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Expansion Bar
AKA: Bar Breaker

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Fig 1: Bar and gauge equipment
Fig 1
Fig 2


A metal bar that expands under direct heating.


There are two main types of expansion bar available, a simple one (see Fig 1) which is similar to the ball and hoop often called the bar and gauge apparatus and a more complex (see Fig 2) device which actually breaks a smaller cast iron bar as the main bar contracts.

The more simple apparatus consists of a metal bar (sometimes cylindrical) and a frame. When both at the same temperature, the bar should fit snug within the frame. The bar is then heated with a Bunsen burner until it has expanded sufficiently. It will then become too big to fit within the frame demonstrating how much the metal can expand during heating.

The more complex apparatus works in a slightly different way but utilises the same principles. Small cast iron bars are used in this experiment, replacements can be found through many lab suppliers. Cast iron is used primarily because it is easier to snap than say steel.

The centre of the main bar may be heated directly with a bunsen burner. During heating, the main bar should expand slightly allowing the small bar to be inserted into the gap at the end of the large bar or on other versions, allowing a srew to be tightened, holding the small bar in place. When the bar cools down, it will contract to its original size putting immense pressure on the small bar until it snaps.

More complex types have become available in recent years containing micrometer type instruments which can be used in an investigation scenario. Testing the specific expansion of various types of metals and plotting graphs accordingly is much easier with this new apparatus.


 CautionThis equipment remains very hot afer use.

wear eye protection

The contents of this page are for information only. Please refer to CLEAPSS or ASE safety advice and/or publications before undertaking any preparation, practical experiment or using any equipment featured on this site or any other.