William Thomson was born in Belfast in 1824. Most people know him as Lord Kelvin, a physicist whose work helped to formulate the laws of thermodynamics. He is best known for calculating ‘absolute zero’ and so, the unit of temperature Kelvin (K) was named after him. Through the story of William Thomson, we’ll explore the measurement of physical quantities in physics.

William Thomson was a mathematical physicist and engineer born in 1824 in Belfast, Ireland. He contributed to some of the most important scientific discoveries in history. At the University of Glasgow he did important work in the mathematical analysis of electricity and formulation of the first and second laws of thermodynamics, and did much to unify the emerging discipline of physics in its modern form. He also had a career as an electric telegraph engineer and inventor, which propelled him into the public eye and ensured his wealth, fame and honour. For his work on the transatlantic telegraph project he was knighted in 1866 by Queen Victoria, becoming Sir William Thomson.

His most important work was the calculation of absolute zero (-273.15 ^{o}C), the temperature at which all atoms and subatomic particles stop moving completely. This is the coldest temperature that can possibly be achieved and scientists are still working around the world to reach absolute zero. Because of his calculations, the S.I. unit of temperature is named the Kelvin (K). S.I. is the international system of units that defined the worldwide standard units for things like time, mass, length, temperature etc. Some of the units are named after the scientists who first discovered the properties.

*Physical Quantities*

To learn about measurement, we first need to understand the term ‘physical quantity’. This is a scientific term for something that we can measure, such as **length**, **temperature** or **time**. Another important word we may be using is the term ‘quantified’. This is the scientific term for saying a quantity was measured. So if we use our ruler to measure the length of a science book, in scientist speak we have “quantified a quantity”. Luckily nobody actually talks like that, but it is good to know the scientific terms because you will hear them throughout the physics section. In physics we like to look at quantities like mass, speed, distance, energy and we have different ways to measure and understand them.

*Measurement*

What does it mean to measure something and why is it
important? Imagine you wanted to rank the students in your science class in
order of height, how could you do this? Obviously you could stand everybody
beside each other, use your eye to judge who is taller than who, and then move
people around. This method would work for a small class, but if we wanted to do
this for your whole school it would be nearly impossible. An easier way to do
this is to measure the height of every student using a ruler, or metre stick
and having a number for the height of every student. This is the basis of
measurement in science. Measuring something gives us a **numerical value** for a **physical
quantity**. Using these numerical values (i.e. numbers) we can easily compare
two objects. In the case of measuring student’s height, we need to use a common
unit. If we measured one person to be 5 ft and another to be 1.8 metres, how do
we know who is taller? In science, we need to make sure that all measurements
are given in the same units. For this, scientists created the International
System of Units (SI units). Having an agreed upon system of units means that if
you do an experiment here in Ireland, somebody in Australia should be able to
repeat that experiment and get the same results. The most common SI unit we
will come across in physics are the metre (m), for distance, the second (s) for
time, the ampere (A) for current and the kilogram (kg) for mass.

*Error*

When we make any measurement, we need to take into account slight mistakes that could be made. We call this error. Error tells you how far off the correct value you are. Imagine you wanted to measure the width of your thumb and you have a metre stick and a small ruler. Which do you think would have the smaller error? Well your small ruler has increments in millimetres (mm), while a metre stick only goes up in increments of centimetres (cm). So for that measurement, the ruler would have a smaller error. If a measurement has small error, that means we can trust the measurement.

For every measurement we want to make, there are specific
tools that we can use. If we want to know the temperature of a room, we use a
thermometer that gives us a numerical value for the temperature in degrees (^{o}
C). If we want to know the mass of an object, we use a mass balance which gives
us a numerical value in kilograms (kg). It is very important to choose the
correct measurement tools.

Before you begin your life as a physicist, you need to know how to make correct measurements that you trust and can stand by.