Mass measurement involves determining the amount of matter in an object. This differs from weight, which depends on the force of gravity. If you were to travel to Mars, your weight would change, but your mass remains the same.
People weigh things all the time with a balance, but they are measuring their mass rather than their weight. Weighing scales are calibrated to take g into account and display the object’s mass.
Compasses
A compass is a magnetised needle or dial suspended on a pivot and housed in a capsule filled with liquid (lamp oil, mineral oil, white spirit, purified kerosene or ethyl alcohol). Modern compasses also have phosphorescent or photoluminescent markings on the north end of the needle.
The Earth has two centres of magnetic force, called the north and south poles. The compass needle detects these and points to the direction of magnetic north when the compass is held level in your hand.
The compass is delicate and should be handled carefully. It should be kept away from articles containing iron or steel, and other items that generate their own magnetic field. In addition, the compass can behave erratically when close to the North Magnetic Pole. This is due to the daily fluctuations in the horizontal force of the magnetic field caused by the varying electric currents causing magnetic declination. This effect increases as one gets closer to the North Magnetic Pole.
Compass needles
In its basic form, a magnetic compass is just a needle on a pivot. When it is left alone, it tends to align itself with the Earth’s magnetic field, pointing one end of the needle toward the Earth’s north magnetic pole and the other away from it. The needle is usually marked so that the user can distinguish which end of the needle points north.
The fact that the Earth acts like a giant magnet is why compasses work. But the Earth’s magnetic field is not exactly stable; its north and south poles are constantly shifting. The resulting variation in the direction that a compass needle points is called magnetic declination. Using charts of magnetic declination and local calibrations, users can compensate for this variation to use their compass accurately. The result is that the compass will point in roughly the right direction, but will not be perfectly accurate in every situation.
Scales
Scales function to quantify weight through resistance to acceleration caused by gravity. They are most commonly used to weigh bulk materials and products in industrial settings, though they are also often employed in commercial and household environments to measure smaller masses such as groceries or baking ingredients. Scales fall into two broad categories: mechanical and digital. Mechanical scales employ a spring or hydraulic transducer to convert the force of an object into a readout on a dial, while digital scales use a strain gauge to determine mass via electronic circuitry.
While traditional mechanical balance-beam scales intrinsically measure mass, ordinary electronic scales must be calibrated for each location to take into account variations in gravity. From a pragmatic viewpoint, once a scale is calibrated for a particular location, it is generally assumed that gravity will be the same at all times, and measurements are reported in mass units such as kilograms, even though weight technically refers to a measurement of force using newtons.
Weights
It’s important to understand that weight and mass are not the same thing. A 15-gram bird will still be the same on Earth, the Moon or the International Space Station, but its weight will vary based on the strength of gravity acting on it.
While scientists and engineers are well aware of the difference between mass and weight, many people use the terms interchangeably. This can be confusing, especially in fields like physics where the distinction is important.
Since the time of Newton, physicists have made a clear distinction between mass and weight. However, the word “weight” has not disappeared from common usage, even in scientific journals and publications. For this reason, the 23rd General Conference on Weights and Measurements has decided that the word “weight” should be phased out in favor of the term mass for official use. This will allow scientists and engineers to correctly specify the amount of matter an object contains, while using the word “force” for gravitational acceleration.