Atmospheric-Electric Observations, 1915-16 



391 



a particles is cut short by their traversing, for example, a short cord of the cylinder, the 

 average reduction of range brought about in this way is a definite and calculable function of 

 the radius of the cylinder and of the ti-ue range of the a particles. It is independent of the 

 distribution of the active material on the foil, a point of some importance, since the distribu- 

 tion of active deposit on the foil is by no means uniform. 



The central system of the ionization chamber is attached to a single-fiber electroscope 

 adjusted to a sensitivity of 5-10 divisions per volt, and the potential is applied to the outer 

 vessel, the whole being mounted on a gimbal. The method of allowing for leakage is 

 exactly analogous to that adopted in the case of the conductivity apparatus, except that it is 

 not readily possible to make a leakage test at the end of the experiment, since the whole of 



Fio. 26. — Diagram of Water-Dropper. 



the internal surface of the ionization chamber is then likely to be covered with the dis- 

 integration products of the material originally collected. 



A picture of the ionization-chamber system is shown to the right of Plate 22, Figure 2, 

 and in Plate 22, Figure 3 , will be seen the collecting apparatus with the outer cyUnder removed 

 so as to show the central system. The collecting apparatus is mounted by itself aft of the 

 galvanometer house, and the fan for drawing the air is worked by a motor. For driving 

 this motor and the one in the conductivity apparatus, a 30- volt battery of 300 ampere-hour 

 Edison primary cells has been used, as it has not been practicable so far to install an accu- 



