204 Dike — Recent Observations in Atmospheric Electricity. 



Sobering at Gottingen lias worked on the problem of a self- 

 recording apparatus for the measurement of specific conduc- 

 tivity, avoiding saturation currents by removing the charged 

 body from the neighborhood of any earthed conductor. A 

 suspended sphere of 5 cm radius connected by a fine wire 50 cm 

 long to an electroscope was found to answer this requirement 

 and to give results following Ohm's law and independent of 

 air currents. This form of apparatus is not convenient for 

 field work but may be made useful for observatory work by 

 using a long tbin wire in place of the sphere. This must be 

 carefully screened from the earth's field by surrounding it with 

 a cylinder of wire netting. A sort of arbor of wire netting 

 20 cm long was constructed at the Gottingen Geophysical Insti- 

 tute in the open and a wire 0'14 mm in diameter was stretched 

 inside this so as to be at least # 5 m from the netting, and sup- 

 ported by specially constructed insulators to make possible its 

 continuous use out of doors. One end was connected with an 

 electrometer indoors. Comparisons between the sphere disper- 

 sion apparatus inside the screen and Gerdien's conducting 

 apparatus outside gave the same difference as with both instru- 

 ments outside. The capacity of the wire was found to be 104 cm 

 and with the normal conductivity of the air with a potential 

 of 100 volts on the wire the current should be 1 X 10 " amperes. 

 This being too small to measure with a galvanometer, a 

 uranium cell was used as a high resistance to adapt the arrange- 

 ment to continuous registration by electrometer methods. The 

 uranium cell gave a saturation current, hence a constant cur- 

 rent, from 50 to 300 volts and was arranged so as to be adjust- 

 able to give different currents by screening some of the uranium 

 deposit. If the inner electrode of the uranium cell is connected 

 with the dispersion wire and the outer electrode with a poten- 

 tial of some hundred volts, the inner electrode and wire will be 

 charged to the potential at which the current from the wire into 

 the air is equal to the current in the uranium cell. If V is 

 this potential and Z the capacity of the effective portion of 

 the wire and the current in the uranium cell is i, the specific 



i 

 unipolar conductivity of the air is \= . 



The value of i being constant, it is only necessary to record 

 "Fto follow the course of the conductivity. This was done 

 photographically by means of the electrometer, the wire being 

 connected with the needle and the quadrants charged by means 

 of a storage battery of 20 elements. In the registration- 

 various difficulties were encountered, mainly in failure of insula- 

 tion of tbe wire through moisture or spider webs. 



The apparatus charges slowly through the uranium cell after 

 being discharged, so that it is only suitable for slow registra- 



