A hydraulic lift is a machine that uses pressurized liquid in a confined space to transfer pressure. The pressure of the fluid is transferred from one end of the hydraulic system to the other undiminished – allowing force to be magnified by transferring it from a smaller piston to a larger one. The principle of hydraulics is used in many systems, and is found in machines as varied as car brakes and the human circulatory system.
You can show the basic principle of hydraulics with a simple demonstration. One principle of hydraulics is that a fluid always seeks to stay horizontal – with the surface of the fluid parallel to the horizon. You can demonstrate this simply by filling a glass half full with water. Now tilt the glass back and forth. The surface will stay level. This is the idea behind a ship's compass, which floats on a liquid so it always remains steady.
This experiment demonstrates the way that pressure works in a hydraulic system, and that fluid cannot be compressed beyond a certain point. Push one end of a thick drinking straw into a one-inch slice of potato. Pull the straw out, leaving a plug of potato in the straw. Use a skewer to push the potato plug into the middle of the straw. Fill the straw with water and push the straw into another piece of potato to make a second plug, trapping the water in between the potato plugs. Use the skewer to push on the new potato plug. You will notice that the other plug moves as well. The trapped water cannot be compressed any further and pushes against the potato piston.
Hydraulic jacks are used to lift very heavy objects. You can build a model of a hydraulic jack very easily. Tape a plastic sandwich bag over the end of a length of plastic tubing to make an airtight seal. Tape a plastic funnel to the other end of the tube. Lay the plastic bag flat on a table or other surface and place a book on top of it. Hold the funnel up so it is higher than the bag and slowly pour water into it. The bag will fill with water, lifting the book.
Demonstrate hydraulic pressure used in hydraulic lifts using two blunt-tipped syringes – like the kind used in cooking. Connect a short length of plastic tubing (about 2 or 3 inches long) to one syringe. Fill the other syringe with water or vegetable oil and attach it to the other end of the tubing. As you push down on the plunger of the water-filled syringe, the water will flow into the other syringe and push its plunger up by the same amount. Repeat this using one syringe that is larger than the other. You will see that the ratio of movement changes – the smaller plunger will move farther than the large one for the same volume of water movement.