The need for an international unified system of units has led to the adoption of the SI system (Système International d’Unités). This system adopts certain basic units for length, mass, time, electric current, absolute temperature and luminous intensity. A list of the units, together with their basic symbols is given in table 1.1. The value of many quantities varies with time. For example, after switching a central-heating system on, the temperature of the radiators in the system gradually increases from the room (cold) temperature to their final (hot) temperature. The temperature of a radiator at a particular time as it warms up is known as its instantaneous temperature, and is assigned the lower-case (small) alphabetical character appropriate to temperature, that is, t or θ (Greek). The final temperature is assigned the upper-case (capital) character, that is, T or Θ. A range of these symbols is shown in table 1.1
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