Nolan — Mobilities of Ions Produced hy ISpraijing. 1 1 



The theory of the coaxial cylinder method of measviring mobilities of ions 

 is well known. Ionised air is drawn through at a steady rate, and a ciirrent- 

 voltage curve is plotted. Saturation for an ion of mobility lo is produced at a 

 voltage P^when 



2nLu 



a and h being the radii of the inner and outer cylinders, Q the quantity of 

 air drawn through .per second, and L the length of the inner cylinder. " If, 

 therefore, ions of only one mobility are present, the current-voltage curve 

 will be a straight line bending over and becoming parallel to the voltage axis 

 at the saturation voltage. If a number of different classes of ions are present, 

 each class having a definite mobility, the current-voltage curve is made up of 

 a number of straight lines, each intersection representing a voltage at 

 which one class of ion is saturated, and each succeeding section of the curve 

 making a smaller angle with the voltage axis until saturation is finally 

 reached. If the number of different classes of ions is great, the intersecting 

 straight lines are short, and the result is, in practice, a smooth curve. But if 

 the interval between the mobility of one class and that of the next is not too 

 small, and if the quantities concerned are big, it is possible to use the current- 

 voltage curve as a method of measuring the mobilities of all the ions 

 concerned. 



This is the method of determining mobility that has been used throughout 

 the woi'k described in this paper. The ordinary procedure, therefore, was to 

 find the rate of charging of the inner terminal for various voltages, the air 

 being drawn steadily at a suitable rate and the pressure on the sprayer being 

 also kept steady. In the course of the work the widest possible range of air 

 velocities was employed, and measuring- cylinders of various dimensions were 

 used. 



It is clear that the success of this method depends upon the accuracy 

 with which the current-voltage curve can be drawn. As far as the electro- 

 meter readings are concerned a high degree of accuracy can be reached, 

 especially when, as in most of this work, the electrical quantities to be dealt 

 with are quite considerable. Variations of the spraying apparatus constitute 

 a possible source of error ; but it was found that, except on very rare occasions, 

 the sprayer was remarkably steady. But there is still the disadvantage 

 that unless the quantity of ions of any one class is big, the bend of the 

 curve due to the saturation of that class cannot be accurately defined, and 

 consequently the value deduced from the mobility is subject to considerable 

 error. This method, however, has the advantage that it can be used, without 



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