Interactive Primary Newsletter 31 Under pressure

Feeling the pressure? Another way of ‘weighing’ the air is to measure atmospheric pressure, using a barometer. We’ll use the balloons this time for a different purpose. This is another idea that has been widely published but, this time, it usually ‘works’. Fig.2 - DIY barometer from above Take one of the balloons left over from the coat hanger debacle. Remove the ‘neck’. Stretch the balloon (Fig.2) over the mouth of a jam jar or similar container (we used an empty pillbox from the local pharmacy). Put sticky tape around the balloon and the container, to keep everything airtight and tidy. Finally, glue a drinking straw to the centre of the stretched balloon. Test the glue on the material before attaching the straw. We used good old-fashioned Gloy. The picture in Fig.3 shows the finished barometer. This has a plastic ruler as a scale to read changes in pressure (the force per unit area exerted by the column of air above the container). A barometer proper is used primarily as an aid to weather forecasters – this is done by logging trends in air pressure over time. Here, however, we are trying to show that air exerts a force. From there to understanding that air has mass and thus weight is still a bit of a leap, especially at primary level. Fig.3 - Finished barometer Magic Card Tricks