254 
PHYSICS: 0. BLACKWOOD 
Proc. N. a. S. 
5 cm. internal diameter, which is fitted with two co-axial electrodes 2 cm. 
in diameter and, respectively, 105 and 35 cm. long. (See fig. I.) The 
electrode V is grounded, while is connected to a pair of quadrants of 
an electrometer. Any desired potential may be established between the 
cylinder and the electrodes by means of a battery of dry cells. 
EAFLTH 
m 
STREAM 
Eanih 
Flq.I 
The ions to be tested are carried by a blast of air through the cylinder 
in a direction parallel to its axis. If there is no electric field, none of the 
ions are driven to the electrodes and the electrometer current is zero. 
When, however, a field is established and is increased progressively, 
more and more ions are driven from the air stream; the current increases 
and achieves its maximum value at a critical voltage V at which all the 
ions are caught. As the field is still further augmented, the long, grounded 
electrode catches more ions, fewer reach the second electrode, and finally 
— at a voltage V" — the current again becomes zero. The voltages V 
and V'\ corresponding to the maximum and zero currents, respectively, 
may be computed for ions of mobility k by means of the formula : 
2irUV' 
log, h/a = 
27r(L' + L')V 
, log. b/a 
a and h being the radii of the outer and inner cylinders, Q the volume of 
air traversing the tube each second, L' and L" the lengths of the long and 
short electrodes. 
Figure I shows the forms and relative positions of the three theoretical 
current-voltage curves for ions of respective mobilities, 0.0043, 0.0010, 
and 0.0004 cm. per second in a field of one volt per cm. These are the 
values found for successive groups of ions by the observers mentioned 
above. ^ The dotted line indicates the summation curve produced when 
the three classes of ions are simultaneously present in equal concentra- 
tion. It will be noted that this curve has three well-defined peaks j each 
corresponding to a certain class of ions. 
