Early Measurements and Units of Measurement, and How We Obtained the Systems We Use Today

Abstract

In a previous article I discussed the growth of our ideas of length and of time, and how our present British Standards were fixed by Act of Parliament in 1855. Now we require to consider our units of mass, or quantity of matter, and we find that all untrained people confuse mass and weight, and many people have no conception at all of "mass," because they have never thought about it. Everyone is familiar with the idea of length; everyone thinks they are familiar with the idea of time; but very few people know, even generally, what they mean by the mass of a body. It is not weight; it is not volume. Unfortunately, having confused the ideas of weight and mass in our earlier years, we find great difficulty in separating them again. If I take a lump of iron and hold it in my hand, I can say "I estimate that weighs about four pounds." I can put it on a beam type balance, and find that it weighs just over four pounds, by comparing it with some sub-standard masses -" weights " as you call them - which I have in the laboratory. In each case I am considering the pull of the big world mass on the lump of iron-the force with which it is pulled towards the ground-its weight, that is. We know that at a given spot the pull towards the earth on different bodies, their weights, are, provided we weigh them in a vacuum, proportional to the amount of matter in them, which amounts of matter are termed their masses. So that, if under those conditions, and under those conditions only, we compare weights, we are at the same time comparing masses. Even if the materials are not in a vacuum, but in air, the error introduced is generally small. We have grown so accustomed to gauging the relative quantity of " matter " in a body by its weight, that the very idea of mass has become hidden behind our conception of weight. A pound weight is a force, the pull on a one pound mass of matter towards the earth.