The kilogram, the actual kilogram, sits in a vault in Sèvres, France under numerous bell jars. It is the last SI unit to be defined based on a physical quantity—in this case one kilogram of platinum ...

I love thinking about things I can’t see with just my eyes. Like the atoms that make up everything. I asked my friend Fred Gittes how to figure out the number of atoms in a leaf. He’s a physicist at ...

How many atoms are there in a kilogram of silicon? This number is Avogadro’s constant and working out its value is one of the more important tasks that materials scientists face. Today, a group of ...