2 BULLETIN 1379, U. S. DEPARTMENT OF AGRICULTURE 



On fine days, the potential gradient in the atmosphere is almost 

 invariably positive in sign (that is, a positive charge tends to move 

 downward), and the magnitude of the vertical gradient is of the order 

 of 100 volts per met er, t 'hough it is continually varying. When thun- 

 derstorms are in the neighborhood, the potential gradient may be 

 either positive or negative and changes sign frequently. The magni- 

 tude of the potential gradient also undergoes wide fluctuations, 

 during stormy weather frequently attaining values of 10,000 volts 

 per meter, 100 times the normal gradient. 



A further examination of the lower atmosphere shows that charged 

 particles or ions are always present. Both positive and negative 

 ions are found, the positive ions generally being somewhat more 

 numerous. They consist of groups of molecules loosely bound 

 together and carrying a charge. Frequently these small ions attach 

 themselves to dust particles, thus becoming large ions, which move 

 much less rapidly than the small ions. 



When the potential gradient is positive, the negative ions move 

 upward and the positive ions downward to the ground, thus con- 

 stituting an electric current flowing from air to earth. This current 

 is due almost entirely to the small or free ions, the mobility of the 

 large ions being so low that their influence on the conductivity of the 

 air can be disregarded. The magnitude of the current from the air 

 to a unit area on the earth's surface is extremely small, being only 

 2X10 -12 amperes per square meter or 5X10 -8 amperes per acre. 

 The strength of the current is proportional to the potential gradient, 

 to the number of ions per unit volume, and to their mobility. The 

 average number of free ions is of the order of 1,000 per cubic centi- 

 meter, the positive ions constituting somewhat more than one-half 

 the total number. Their mobility is such that they migrate with a 

 velocity of about 1 centimeter per second when subjected to a poten- 

 tial gradient of 100 volts per meter. 



Although t he air-earth current per unit area is extremely small, 

 it is sufficient when applied to the whole of the earth's surface to 

 reduce the negative charge of the earth to one-half its initial value in 

 about 10 minutes. The explanation of tiie maintenance of the 

 negative charge of the earth under such extraordinary conditions is 

 one of the outstanding problems in atmospheric electricity {12, 47). 2 



ELECTRICAL FIELD EMPLOYED IN ELECTROCULTURAL EXPERI- 

 MENTS 



In most of the field experiments conducted at the Arlington 

 Experiment Farm, the standard height of the network was 5 meters, 

 and the potential of the network was approximately 50,000 volts. 

 The average potential gradient under the network was therefore of 

 the order of 10,000 volts per meter, or about 100 times the normal 

 gradient in fine weather. This would produce an air-earth current 

 about 100 times the normal current as long as the ion content of the 

 air remained normal. However, a marked ionization occurred at the 

 network, so that the number of positive ions per unit volume under 

 the network was much higher than normal. This was shown by 

 means of measurements made when the network was charged and a 

 gentle breeze blowing. On the windward side of the network the 

 conditions were normal, but on the leeward side a decided increase 



2 The serial numbers (italic) in parentheses refer to "Literature cited," at the end of this bulletin. 





