The SI system of units and conventions

The initials SI are an abbreviation for Systeme International d'Unites (International System of Units), the modern form of the metric system, finally agreed upon at an international conference in 1960. It is now being adopted widely throughout the world and is likely to become the primary world system for units and measurement. As we shall discuss below, the system rationalises the main metric units of measurement and standardises their names and symbolic representation. It also rationalises certain mechanical principles and conventions.
The British system of weights and measures is many centuries old, and the derivation of its units with their multiples and sub-divisions is often obscure. The system has been refurbished from time to time but the yard and the pound with their multiples (e.g. mile) and sub-divisions (e.g. ounce) have persisted; so have such ridiculous measures as 5+ yards = 1 rod, pole or perch, 4 roods = 1 acre or 141b = 1 stone remained with us to try the mental agility of generations of students, not to mention the more mature, and less mentally agile population.
The metric system was founded during the French Revolution and has been adopted for use by most countries with the notable exceptions ofthe British Commonwealth and the U.SA., but even in these countries it is used for precise scientific measurements.
The basic units of the SI metri,c system are the metre and the ki10gramme and it is exclusively decimal, so that all multiples and subdivisions of the standard are found by applying factors of 10 (1 kilometre = 1000 metres; 1 ki10gramme = 1000 grammes; 1 hectare (area) = 100 X 100 square metres, and so on). In the English system, of course, there is no such orderly pattern and indeed, the foreigner might well question our sanity when he hears us refer to 112 pounds as a hundredweight.
However, we have, at last, been caught up with the progress of the times and as a nation we have decided to change over to the metric system. The entire text of this book conforms with the SI system and the object ofthis introductory chapter is to provide help and reference for the reader as he finds his way into what may seem, at first, to be a complexity. The best advice that can be given for achieving rapid progress in coping with the change is to become familiar with the new measures and to learn, as soon as possible to think in terms of them, and not to persist in making mental conversions back to the old units. This process can be speeded up by acquiring, as soon as possible, a mental appreciation ofa range oflengths, weights, capacities, etc. Some of these could be (say) the mental judgement of 25 millimetres, 1 metre, the weight of 1 kilogramme, the amount of fluid comprising 1 litre (1000 cm3) and the pressure corresponding to 1 bar (approx 1 atmosphere)*. In this way it will soon be possible to think of these measures in their own right and not grope around converting them to their English equivalents (1 kg = 2.21b and so on). A similar process is concerned in the learning of a foreign language where fluency will never be achieved until a student thinks in terms of the language concerned and abandons all attempts to interpret mentally from, and into, English. It is well known that another language is quickest learned by living amongst those who speak nothing else.lfthe reader can approach this new mathematical and scientific language in this frame of mind he will find that the former system will rapidly recede, so increasing the ease with which he can cope with the problems involved.