Some years ago, during my stay in Rio de Janeiro, I busied myself during spare time in determining the value of the normal pressure (by means of a plane mounted on a rotary arm) in such a way that no disturbing induced air current could arise.

For this purpose I had at my disposal a room 12 metres square by 8 metres high. I employed an apparatus by which a rotating arm 9 metres long described a quadrant or quarter circle. The test surface was 0.25 square metres in size. Since the surface traversed the quarter circle only once it encountered only still air. Uniform motion was controlled by the curve described by a vibrating spring on the end of which was fastened a small paint brush with colour. The spring was fastened one meter from the turning centre and vibrated over a paper spread on a measuring table. (Figures 1 and 2, not reproduced.)

The results were however not reliable enough on account of the shortness of the observations and the small size of the room. After my return to Germany, in the spring of 1913, I therefore undertook new experiments on a larger scale. Through the kindness of the director of the South–West Electricity Works, there was placed at my disposal a large empty machinery hall in Schöneberg of 32 metres clear width, 60 metres long and 15 metres high.