Edward Willett

Measurement

[podcast]http://edwardwillett.com/wp-content/upLoads//2009/11/Measurement.mp3[/podcast]

“Inch-worm, inch-worm, measuring the marigolds…”

Despite that line from a popular song, the fact is, inch-worms don’t measure anything. Neither to cockroaches, bulldogs, llamas or horned toads…because measurement is the process of counting how much of a sensory signal exists, and so far as we know, no other animals can count.

Simply counting things wouldn’t itself count for much if we couldn’t communicate, though. Through language, we’re able to tell others what we have measured, which enables us to describe things we’ve seen, contract with others for trade or exchange, and control various processes.

Just think about all the things you rely on measurement for. Your clothes were measured to fit your body. Your food is stored in a refrigerator whose temperature is measured and controlled by a thermostat. The radio you listen to is tuned to a station identified by the measurement of its broadcast wavelength. Your car is built from standardized parts, each carefully measured. You are careful (aren’t you?) not to exceed the speed limit, measured in kilometers per hour by your speedometer…and so it goes.

Our technological world is a product of our measuring ability. But measurement is a lot older than modern technological civilization.

Time was one of the earliest things to be measured because the passage of time can easily be marked by counting existing units–days, lunar months, seasons, years. Space can also be easily measured using our own bodies, which is why the earliest units were things like spans (thumb to little finger on a spread hand), cubits (elbow to finger tip), and, of course, the foot. Once you have a unit of length, you can measure four fundamental qualities of space: distances, areas, volumes and angles. Early civilizations did just that, even though those human-based units of length aren’t exactly what you’d call standardized. (My foot and my daughter’s, for example, aren’t exactly the same size.)

Finally, our own senses also enable us to easily measure force. We can count, for example, how many stones of a particular size one person can carry, and thus judge how much force that person is capable of exerting.

But our own senses could only take us so far. It wasn’t long before we began creating tools that were more accurate and more sensitive.

Every measuring instrument has three main components: a sensor, which detects the phenomenon to be measured, a counting mechanism, and a display, which enables us to know what the count is, or at least when a certain count is reached. Sometimes there is also a transducer, which converts the phenomenon into some other phenomenon which is more easily measured. Thus, in a speedometer, a flexible cable rotates at the same speed as the wheel, and a magnet connected to the cable creates a field that varies according to how fast the cable is spinning. That field then deflects the speedometer needle.

Converting the response of the instrument to some kind of agreed-upon units is called calibration. How well that conversion is carried out is called accuracy.

For most of mankind’s existence, lack of accuracy hasn’t been a problem. The ancients didn’t worry about being off a half-hour here or a centimetre there. Even in the steam engines that powered the Industrial Revolution accuracy was of little concern; as long as adjacent parts were compatible, who cared if the same parts in other engines were a completely different size? But those mutually incompatible engines made mass production possible, and mass production means interchangeable parts, which do require accurate measurement.

So does science, among other things to ensure that results obtained by one scientist can be repeated by another. Greater and greater sensitivity of measurement was needed, as well, as the material world was examined in finer and finer detail.

Today we can measure things with mind-boggling accuracy, which has lead to more and more precise definitions of units of measurements.

So the next time someone says to you, “Wait a second,” impress them by saying, “Of course. Have you a device by which I might measure the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom?”

Then I suggest you run away, as fast as your feet—whatever size they may be—can carry you.

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