The Permicron: a Unit of Wave Number

Abstract

IN spite of the great theoretical advantages of recording spectra in terms of frequency or wave number, most data on visible and ultra-violet spectra, more particularly spectra of solutions, are in practice expressed as wave-lengths with the millimicron as a favourite unit. One reason for this preference, apart from habit, is found in the inconvenience of sec.⁻¹ and cm.⁻¹ as units. Frequencies expressed in sec.⁻¹ involve large powers of ten (∼ 10¹⁵), while the fresnel¹ (= 10¹² sec.⁻¹) has never become popular. Wave numbers in cm.⁻¹ are of the order of 10⁴, and require the use of written or spoken powers of ten if the recorded values are to express the relatively low precision of measurement in spectra of solution. Thus, to say that the absorption maximum of bromine is at 405 mµ. implies a reasonable uncertainty of about 10 A. in locating it. To express the value as 24,691 cm.⁻¹ or even as 24,700 cm.⁻¹ implies a much greater precision than is obtainable experimentally.