Many of the laws of science—for example, all the ‘conservation’ laws, the laws of thermodynamics, Newton’s first and third laws of motion, Lenz’s law in electromagnetism and Le Chatelier’s principle in physical chemistry—are special and more precise statements of the everyday phrase, ‘You do not get something for nothing.’ The same applies to effects produced by ionising radiations and it has therefore always been obvious that these effects can only be brought about if the radiation transfers some of its energy to the material in which the effect is produced. The process of energy absorption is important even if it is later followed by complex physical, chemical or biological processes. Thus there has long been a desire to determine the energy absorbed by irradiated materials as a step in obtaining a quantitative correlation between the radiations and the effects they produce. The heating effects of x-rays on metal foils were measured as early as 1897, and Curie and Laborde (1904) used a calorimetric method to determine the rate of energy release by radium. By 1913 Christen was advocating the use of a quantity he called ‘dose’ and which he defined as the radiant energy absorbed per unit volume. (An account in English (Christen 1914) may be more readily available.) He recognised that measurements would probably have to be made of the number of ions produced in a specified volume of dry air under standard conditions of temperature and pressure, but thought it would be better to calculate the energy required to set free this number of ions so that ‘the unit of dose would be expressed by ergs per cubic centimetre’. He was ahead of his time. Zimmer (1938) and Gray and Read (1939) were faced with the problem of measuring neutrons. As these particles could not be measured in terms of the quantity exposure which had by that time been brought into use for x-ray and γ-ray measurements (see §5.1), they introduced an ‘energy unit’ for the purpose. Like Christen’s unit it was expressed in energy per unit volume and was calculated relative to the energy absorbed per unit 48volume of water exposed to one roentgen (§5.1) of γ rays. But again like Christen they had to measure it in terms of ionisation in gases because, as will be seen later, there are severe practical difficulties in the direct measurement of ionising radiations by the heating effect they produce. It was not until after its meeting in 1950 that the ICRU (1951) felt it prudent to say Tor the correlation of the dose of any ionising radiation with its biological or related effects the ICRU recommends that the dose be expressed in terms of the quantity of energy absorbed per unit mass (ergs per gram) of irradiated material at the point of interest.’ Even then they went on to say ‘Inasmuch as calorimetric methods are not usually practicable, ionisation methods are generally employed.’ The ICRU (1954) gave this quantity the name absorbed dose and said ‘The rad is the unit of absorbed dose and it is 100 ergs per gram.’ At its 1956 meeting the ICRU (1957) said the ‘absorbed dose of any ionising radiation is the energy imparted to matter by ionising particles per unit mass of irradiated material at the point of interest.’ The 1959 report (ICRU 1961) gave guidance on the term ‘energy imparted’ and the 1962 report (ICRU 1962) said ‘The absorbed dose (D) is the quotient of ΔED by Δm where ΔED is the energy imparted by ionising radiation to the matter in a volume element, Δm is the mass of matter in that volume element.’ The Δs were introduced to indicate that Δm was sufficiently small to define the point of measurement but sufficiently large for ΔED to be made up of so many energy deposition events that a good average value was obtained. Absorbed dose was therefore a macroscopic quantity needing to be averaged over a volume, in the same way as other macroscopic quantities such as density have to be averaged. The ICRU (1971a) changed absorbed dose to a quantity defined at a point by a differential quotient dε̄/dm, where dε̄ was the mean energy imparted, with the meaning given in the present day definition below.