The first iron lung used in the treatment of polio victims was invented by Philip Drinker, Louis Agassiz Shaw, and James Wilson at Harvard, and tested October 12, 1928, at Children's Hospital, Boston. The original Drinker iron lung was powered by an electric motor attached to two vacuum cleaners, and worked by changing the pressure inside the machine. When the pressure is lowered, the chest cavity expands, trying to fill this partial vacuum. When the pressure is raised the chest cavity contracts. This expansion and contraction mimics the physiology of normal breathing. The design of the iron lung was subsequently improved by using a bellows attached directly to the machine, and John Haven Emerson modified the design to make production less expensive. The Emerson Iron Lung was produced until 1970. Other respiratory aids were used such as the Bragg-Paul Pulsator, and the "rocking bed" for patients with less critical breathing difficulties.