually denoted by g, is proportional to the distance an object falls divided by the time it takes to fall. This is why the pendulum experiment is so familiar to physics students. However, to obtain accurate measurements of g one must be able to measure both distances and times to within a few parts in 108 or 109; not a trivial task!
The value of g changes slightly with location on Earth due to the rotation and bulging of the planet. The effect is small, but the variations can cause noise that afflicts delicate measurements. A more recent method of measurement has been to use interferometers that detect interference between atoms that have been subjected to different gravitational potentials. This approach allows very accurate determinations of g. However, these methods are not yet used routinely for mass measurement because the precision needed would be beyond what is available today. This is expected to change soon, as technology improves.
Balances are used for very precise mass measurements because they are not affected by changes in gravity between locations. They are also less sensitive to vibration than spring-based scales. In its most basic form, a balance has a beam with a fulcrum that rests on a sharp V-shaped knife edge. The fulcrum is attached to a container of known mass, which in turn is connected to a pan that holds the test substance.
Analytical balances are used for very accurate, quantitative measurements of mass down to the nearest 0.001 g. These instruments are delicate and require careful use to minimize errors. It is important to clean up spills on the balance immediately and never add chemicals directly to the weighing pan or to a piece of weighing paper placed on it. You should also perform daily inspections of your analytical balance to check for sensitivity error. This is done by loading a weight that is close to the maximum capacity and resetting the display to zero.
Transducers are devices that convert energy into a signal. They can also be called sensors or actuators. They are a crucial first step in any mass measurement system. Depending on the type of transducer, the output signal can be used to read the physical quantity such as pressure or temperature. The ability of a transducer to produce an identical output signal on application of the same input signal is known as repeatability. This is important in obtaining consistent results in repeated tests.
The sensitivity of a transducer is the ability of the device to detect the smallest change in a physical quantity and translate it into an electrical signal. This is measured in terms of the ratio between the electrical signal and the physical quantity.
For example, a photomultiplier tube generates electrons by passing a single photon through a series of individual dynodes, each of which is capable of producing 106 to 107 electrons. These electrons are gathered in a central plate and converted to current. This process is similar to what happens in a mass spectrometer.