The future of the kilo: a weighty matter

In a vault inside the elegant Louis XIV Pavillon de Breteuil, outside Paris, a small metal cylinder rests on a shelf beneath a double set of bell jars. It has lain there for more than a century, its repose only occasionally disturbed when the vault’s three key holders perform a co-ordinated opening ceremony to let technicians enter and clean the ingot.

First, the platinum-iridium cylinder is rubbed with a chamois that has been soaked in alcohol and ether. Then it is steam rinsed using boiling, double-distilled water. Finally, the 1kg cylinder is returned, carefully, to its resting place.

Such reverence for a lump of metal is unusual, but has a purpose. The pavilion houses the International Bureau of Weights and Measures and that piece of platinum-iridium alloy is its holiest relic. It is the defining mass against which all other kilograms are measured. This is the international prototype of the kilogram. The IPK, in short.

Dozens of copies of this carefully calibrated piece of metal have been made. They are stored around the world and used to standardise individual nations’ weights and measures systems. Britain’s copy is kept at the National Physical Laboratory (NPL) at Teddington, near London. But the Parisian version is the king of the kilograms. “All mass measurements, anywhere on the planet, are traceable to that one unit in the Pavillon de Breteuil,” says NPL scientist Tim Prior.

But the days of le grand K, as it is known, are numbered. Later this month, at the international General Conference on Weights and Measures, to be held in France, delegates are expected to vote to get rid of this single physical specimen and instead plump to use a fundamental measurement – to be defined in terms of an electric current – in order to define the mass of an object. The king of kilograms is about to be dethroned.

And crucially much of the key work that has led to the toppling of the Paris kilogram has been carried out at the National Physical Laboratory where the late Bryan Kibble invented the basic concepts of the device that will replace that ingot in the Pavillon de Breteuil. The Kibble balance works by measuring the electric current that is required to produce an electromagnetic force equal to the gravitational force acting on a mass. A second stage allows the electromagnetic force to be determined in terms of a fundamental constant known as the Planck constant which will, in future, be used to define a kilogram. These machines will provide the standard for weighing objects – and that means no more dusting of old lumps of alloy to ensure they stay pure and accurate.

Developing the Kibble balance was a highly complex business, however, for balances – when they come into use – will have to achieve highly demanding levels of accuracy. A former colleague of Kibble is Ian Robinson, an NPL Fellow in the laboratory’s engineering, materials and electrical science department. He is leading an international team of researchers to create simple and accurate versions of the Kibble balance to fulfil that need.

“One key reason for doing this work is to provide international security,” says Robinson. “If the Pavillon de Breteuil burned down tomorrow and the kilogram in its vaults melted, we would have no reference left for the world’s metric weights system. There would be chaos. The current definition of the kilogram is the weight of that cylinder in Paris, after all.”

That vulnerability will soon be a thing of the past. “We are going to change all that,” says Robinson. “We are going to create a method for weighing the kilogram completely accurately until the end of time. We will have released ourselves from a single point of failure.”

Another major motivation for the replacement of le grand K is the need to be able to carry out increasingly more and more precise measurements. “Pharmaceutical companies will soon be wanting to use ingredients that will have to be measured in terms of a few millionths or even billionths of a gram,” says Prior. “We need to be prepared to weigh substances with that kind of accuracy.”

In replacing the Paris kilogram with units measured in terms of Planck’s constant, scientists have had to push technology to its limits, though in one sense they are merely catching up with all the other methods now used to define the other basic units with which we measure our existence, such as time and length. These are determined today in terms of fundamental processes.

Consider the basic unit of length, the metre. It was originally deemed to be one ten-millionth of the distance from the equator to the north pole, before being replaced by a platinum bar held in Paris. Today, it is designated as being the length that is travelled by light in a vacuum in 1/299,792,458th of a second.

Similarly, the basic unit of time is the second. It was once stipulated to be 1/86,400 of an average day. However, Earth’s rotation is variable, which made it difficult to measure time precisely this way. As a result, the second is now established as being the time taken for a caesium atom to vibrate 9,192,631,770 times, no more and no less.

In fact, with one exception, all the units we use to measure the world around us – and which make up the International System of Units – are today specified in terms of fundamental universal concepts. These include the kelvin, the basic unit of temperature, as well as constants that define electric current and luminous intensity.

It is only the kilo that has continued to be defined by a manufactured object, that lump of alloy in Paris. Its existence can be traced to the decision by Louis XVI to support a new system of weights and measures, one that would be independent of vagaries such as the length of a monarch’s arm. This metric system was put forward in 1791 with the kilo being defined in terms of the mass of a given volume of water at 0C.

For the next 100 years, the kilogram was redefined over and over again, while a gradually increasing number of nations joined France in adopting the metric system, stimulated by growth in international trade and the need to standardise the weights of manufactured goods. Eventually, the international metre convention was signed in 1875 by 17 nations. (It is worth noting that Britain did not sign until several years later, revealing another constant.)

Items such as the prototype metre and kilogram were all kept in Paris and then copied and sent to other nations. And by and large it was a fairly accurate process, says Prior. “All the other kilogram ingots that are used across the planet were copied from the IPK in Paris and are very close in weight to that standard. They all weigh within a milligram of the main ingot that is kept in the Pavillon de Breteuil. However, we need a higher level of precision these days and that will be provided by the Kibble balances that will replace the Paris ingot.”

Measure for measure

Apart from the kilogram, the Système international d’unités (SI) has several other base units of measurement, all now derived from physical processes. These include:

Time The base unit of time is the second, which was once defined as 1⁄86,400 of an average day. However vagaries in the Earth’s rotation made it difficult to measure precisely. It is now defined as being the time taken for a caesium atom to vibrate 9,192,631,770 times.

Length The metre was first defined as one ten-millionth of the distance from the equator to the north pole. Then it was replaced by a platinum bar held in Paris. Now it defined as the length travelled by light in a vacuum in 1/299,792,458th of a second.

Temperature The basic unit of temperature is the kelvin, which is defined as being 1/273.16th of the temperature of water at 0.01C.

Electric current The ampere is defined in terms of the current required to generate a certain force between two wires set a metre apart.

Two other units are included in the list: the amount of a substance in a given sample is measured in moles and luminous intensity is measured in candelas.