In the ballistic use of an electrometer, for ionization measurements, the chamber is exposed to rays for a predetermined time, and the reading ordinarily taken is that of the electrometer after it stops swinging. The present method, more strictly ballistic, is to read the farthest point of swing. This not only saves time, but avoids serious complications from insulation leakage. The validity of this method depends on four conditions: (1) linearity of the differential equation of motion; (2) its homogeneity, except for the term proportional to the ionization; (3) initial conditions, deflection and velocity zero; (4) constancy of exposure time. (4) is readily satisfied; (3) and (2) demand cancellation of natural ionization by a constant opposite current; (1) is the most questionable. The present tests, on an electrometer of 12 seconds period with exposures from 2 to 40 seconds, demonstrate the possibility of satisfying all these conditions, so that the strictly ballistic method has no systematic errors large enough to detect in the presence of a 1% erratic error. For large deflections, ballistic applications of a potentiometer show departures of about 1% from linearity, but even then, calibration by the potentiometer enables this electrometer to compare ionization currents accurately.
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