In order to make a vacuum in airplanes, these devices were equipped with additional Venturi
This is my Mom in a glider .. I remember talking to her a lot when she showed me these photos .. I was 6 years old then , and I was very scared because in our family one pilot died in 1924 ... and I knew this story ..
And here's an animation of how the gyro drive works on this negative pressure in these aviation instruments. He is so precise that they are now using the vacuum for his drive, but that is generated by the electric motor ... But the gyroscope continues to drive the vacuum ...
Andrew
https://www.aircraftspruce.com/categ...in/vacuum.html