What material are hydraulic jacks often filled with?

Hydraulic jacks are commonly filled with hydraulic fluid, which is typically oil. This fluid is specifically chosen for its properties that allow it to transmit force efficiently while also being stable under pressure. When the piston of the jack is operated, the fluid is forced through the system, enabling the hydraulic mechanism to lift heavy loads. The characteristics of hydraulic fluid, such as its viscosity and resistance to compression, make it essential for the effective functioning of the hydraulic system.

Other options like air or water do not provide the same level of performance and safety. Air could compress under pressure, making it ineffective for proper lifting, while water can cause corrosion and has a lower boiling point, which could lead to vapor formation under certain conditions. Glycerin, although a liquid, is not typically used in hydraulic systems due to its different properties and potential issues with freezing and viscosity under varying temperatures. Thus, hydraulic oil remains the standard for ensuring efficient and safe operation of hydraulic jacks.