DEPARTMENT OF TERRESTRIAL MAGNETISM. 279 



gradients or strong winds the former region would be evanescent, or prac- 

 tically so. In any case, however, for a region outside the latter region, n- 

 should be normal. Theory also shows that n+ has its normal values at all 

 points of the space around the tower. 



A. Gockel has made two sets of measurements, one on the top of a tower 

 13 meters high, and the other on a platform shielded from the potential 

 gradient, and situated 2 meters below the top of the tower. For the measure- 

 ments in the region exposed to the field he finds 7i+/n- = 2.03, while for 

 the shielded region n+//?_ = 0.92. He considers these measurements incon- 

 sistent with the above-cited views as to the action of the tower. It must be 

 remarked, however, that we must here carefully distinguish between two dis- 

 tinct phenomena. As already stated, theory shows that /t+ and ?i_ have 

 their proper values in the space around the tower for points outside the critical 

 region; but when we introduce an instrument to measure the ionic densities, 

 the error caused by the induced charge on the instrument produces its effect 

 just as it does when measurements are made over the surface of the ground. 

 It would thus appear that the difference between the values of n+/n_ for 

 the shielded and the unshielded positions is attributable to the induced charge 

 on the instrument in the latter case, and there is no evidence to show that it 

 is due to the tower directly. In order to reduce the instrumental error, it is 

 necessary to shield the instrument with wire netting, as discussed on pages 

 210-212, and on page 215 of the paper already cited. ^ 



An apparatus for automatically recording the electrical conductivity of air. W. F. G. 

 Swann. 



The apparatus is a modification of that of Gerdien, in which the conduc- 

 tivity is measured by passing the air through the space between two concentric 

 cylinders, and noting the rate of fall of potential of the central member, which 

 is initially charged to a potential of the order of magnitude of 100 volts. 



In the present apparatus, the central cylinder is connected to one quadrant 

 of a quadrant electrometer, the other quadrant being connected to the case of 

 the electrometer which is insulated and maintained at a constant potential 

 of say 100 volts. The two quadrants are permanently connected through a 

 high-resistance radioactive cell, of the type developed by S. J. Mauchly and the 

 author (Terr. Mag., vol. 22, pp. 1-21, 1917). Under these conditions, when 

 ail' is passing through the instrument, the electrometer experiences a steady 

 deflection, determined by the conductivity, the potential applied to the elec- 

 trometer, and the resistance of the radioactive cell. The deflections are photo- 

 graphically recorded upon a rotating drum. 



The usual form of Gerdien apparatus is subject to error on account of the 

 collection of radioactive material from the atmosphere by the two concentric 

 cylinders. In the present apparatus this error is avoided in the following 

 manner : Before passing between the above two concentric cylinders, the air is 

 caused to pass through the space between a set of concentric cylinders, which 

 are divided up and connected into two groups in such a way as to cause the 

 complete set to form one cylindrical condenser of capacity much greater than 

 that of the two principal concentric cylinders. Once every hour a potential 

 difference of 250 volts is applied for 3 minutes between the two units of this 

 subsidiary set of cylinders, and thus, during these intervals, the ordinary 

 small ions, which are usually the main agents in producing conductivity, are 

 cleared out of the air before it passes through the main pair of concentric 

 cylinders. On the other hand, the ions generated between the main cylinders 



iTerr. Mag., vol. 19, pp. 205-218, 1914. 



