Page:Radio-activity.djvu/187

 between A and B varies as e^{-[Greek: lambda]x}, i.e. it decreases according to an exponential law with the distance traversed.

Fig. 37.

The variation of the current between A and B with the distance from a thin layer of uranium oxide is shown in Fig. 37 for different gases. The initial measurements were taken at a distance of about 3·5 mms. from the active surface. The actual values of this initial current were different for the different gases, but, for the purposes of comparison, the value is in each case taken as unity.

It will be seen that the current falls off with the distance approximately in a geometrical progression, a result which is in agreement with the simple theory given above. The distance through which the rays pass before they are absorbed is given below for different gases.

Distance in mms. to    Gas         absorb half of radiation Carbonic acid              3 Air                        4·3 Coal-gas                   7·5 Hydrogen                  16

The results for hydrogen are only approximate, as the absorption is small over the distance examined.