Early career In 1924, Penning was hired as an experimental physicist at the Dutch section of the Philips research department located in the Strijp district of Eindhoven. Gilles Holst, former assistant to Heike Kamerlingh Onnes, and head of the
Philips Natuurkundig Laboratorium (NatLab), tasked Penning with continuing research on gas discharge phenomena to develop new lamps. Penning investigated breakdown energy potentials of low pressure inert gas discharges. In 1926, Penning observed that electrons under direct voltage current in low-pressure mercury discharges reach high velocities due to high-frequency oscillations in the gas. This observations challenged Irving Langmuir's observations. Penning also investigated the impact gas density and electrode distance have on the voltage and stability of noble gas mixture discharges by measuring Paschen curves. The Penning effect, as it is often called, describes an ionization chain reaction caused by high energy collision between excited inert gas (i.g. argon) metastable atoms and trace gas impurities whose ionization potential is less than the stored potential of the meta-stable molecule. These collisions cause the release of electrons; which can interact with other stable noble gas molecules to create more meta-stables, resulting in more ionizing reactions. Penning first reported the effect in 1927. The
Penning effect is used in gas-discharge
neon lamps and
fluorescent lamps, where the lamp is filled with a
Penning mixture to improve the electrical characteristics.
Penning gauge Penning invented a type of
cold cathode vacuum gauge known as the
Penning gauge, which Philips commercialized. Penning investigated the effect of magnetic fields on low pressure gas discharges. He determined that the energy current through a discharge tube in a magnetic field could be used to reliably measure the pressure. In his first attempt to measure the energy current, Penning used a triode ion gauge system with a linear electron flight path. This method worked, but was fragile and ineffective at very low pressures. In his second attempt, he equipped his vacuum tube with a ring shaped cathode and two anode plates above and below in a magnetic field. Electrons now traveled a longer flight path out of the ring, between electrodes in a spiral orbit. Increasing the electron flight path allowed for more collisions with meta-stable atoms. This amplified the electron signal, which was sufficient to measure gas pressure. The Penning manometer (vacuum gauge) emerged as a practical application. Penning traps are currently used for magnetic measurements and are an active research topic. The introduction of this trap has led to new approaches in high resolution mass spectrometry with mass spectrometers such as the ion cyclotron, resonance spectrometer, or ion trap.
Late career For a short period during World War Two Penning worked at the Philips Tube Factory developing high frequency electron tubes. Penning, with M. J. Druyveseyn, wrote a review of his work on gas discharges that was published in 1940 in the American journal
Reviews of Modern Physics. Due to the German occupation of the Netherlands, Penning did not see the published text until 1946. After the war he focused on the phenomena of cathode fall during glow-discharge reactions. Penning created a cathode tube with a more stable voltage by removing the oxide layer from the cathode; this allowed for consistent measurement of cathode fall. In the 1950s Penning’s health began to deteriorate. He died on December 6, 1953, at Utrecht. ==Selected patents==