Carbon dioxide expired by the fowl is absorbed in the KOH solution, pressure then falls in the system and a solution of NaCl in the glass manometer (V) rises inthe closed arm. When the solution reaches the stainless steel contact (W), the electrical circuit is completed and a relay system (X) opens a solenoid valve (Y)permitting oxygen to enter from the cylinder. The solution rises in the open arm until it reaches the stainless steel contact(Z) and the relay system shuts off thesolenoid valve.