 At first glance, the distinction between fundamental and derived measurement may seem reminiscent of the distinction between extensive and intensive magnitudes, and indeed fundamental measurement is sometimes called “extensive”. Nonetheless, it is important to note that the two distinctions are based on significantly different criteria of measurability. As discussed in  Section 2,  the extensive-intensive distinction focused on the intrinsic structure of the quantity in question, i.e., whether or not it is composed of spatio-temporal parts. The fundamental-derived distinction, by contrast, focuses on the properties of measurement operations. A fundamentally measurable magnitude is one for which a fundamental measurement operation has been found. Consequently, fundamentality is not an intrinsic property of a magnitude: a derived magnitude can become fundamental with the discovery of new operations for its measurement. Moreover, in fundamental measurement the numerical assignment need not mirror the structure of spatio-temporal parts. Electrical resistance, for example, can be fundamentally measured by connecting resistors in a series (Campbell 1920: 293). This is considered a fundamental measurement operation because it has a shared structure with numerical addition, even though objects with equal resistance are not generally equal in size.