From ‘Analysis of the ventilatory defect by timed vital capacity measurements.’ By Edward Gaensler, American Review of Tuberculosis, 1951; 64(3): page 260.
“Fig. 1. Vital capacity spirometer with timed capacity attachment and electronic timer. The insert shows the timing device with the cover removed. On the left is the micro-switch which initiates the timed interval when the spirometer bell begins to rise. On the right is the midget solenoid which moves the added second pointer only while it is activated by the timer. “