abstracts: atmospheric electricity 13 



based on the initial rate of production of ions in the ionization chamber, 

 corresponding to unit charge on the collecting wire when exposed. The 

 employment of an Einthoven electrometer in a manner analogous to 

 that adopted in the case of the ion-counter described above renders 

 it possible greatly to increase the convenience, rapidity, and accuracy 

 of the measurements. 



The third portion of the paper describes a piece of apparatus suit- 

 able for obtaining relative values of the potential gradient. Its action 

 is based on recording the change of potential which a metal disc under- 

 goes when, after being earthed and insulated, it is caused to alter its 

 position in the Earth's field. The chief advantage of the method is 

 that high insulation is not necessary, since it is only while the disc is 

 changing its position that leakage is operative. In view of the slow 

 action of the usual forms of collectors, leakage usually becomes a serious 

 consideration in determining the final potential obtained, especially 

 at sea. Another advantage of the present method is that it enables 

 the potential-gradient to be obtained at any instant. 



W. F. G. S. 



ATMOSPHERIC ELECTRICITY.— Ow certain matters relating to the 

 theory of atmospheric electric measureynents. W. F. G. Swann. 

 Terr. Mag., 19: 205-218. 1914. 

 The earthed portions of pieces of apparatus exposed to the atmos- 

 pheric potential gradient have to take a negative charge in order that 

 their potential may remain zero. The effect of the charge in modify- 

 ing the measurements is discussed for various cases. In the Ebert 

 ion-counter the effect is to decrease the measured ionic density in the 

 case of the negative ions, while in the case of the positive ions the 

 results are unaffected, though the paths of the ions are of course influ- 

 enced by the charge. It is shown that the effect is expressible very 

 simply in terms of the charge induced by the potential gradient on the 

 inside of the opening where the air enters. By measuring this charge 

 it is possible (laiowing the rate of the air flow) to estimate the order of 

 magnitude of the effect. The effect is greatest of course when the 

 apparatus is mounted on a tall slender support. It was found by 

 measurement that it might easily amount to 20 per cent of the value 

 of the quantity measured. The analogous effect is discussed for the 

 case of the Gerdien conductivity apparatus, and it is shown that even 

 in the case of the negative ions no error is introduced, provided that 

 the potential of the central conductor is not too high. In fact the 



