A Brief History Of Measurements

Almost without noticing, we constantly use measurements in every aspect of our lives: we measure the speed of our cars in kilometers per hour and the temperature outside - in Celsius degrees; when we prepare a cake we measure ingredients in grams or milliliters; we measure electricity in the socket in volt or ampere, and the efficiency of our electric appliances in watt-hour. These are only a small part of the measurements and units that we require and make use of to measure sizes in our everyday lives. These measurement units, for example meter of kilogram, were made so that we will have a common language. When science became international, the international unit system created a uniform frame so that we can all measure in the same way, with the same standard, everywhere. But how do you determine what is a meter? and who decides which measurement units exist?

How do you determine what a meter is? A standard meter on a wall in Paris | ויקימדיה, LPLT 

The Higher Measurer

In the past there was no global uniform unit system, but only measurements determined by different communities. In the bible, for example, the unit cubit is mentioned, which represents the length between the elbow and the tip of the fingers. The length of a cubit is not uniform as there are people with longer and shorter cubits, but the cubit was a common measurement that people could rely on and know that they are generally referring to the same quantity.    

In the 18th century, as part of the enlightenment era in Europe, defined measurement methods began to be founded. For example, France began defining sizes such as a meter and a kilogram, and distributed them to other countries in Europe so that they could run trade in an accurate and pre-agreed-upon manner and also easily share scientific knowledge. In 1875 seventeen countries signed a treaty where they agreed on the meter and kilogram as measurement units and the foundation of the International Bureau of Weights and Measures (BIPM), which is meant to design and coordinate the determining and preservation of accepted measurement units. To this day the BIPM is the sole organization in charge of determining internationally accepted measurement units.  

How were the units determined? The method was simple: they picked a size that was commonly available and used it as base for the unit. For example, the meter was determined as the ten millionth part of the distance between the north pole and the equator, and the gram was determined as the mass of a cube of water that each of its sides is one centimeter. These definitions are accepted but problematic, as they require a highly accurate measurement of sizes that at the time were difficult to measure. Thus, when the BIMP was founded the method to determine units changed and began relying on physical references. For example, the BIMP produced a metal rod at a certain length and determined that from that point forward this is the length of a meter. Copies of the rod in the same length were produced and distributed worldwide. In this way they created a ruler that is based on the official meter and used it to measure other things that are also measured according to the original one meter rod. Similarly, the BIMP casted a weight that determined the kilogram. The second was also based on a size that is commonly available - about 1/86,400 of the duration of a day, which could be measured using a mechanical clock at the time.

The BIPM also casted weights that the kilogram was determined according to. The weights, within protective glass bells | BIPM

This method is simple, but also has a problem: the sizes that the unit system is based on are not completely fixed. The one meter rod appears to us as a constant size, but it can also expand and contract according to temperature, and the one kilogram weight can change its weight due to dust and other particles accumulating on it. The changes will indeed be minor, but when dealing with sizes that scientific research is conducted according to, even slight changes can result in mistakes. Even the second can change, as the earth’s rotation speed varies, and accordingly the length of a day. As a result, in the mid 20th century it was decided to alter the unit system again and base it on physical constants. The new measurement system, International System of Units (SI), was determined during the 1950s and has been updated occasionally since. 

The Way Nature Measures

The SI system is based on seven physical constants that serve as the basis for seven basic measurements. The use of physical constant, fixed and unchanging sizes that are based on the laws of nature, solves the problem of uncontrolled changes to the unit of measurement. The length of a metal rod could change from time to time, but the physical constants in nature, the speed of light for example, do not. The seven basic units in the SI system are meter, kilogram, second, ampere (a measurement unit for electric current), Kelvin (temperature unit), mol (matter unit) and candela (light unit). The rest of the measurements we know could be expressed using these units.

The seven basic units in the SI system are meter, kilogram, second, ampere (a measurement unit for electric current), Kelvin (temperature), mol (matter) and candela (light) | Shutterstock, Kooto

The seven units are determined by the sizes of seven physical constants: the speed of light in vacuum, the electric charge of an electron, the Planck constant that links between light frequency and its energy, Boltzmann constant that converts temperature to energy, Avogadro number that measures the number of atoms in matter, the frequency of light that is discharged when an electron of a Cesium-133 atom passes from one orbital around the nucleus to another and the intensity of light that is emitted from a 540 tera-hertz light source. 

How can we use this system to determine the sizes of the units? Let us take a look at some examples. The frequency of the light that is discharged during a shift of an electron in a Cesium-133 atom is 9,192,631,770 hertz, which is a little over 9 billion beats per second. This means that using the SI system, one second has 9,192,631,770 cycles of the light emitted from the shift in electron movement around the aforementioned Cesium atom. The atomic shift is measured by precise atomic clocks around the world. Similarly, the speed of light in vacuum is 233,792,458 meters per second. We already know the length of a second, so we can determine that a meter is the distance that light covers during 1/299,792,458 of a second. Using meter, second and Planck constant we can determine a kilogram, second and the electric charge of an electron define ampere, and so on.

The SI system, one second has 9,192,631,770 cycles of light emitted from the change in electron movement around a Cesium atom. The Cesium clock was constructed in 1955 and enabled the determination of a second using the new system |  National Physical Laboratory © Crown Copyright / Science Photo Library

The Length Of A Meter On Mars

It seems that the method is too complicated: it requires highly accurate measurement of certain sizes. Would it not be easier to measure a one meter rod? There are enough arguments in favor of this system: first, we can measure these fundamental constants with unprecedented maximum accuracy. The error in their measurement is smaller than one millionth of a percent, and so they are well defined. Second, while the measurement is indeed complicated and difficult, we also gain a fixed unit system, which is rational and consistent everywhere, also in space or on another planet, also in the past or in the future. And finally, a consistent measurement unit system provides help in a practical everyday manner: for example, internet speed is dependent on international communication systems, which are based on fast components that have to operate in a synchronized and accurate manner. Such synchronization is made possible by the uniformity of the duration of a meter and the value of an ampere, a uniformity shared by all components worldwide. Similarly, the SI system enables commercial, technological and scientific cooperation across the world - and maybe, in the future, outside of it.