This demonstration consists of a transparent cylinder with a flared base, and a plunger that can be pushed into it. A small (very small) piece of cotton is pushed into the bottom of the tube using the wire provided, and the plunger is sealed into the tube. The plunger is pushed down sharply, compressing and thereby heating the air within. The temperature rises high enough to ignite the cotton with a flash, which can be readily seen through the plastic tube.
• Consider inviting a volunteer from the audience to try the demonstration. This will require careful supervision, but is safe. Just ensure that the syringe isn't knocked off the table by an overenthusiastic student!
• This demonstration works best with a very small amount of cotton to ignite, no more than a few millimeters at most. Consider showing the device with different amounts of cotton, and how the results change. Encourage students to discuss reasons for this.
This demonstration illustrates that an essentially fixed mass of air will increase in temperature when its volume is reduced, i.e. it is heated when compressed. The fire syringe is a simple piston, and can be used to introduce a discussion of the use of pistons in engines.
Consider using this demonstration in conjunction with both other thermodynamics demonstrations from section I5, and relating it back to general gas behaviour with demonstrations from section I3.