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292 H. E. EDGERTON AND K. J. GERMESHAUSEN [J. S. M. p. E. at the junction of the mercury and the glass. Such a high potential applied suddenly at that point starts a cathode spot in the liquid mercury where it touches the glass, which supplies the electron emis- sion for the large peak currents in the mercury-arc stroboscope tube. Immediately after the condenser C discharges through the trans- former and thyratron, a negative bias appears upon the grid of the thyratron, which thus regains control. Between flashes the con- densers throughout the circuit accumulate charges, so that they are prepared for the next flash. Uses of High-Speed Motion Pictures, and Examples. — The most obvious use of the high-speed camera is for taking motion pictures of fast or complicated motions in order that they may be slowed down, when projected on the screen, to such a speed that the eye is able to see and the mind to comprehend. Subsequent showings often bring out obscure but important details that were not noticed during previous projections. The motion picture can be kept for reference to refresh the memory or as a record of the motion at the particular time of the exposure. A second use of the high-speed motion picture camera, perhaps more important than the first for engineering purposes, is its ability to permit making measurements of position as a function of time. The individual frames of the moving-picture film record the instan- taneous position and form of the object being photographed with time interval between frames depending upon the rate at which the camera is run. The stroboscopic-light camera, when used for .such work, may be arranged so that the time between flashes is accurately determined by a constant-frequency source of power instead of by the synchronizing commutator on the camera. The advantage of using an accurately timed flashing light lies in the fact that the speed of the film does not enter into the measurements. Needless to say, the pictures can not be projected, as they are not placed in the proper relationship to the sprocket holes. However, should it be desired to project them, they may be thrown upon a screen with respect to a stationary reference, and recopied frame by frame. The velocity of a moving object may be determined from a motion picture film by measuring the difference in the position of the image between two successive frames and dividing by the time interval be- tween the pictures. The accuracy of the results is influenced both by the accuracy of measurement of the displacement and the accuracy of determination of the time interval. Usually the former involves