Study of time lag of the needle gap

The study of high-voltage, steep-wave-front transients is difficult from the experimental standpoint because of the very short times involved. Due to the improvement which the cathode-ray oscillograph has enjoyed in recent years, a device is now available, by the use of which transients occuring in times as short as one-millionth of a second or less may be photographed. In the paper, the authors used an oscillograph developed by Dufour in France, with which a brief study was made of the time lag of needle gaps and of a needle to a plane. A description of the oscillograph is given including a discussion of the method of operation. The photographic film is placed inside the tube so that the electrons impinge directly on the film. The wave is drawn out along a time axis by the combined action of a sweeping motion and a perpendicular oscillating motion imparted to the electron stream by the action of proper electromagnetic fields. Tests were made with a wave which was nearly perpendicular, reaching its maximum in about one microsecond. Such a wave was obtained by the discharge of a condenser through a suitable circuit. An oscillogram which shows the wave front used is given, and attention is directed to the 20,800 kilocycle oscillation which appears superimposed on the wave front. The results of tests in which this wave front was applied to gaps are given and it is shown that with any given gap setting and sparking voltage that the time lags vary through wide limits. It is also shown that, for the same voltage, increased gap settings mean increased lag. The per cent over voltage, required to keep the lag to two microseconds or less, decreases as the gap spacing increases.