ABSTRACT
It is, of course, meaningless to say that the distance from point X to point Y is 300 without saying whether we are speaking of miles, kilometers, yards, meters, feet, and so on. The magnitude of a physical quantity is described as the ratio of the quantity in question to that of a specific standard quantity called its unit. For example, to say that a room is 20m deep means that the ratio of the depth of the room to the length that is conventionally called one meter is 20. If the unit is changed, say, to em, the ratio changes, in this case to 20 x 100 = 2,000 em.
