. Carnegie Institution of Washington publication. 236 FLUORESCENCE OF THE URANYL SALTS. per second. By means of a small set-screw the disk could be clamped at various positions on the shaft, corresponding to various times in the decay process. Without the star-wheel SS the discharge of the condenser KK produced an exciting spark at E, which, with the aid of a revolving mirror, was found to consist of a pilot-spark, followed by five or six smaller sparks; hence the zinc star-wheel SS was mounted on the shaft to reduce the discharge to one spark. By experimenting with small and large capacities
. Carnegie Institution of Washington publication. 236 FLUORESCENCE OF THE URANYL SALTS. per second. By means of a small set-screw the disk could be clamped at various positions on the shaft, corresponding to various times in the decay process. Without the star-wheel SS the discharge of the condenser KK produced an exciting spark at E, which, with the aid of a revolving mirror, was found to consist of a pilot-spark, followed by five or six smaller sparks; hence the zinc star-wheel SS was mounted on the shaft to reduce the discharge to one spark. By experimenting with small and large capacities it was found that resonance must be recognized, and the proper amount of capacity to produce a regular, strong spark was finally discovered. The measurements of time were read with the aid of the light yielded by the exciting spark by noting the position of the edge of the sectors on a protractor mounted rigidly in front of the machine. The range of times accurately measureable include those from to second by steps. The photometer, spectro-photometer, camera, and spectrograph have been successfully used with this FIG. 6. In their preliminary study of the cathodo-phosphorescence of the rare earths, Nichols, Wick, and Wilber1 employed a phosphoroscope with a disk mounted on the shaft of a motor. The primary of an induction coil was interrupted by a plunger attached to a crank on the shaft of the motor. The plunger, once in a revolution, dipped into the mercury cistern, while the opaque portion of the disk hid the specimen from view, and as a result the specimen was excited by the discharge of the secondary coil through a cathode- ray tube, much after the manner of Crookes's device. The change in time between excitation and observation was accomplished by changing the speed of the motor, and an ammeter in the field circuit was calibrated to measure the angular velocity. It is evident that with only one excitation per revolution only one ope
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