The electro-optical shutter is used to study spark break-down in nitrogen, hydrogen, oxygen, carbon dioxide, helium and argon at atmospheric pressure and for gap lengths up to about 1 cm. Four distinct types of break-down, as characterized by the appearance and growth of the luminous streamers, are recognizable in these gases. The velocities of the streamers emanating from the cathode in the cases of hydrogen and nitrogen have been measured and have been found to be about ${10}^7$ cm per sec. which is of the order of magnitude of the calculated velocities of electrons in these gases under breakdown conditions. Pencil drawings representing the course of the breakdown in hydrogen, nitrogen, helium and argon are reproduced. Photographs of single sparks in early stages are shown for nitrogen, oxygen, air and carbon dioxide, the exposure time for some of these being as short as 5×${10}^{-9\mathrm{}}$ sec. The experiments indicate that the major portion of the current passing during breakdown is carried by electrons, and that these, for the most part, come from the cathode rather than from ionization in the gas. The shortness of the spark lag, and the breakdown times of the sparks, show that the phenomenon is essentially due to the motion of electrons, and that the space charges which cause breakdown result from the large difference in the mobility velocities of electrons and positive ions. A theory is formulated to explain the appearance and growth of the streamers observed in certain gases, such as nitrogen.

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