Meitner had a Wilson cloud chamber constructed at the KWI for Chemistry, the first one in Berlin, and with her student Kurt Freitag studied the tracks of alpha particles that did not collide with a nucleus. With her assistant Kurt Philipp she later used it to take the first images of positron traces from gamma radiation. She proved Chadwick's assertion that the discrete spectral lines were entirely the result of secondary electrons, and the continuous spectra were therefore indeed entirely caused by the primary ones. In 1927, Charles Drummond Ellis and William Alfred Wooster measured the energy of the continuous spectrum produced by the beta decay of bismuth-210 at 0.34 MeV where the energy of each disintegration was 0.35 MeV. Thus, the spectrum accounted for nearly, but not all, of the energy. Meitner found this result so troubling that she repeated the experiment with Wilhelm Orthmann using an improved method, and verified Ellis and Wooster's results.