From “Methods in pulmonary physiology”, by Bertels H, Bucherl E, Hertz CW, Rodewald G, Schwab M.  Translated by Workman JM. Hafner Publishing Co., 1963, page 26.

“Anthony and Rohland were the first to use an electro-magnetic device.  In this apparatus, the addition of oxygen is dependent on the expiratory position.  The counterweight of the spirometer bell dips into a vessel of mercury, so arranged that at the highest position of the spirometer (expiratory position) an electrical circuit is closed, resulting in the addition of oxygen through a valve operated by an electro-magnet.  The loss in volume of the O2 gasometer (the second spirometer bell) is recorded providing a measure of the O2 consumption.  The ventilation tracing is horizontal.”