The transmission of pressure in fluids. Hydraulic brake
When the fluid is completely enclosed in a vessel and a pressure is applied to it at an. part of its surface, as for example, by means of a cylinder and piston connected to the vessel, then the pressure is transmitted equally throughout the whole of the enclos fluid. This fact, first recognized by the French scientist and philosopher, Pascal, 1650, is called the principle of transmission of pressure in fluids.
Fig. 11.6 shows a piece of apparatus to demonstrate this principle. It consists of a glass barrel fitted with a plunger and ending in a bulb pierced with he :es of uniform size. It is filled with water by dipping the bulb in water and slowly raising the plunger. When the plunger is pushed in the water squirts equally from all the holes. This shows that the pressure applied to the plunger has been transmitted uniformly throughout the water.
The principle of transmission of pressure has a number of practical applications. Indeed, our lives may often be said to depend on it whenever we ride in a motor vehicle, since the brakes of the majority of road vehicles are worked by hydraulic pressure.
This system of braking is shown diagrammatically in Fig. 11.7. The brake-shoes are expanded by a cylinder having two opposed pistons. These are forced outwards by liquid under pressure conveyed by a pipe from the master cylinder. The piston of the master cylinder is worked by the brake pedal. When pressure on the pedal is released the brake-shoe pull-off springs force the wheel pistons back into the cylinders, and the liquid is returned to the master cylinder.
A very important advantage of this system is that the pressure set up in the master cylinder is transmitted equally to all four wheel cylinders so that the braking effort is equal on all wheels. (See also page 96 for a description of the hydraulic press.)