AND ON THE ELECTRIC FIELD OF THUNDERSTORMS. 
77 
The varying position upon the slit of the image of the meniscus on which the 
microscope is focussed is represented by the curve separating the dark and light 
regions of the record. The fine horizontal lines are due to dust particles or to 
irregularities of the slit; they. are useful as reference lines from which the 
displacement of the meniscus may be measured. The vertical flutings which appear 
in some of the records are probably due to flickering of the lamp. 
Records of the electrical effects of thunderstorms at various distances from the 
place of observation were obtained on ten different days in 1917. The records were 
not by any means continuous throughout the whole duration of a storm : compara¬ 
tively quick runs of the recording apparatus were generally made—varying from 3 to 
50 minutes in duration—and some time had to be spent in changing the photo¬ 
graphic plates and readjusting the apparatus between the successive runs. Again, 
the difficulty of estimating the order of magnitude of the electric effects to be 
expected frequently led to the sphere being exposed when the test-plate would have 
been more suitable, or vice versa ; the readings—which are 40 times larger with the 
sphere than with the test-plate—being in consequence too large or too small to be 
recorded. Thus the records obtained served rather to sample a thunderstorm at 
different stages of its history than to give a complete account of the changes in its 
electric field. 
II. Some Typical Records. 
Enlargements of some of the records are reproduced in Plates 2 to 5. In the 
original negatives a change of one mm. in the ordinates represented a flow of 24 
electrostatic units, or 8'OxlO -9 coulomb through the electrometer: a change of 
potential gradient of 100 or 4000 volts per metre, according as the sphere or the test- 
plate was used, was required to cause the passage of this quantity of electricity 
between the exposed conductor and the earth. 
A fairly typical fine weather record (May 23, 1917, 14h. 17m. to 14h. 51m. 
G.M.T.) is shown in fig. 1, Plate 2 : the sphere was used as the exposed conductor. 
The record begins with a horizontal portion traced before the conductor was exposed 
to the electric field. The small peak near the beginning of the record shows the 
effect of raising the sphere to its maximum height (480 cm.), and immediately 
lowering it again into its protecting case ; it indicates the existence of a positive 
potential gradient of 100 volts per metre. The sphere was raised at 14h. 20m., the 
exposure to the electric field being continued till 14h. 50m. except for regular 
interruptions at 5-minute intervals when the sphere was momentarily lowered into its 
case. The depths of the notches in the curve are measures of the potential gradient 
at the times of lowering the sphere : the potential gradients recorded at intervals of 
5 minutes are, in volts per metre, 120 (at- 14h. 30m. when the sphere was raised), 
110, 120, 90, 75, 80, and finally 90 at 17h. 50m. (when the sphere was lowered). 
The difference in the ordinates of the final and initial horizontal portions of the 
trace (both recorded while the sphere was in its case) is a measure of the integrated 
