Dike — Recent Observations in Atmospheric Electricity. 205 



tion and not for recording rapid changes in the conductivity. 

 For the latter purpose the same wire was used later with an 

 electrometer with a sensitiveness of 0*82 cm per volt and the 

 zero so chosen that with a charge of 88 volts the spot of light 

 fell on the paper. A clock was arranged so that at minute 

 intervals the wire was charged to 88 volts and then insulated. 

 It then discharged itself for a minute to a certain point, the 

 electrometer tracing an oblique line on the paper, the lower 

 end of which gave the fall in potential in one minute. The 

 trace of these points gives the conductivity curve. The curves 

 given show large and rapid variations of the conductivity, 

 apparently indicating that the air is not uniformly conducting 

 but varies rapidly from place to place so that the wind brings 

 air of different conductivities in contact with the wire. The 

 second method of registration is recommended for eclipse work 

 since it gives the mean values for shorter intervals than the 

 usual apparatus. 



Another subject of great interest to students of atmospheric 

 electricity is that of the radio-active emanation in the air, first 

 observed there by Elster and Geitel. Closely related to this is 

 the radio-activity of soils and rocks, sea water, etc., as the prob- 

 able source of the emanation. It cannot be said that it has 

 been proved conclusively what the source of this emanation is, 

 though it seems to be associated with land and to be almost 

 entirely absent over the sea, making it seem probable that its 

 source is in the soil and rocks of the land areas since sea water 

 is only j-^q-q as radio-active as the average sedimentary rock. 

 Several observers have been at work on the problem of the 

 determination of the quantity of the emanation per cubic meter 

 of the air, or what amounts to the same thing, the determina- 

 tion of the radium equivalent per cubic meter, that is the 

 amount of radium required to maintain in equilibrium the 

 quantity of emanation found in the air. The electrical method 

 by which the emanation was first collected is not adapted to 

 these measurements, since some of the carriers of the emanation 

 are very slow moving and are almost sure to get past the nega- 

 tively charged collector. Also some of the emanation appears 

 to be attached to negatively charged particles and would thus be 

 repelled from the collector, while there are also neutral carriers 

 resulting from the union of a positive and negative particle. 



However, there are at least two other methods of collecting 

 the emanation which seem to be quite efficient. One is by 

 means of cooling a current of air in a condenser surrounded 

 with liquid air, the emanation being condensed and remaining 

 behind. The other is by the use of pulverized cocoanut char- 

 coal, which, on allowing a current of air to pass through it, 

 absorbs the emanation contained in it and gives it up on being 



