Page:Radio-activity.djvu/276



149. Emanating power. The compounds of thorium in the solid state vary very widely in the amount of emanation they emit under ordinary conditions. It is convenient to use the term emanating power to express the amount of emanation given off per second by one gram of the compound. Since, however, we have no means of determining absolutely the amount of emanation present, all measurements of emanating power are of necessity comparative. In most cases, it is convenient to take a given weight of a thorium compound, kept under conditions as nearly as possible constant, and to compare the amount of emanation of the compound to be examined with this standard.

In this way comparisons of the emanating power of thorium compounds have been made by Rutherford and Soddy, using an apparatus similar to that shown in Fig. 51 on page 240.

A known weight of the substance to be tested was spread on a shallow dish, placed in the glass tube C. A stream of dry dust-free air, kept constant during all the experiments, was passed over the compound and carried the emanation into the testing vessel. After ten minutes interval, the current due to the emanation in the testing vessel reached a constant value. The compound was then removed, and the standard comparison sample of equal weight substituted; the saturation current was observed when a steady state was again reached. The ratio of these two currents gives the ratio of the emanating power of the two samples.

It was found experimentally that, for the velocities of air current employed, the saturation current in the testing vessel was directly proportional to the weight of thorium, for weights up to 20 grams. This is explained by the supposition that the emanation is removed by the current of air from the mass of the compound, as fast as it is formed.

Let i_{1} = saturation current due to a weight ω_{1} of the standard, i_{2} =     "        "       "        "   ω_{2} of the sample to be tested.

Then      (emanating power of specimen)/(emanating power of standard) = (i_{2}/i_{1})(ω_{1}/ω_{2}).