36 ELECTRICITY IN AGRICULTURE 
in movement afresh there will occur a change of poles. 
This change easily escapes the attention of the observer. 
Such a change of pole might, therefore, have occurred 
during the experiment by which the wire net had been 
charged alternately positively and negatively. 
For this reason it was very important to make a series 
of experiments in which strict control over the direction 
of the current was exercised. The experiments were 
executed in the same way as the experiments mentioned 
on pages 9 and 10, with, however, a few modifications, 
Sixteen burnt clay pots of middle size (19 cm. in 
diameter at the upper side, 14cm. in diameter at the 
bottom, and 19cm. in height) were placed before the 
windows, four in number, facing south. The pots were 
placed four in a line before the window. The earth 
in these pots had been well mixed in a box and distri- 
buted alike in all the pots. In each of 12 pots out 
of the 16 was sown, on May 18th, I901, 12 grains of 
wheat, the same of oats and barley—two grains in the 
middle, and in five symmetrically situated places also 
two grains. In the four remaining pots were set strawberry 
plants in the middle, and around them in three places 
were sown carrots. The pots were placed in four sections, 
one before each window, so as to stand in a row—the pot 
with the strawberries and carrots inside, then the pot with 
wheat, and last that with oats and barley. Every section 
was separated from the other by screens of white paste- 
board, and care was taken that the light was the same for all. 
Above the three first sections (I., II., III.) was suspended 
by a thread of silk a wire net furnished with points 25 cm. 
broad and 120cm. long, at a height of 40cm. above the 
earth in the pots. Through the holes in the bottom of the’ 
pots were introduced equal-sized strips of zinc which were 
in conducting communication with each other and with 
the earth in every section. The pots stood on plates of 
burnt clay, and these on plates of thick glass supported 
