free end of the spring 2 is deflected from the position corresponding to the initial position of the piston 3 until the force exerted by the spring 2 on the piston 3 equals the instantaneous force exerted by the fluid on the piston 3 at which time a position of instantaneous equilibrium is achieved as the pressure varies the piston 3 and the free end of the spring 2 will move toward the positions of instantaneous equlibrium corresponding to the then prevailing pressure in the cylinder 4 as the piston 3 moves in the upward direction as illustrated in fig1 the plunger 8 is displaced in the same direction but the plunger 8 does not share the oscillating movement of the piston 3 and of the free end of the spring 2 rather the plunger 8 is only displaced in one direction that is upwardly until it reaches a displaced position corresponding to the peak value of the pressure during a time interval at which time the brake 13 prevents the return or for that matter any other movement of the plunger 8 regardless of the fact that the pressure in the cylinder 4 may have fallen below the peak value fig2 shows the construction of the gauge 6 in more detail it will be seen that reference numeral 13 identifies a friction brake which is described in more detail in.