880 
In our scematic picture, the gauge consists of two parts: (a) 
the piston, and \(b) the comparatively massive object in which 
the piston moves.) We have so far assumed the baffle to be in- 
finite and at rest. Actually it will move, even if rigid, un- 
less it is very heavy ss well as rigid. Motion of the baffle 
will further diminish the pressure on the piston. In fact, the 
relief pressure due to the moving baffle is 
(19.1) pe [ v(t - 8.) sigs. (t— e,)] 
where Vy is the velocity of the front face of the baffle; 8p 
and a, are helf the times required for sound to cross the pieton 
and baffle, resvectivelf. (19.1) 1s calculated in the same way 
as (11.2). If the action of the gauge is completed before 
t = 6, the exoression (19.1) becomes 
(19.2) 7 ae (t - a 
In thie case, while the gauge is responding, the relief pressure 
from the baffle is proportional to its velocity, but the relief 
pressure from the piston itself is proportional to its accele- 
ration. Their difference in benavior is due to their different 
sizes: the time for sound to cross the piston is small compared 
to the response time of the gauge, while the baffle is so large 
that the time taken by sound to travel from its edge to its center 
exceeds the response time of the gauge. We shall usually assume 
that this is the situation, i.e., 
(19.3) COA) 
576) = 
