Swelling and shrinking
Have you seen a picture of the Eiffel Tower? Perhaps you have been to Paris and stood beside the Tower or even gone up it in a lift. The Eiffel Tower is 320 metres tall. If you could take a tape measure and measure its height in winter and summer you would make an amazing discovery. The tower is seven and a half centimetres taller in summer than it is in winter! What happens to make the tower grow and shrink every year?

The tower does not actually grow in summer. It swells up or expands. In winter it gets a little smaller or contracts. The reason for these changes is due to the material from which it is made. The tower is made from metal girders. The metal is made from tiny particles, all holding on firmly to each other. When summer comes, the weather is warmer and the particles receive more heat from the Sun's rays and the warm air. This extra heat gives the particles more energy and they try to move. As they are held tightly together, they cannot slide around but they can shake themselves. As they shake, they push each other a little further apart. This pushing apart of the particles makes the metal girders expand and makes the tower rise a little further into the air.
As winter comes, the weather cools and the particles in the metal lose their energy. They push less strongly on each other and so move closer together. This makes the metal girders contract and the tower sinks a little closer towards the ground.

The Eiffel Tower is not unusual in expanding and contracting. All materials expand and contract a little so all objects made from them also expand and contract. If you look down your street, objects like lampposts, cars and even buildings are a tiny bit larger in summer than in winter. On a really hot summer day you may hear plastic guttering make cracking noises as it expands in the heat. When you go inside and boil a kettle, even the kettle expands a little. All these changes are very, very small but when large buildings are made they can all add up to make a change that can be measured.

Do all materials expand the same amount when they are heated?
No. Liquids expand about ten times more than solids. Each solid and liquid expands its own certain amount. For example metal expands more than concrete and paraffin expands more than water. In fact different metals expand by differing amounts. For example, brass expands more than iron.

Why does a glass break if hot water is put in it?
You should never put hot water in a container such as a bottle or other container which has a thick glass wall. The inside of the wall becomes hot and expands but glass is a poor conductor of heat and the outside of the wall remains cool. While the hot glass inside the container expands the cool glass on the outside does not. The expanding gas pushes so hard on the unexpanded glass that the whole glass wall cracks.

Do rocks expand when they contract?
Yes, they do. This can be seen particularly well in the desert where there are very hot days and cold nights. The expansion and contraction of the rock makes it crack up and form smaller pieces. These also expand and contract and break up to even smaller pieces. This expansion, contraction and breaking up continues until sand is produced.

Why do power cables sag in summer?
Power cables are carried on metal towers called pylons. In summer the cables sag a little between the pylons. The cables are made of metal and the sagging is due to the expansion of the metal in the heat. During the winter, when the weather is much cooler, the metal contracts and the cables are pulled much tighter between the pylons. If the cables were tightened up in the summer they would get even tighter in the winter and perhaps pull the pylons out of the ground.

Can the pull of contracting metal be useful?
Yes. The hull of a ship is made from huge steel plates. They are held together very closely to prevent water passing between them. Rivets, which look like thick nails, are used to join the plates. Each plate has a hole drilled in it and the holes are lined up one behind the other. A white-hot rivet is pushed into the holes. The head of the rivet is pressed against the upper surface of one metal plate and the end sticks out of the hole of the lower plate. The end is then hammered flat and the rivet is cooled. As the rivet cools the metal contracts and pulls the plates tightly together.

Can metal be used to pull walls together?
Yes, it has been used to pull walls together in the past. You may sometimes see an S or an X on the wall of an old building. The centre of the letter is connected to the end of a long metal bar. If you were to go round the other side of the building you would see a similar letter in the opposite wall. Between the letters, inside the building, is the long metal bar. It was put there when the walls started to bulge outwards. The bar was heated by a fire and expanded. The letters were clamped on the ends of the bar and the bar was left to cool. As the bar cooled it contracted and pulled in the letters and the walls. The letters help the bar by giving it a larger surface on which to pull in the walls.

Do metal bridges expand and contract?
Yes, they do. If they were fixed firmly at each end, the bridge could bend in hot weather. They are not fixed firmly but rest on rollers or a plastic pad. When the metal expands, the bridge simply slides over the rollers or pads. When the metal contracts the bridge slides back again.