Cartesian divers: Pressure on cap sends divers sinking.

Cartesian divers: Pressure on rubber stopper sends divers sinking. Two hollow glass 'divers' are filled with water and air and placed in a tall, water-filled, stoppered hydrometer jar. One diver has slightly more air than the other one. In situation 'A', both divers are at the top of the jar. In situation 'B', pressure on the rubber stopper compresses the air in each diver, sending them both downward. The diver with less air goes down further. The level to which a diver descends is sensitively controlled by the pressure on the stopper. Note: To balance a diver, hold in hot water for a short time, remove, and place in cold water. Sufficient water will be drawn into the diver to make it balance. If too heavy, shake out excess water. FLUID STATICS
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It can be helpful to have a cartesian diver out of the water so you can show it to participants (don't loose it). This is a great opportunity to discuss pressure; ask kids what they feel inside their ears as they dive deep into the pool. Have them make a prediction as to what the divers will do before you demonstrate (it may be helpful to call them something other than Cartesian "divers"). As always when you present a demo make sure you ask the participants questions before you demonstrate and after. Think about the concepts involved beforehand; having a friend ask you questions about it can be very revealing as to if you understand it well or not. Whenever you ask a question count to five (or at least three) in your head waiting for an answer before you stay anything else.