A student knows that some metals are better heat conductors than others.
He investigates rods made of aluminium, brass, copper, iron, lead and zinc.
The diagram shows the equipment which he uses to test three of the rods.
He fits the rods into corks and puts the corks into holes in the lid.
He covers each rod above its cork with wax which is solid at room temperature.
The student pours hot water into the tank and puts the lid on.
After 20 seconds, he measures the length of the melted wax on each rod.
Measure the length of melted wax above the cork in the following diagram.
Length = ........................... mm