Ernest Rutherford (1871-1937)  worked with radioactivity, which he found to be of different kinds, distinguishing between positive, or alpha rays, and negative, or beta rays. In this work he was assisted by the German physicist Hans Geiger , with whom he devised an instrument to detect and count these particles. The instrument consisted of a tube containing gas  with a wire at high voltage along the axis. Each particle entering the tube ionized some gas, causing an electron avalance and a measurable pulse, or click. He discovered that the alpha particles were helium nuclei. By 1909, Geiger and E Marsden used alpha particles to bombard very thin metal foils. Sometimes the alpha particles were scattered backward as if repelled by other positive charges; this led Rutherford to propose the theory of a nuclear atom, containing a very small positive nucleus  surrounded by negatively charged electrons and a great deal of empty space. In 1928 Geiger and W. Muller designed a improved counter to measure radioactivity, the Geiger- Muller counter.

                     

Danish physicist Niels Bohr (1885-1962) (Detail) won the Nobel prize in physics for his model of the hydrogen atom, in which  the electron occupied discrete energy levels, or orbits, around the nucleus, and radiated or absorbed energy only when moving between energy levels, whereas classical theory predicted that an orbiting electron (a moving charge) should radiates energy continuously. This model of circular orbits was able to account for the major series of the hydrogen spectrum, but it  was left to Sommerfeld to explain its fine structure by adding elliptic orbits to the model, resulting in the Bohr- Sommerfeld atom. Bohr is commemorated with circular electron orbits on the Swedish stamp but with an elliptic orbit on the Danish  stamp, which also indicates the quantized nature of the energy transition between orbits. The stamp showing Bohr and his  wife on a garden bench enjoys some minor fame because the bench only has three legs.