We, the public, are encouraged by some politicians, by the DfT and by elements of the media to pick and mix our measurement units – to use both imperial and metric. So why has imperial as a system fallen from favour among those resisting change, and been replaced by a hybrid?
Because, in short, it has been become unfit for purpose.
The imperial system and its US counterpart stopped developing about 200 years ago. They do not contain units to measure more recent discoveries such as electricity or radiation. For example, if you try to find the imperial unit of electrical current, you will be disappointed. So anyone living today will not know a time when only imperial units were used.
The imperial system has a limited range of size of units and is unsuitable for measuring the extremely large and the extremely small. For example, the chemical composition of a bottle of mineral water and the dosage of modern medicines are measured in milligrams, never in imperial. The imperial system’s smallest unit of mass is the grain, which is equivalent to approximately 65 mg. Imperial units of mass do not cover a sufficient range for such tiny quantities so it is no surprise that you never see any imperial conversions for them. Similarly, it is not useful for describing scientific concepts such as the size of atoms and the wavelengths of light. At the other end of the scale, the imperial system struggles with distances in space, the mass of planets or the power output of stars. The resulting strings of zeros can be difficult to comprehend.
But there must be other reasons why many of us can not be bothered with imperial’s quirky complexities.
For forty years, children have learned metric at school. Could it be that as adults we are now comfortable with metric measures and try to apply the simplicity and logical nature of metric to the imperial measures we encounter, not always by choice, from day to day?
Imperial has frequently, for example, several names for the same physical quantity. For volume, there are the imperial and US dry and liquid measures. Quantities in these systems are different, even for units that share the same names. Common unit names that represent different quantities create ambiguity and obscure simple direct comparisons. So the litre gradually supplants its imperial rivals in day to day activities.
On the other hand, some imperial units for measurement have limited applications. For example, British thermal units are not suitable for measuring food energy intake, nor are pounds per square inch suitable for barometric pressure in weather reports. Fortunately, there are metric alternatives in common use.
Furthermore, some imperial unit names have different meanings, depending on where they are used. For example, when miles are used on land, we think of the statute mile but when miles are used at sea or in aviation it is the nautical mile. In all cases, the word “mile” is used and it is assumed that the type of mile is implicitly understood. The barrel also varies in size, depending on the commodity being measured. Pounds and ounces have different meanings for grocers and jewellers. All interesting for the professionals, but perhaps confusing for the person in the street.
And so, the use of the imperial system fades away. Our collection of medieval measures, which was overhauled in the 1820s to meet the needs of Britain’s industrial revolution, is seen as no longer fit for purpose. That is why those who would like the UK to have its own unique traditional measurement system, and their friends among politicians and the media, now favour the use of a hybrid of imperial and metric which they have called “British weights and measures”. Traditionalists will be relieved that the horsepower is likely to be around for a while, but may regret that the foot-candle is almost forgotten.