How Many Rods Are There in a Chain?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

09 January 2005

Kenyan tailors can be confusing. When they order clothing materials, the measurements are made in metres, but when they make the clothes, they take measurements in inches. Estate agents are also in the same boat – land is sold in acres while surveyors work in hectares. The questions then arise: how many inches are there in a metre and how many acres make a hectare?

Unlike the metric system where units are expressed in multiples (or fractions) of ten, the imperial system is cumbersome. The inch is divided into halves, quarters, eighths, etc. There are 12 inches in a foot, three feet make a yard, five and a half yards are equal to a rod, four rods are a chain… No wonder the world moved away from the system.

In the metric system, the metre is defined as the distance travelled by a beam of light in a vacuum in the time interval of 1/299 792 458 of a second. In 1958, the length of one inch was fixed as 25.4 millimetres exactly.  Therefore, there are roughly 39.37 inches in a metre. Note; this is an approximation.

Now, one acre is the area measuring 70yd by 70yd; that is 4,900 square yards. A hectare on the other hand measures 100m by 100m, or 10,000 square metres. How are the two related?

One yard is equal to 36 inches (three feet times 12 inches per foot), thus 70yd = 2,520 inches or 64,008mm or 64.008m. Therefore one acre is 64.008m by 64.008m = 4,097 square metres, approximately. But since one hectare is 10,000 square metres, then one acre = 0.4097ha. Put the other way round, there are roughly 2.441 acres in a hectare.

Despite its elegance, the metric system has very strict symbols for its units. For example, a small letter ‘m’ alone means metre, but when used as a prefix it means “milli” or thousandth. Capital ‘M’ on the other hand means “mega” or millions. Contrary to popular usage, the symbol for “kilo” is a lower case k – the upper case K means kelvin, which is a unit for measuring temperature. A small letter ‘s’ stands for seconds it does not indicate plural! A capital S means siemens (as in the mobile phone), the unit for electrical conductance.

Therefore, when someone puts up a road sign that reads, say “XYZ School, 25KMS” what they are saying is that the school is 25 kelvin-mega-siemens, which is total nonsense! The sign should actually read 25km, meaning 25 kilometres.

But this abuse of symbols is not as great a crime as that committed by toothpaste manufacturers in Kenya. Some tubes show the quantity in millilitres, while others give it in grams. The former measures volume while the latter is mass. Yet volume and mass are like cows and goats – how many goats make one cow? Surprisingly, the department of weights and measures and the bureau of standards haven’t seen anything wrong in this.