IOWA ACADEMY OP SCIENCE 
161 
lowed by three observations without the field and then by three with the field. 
The times between the different observations were as nearly equal as could 
be obtained by a single experimenter under the conditions of work. It is not 
likely that the small differences of the intervals affected the results to any ex- 
tent. By making the observations rapidly so that compressions in the fog 
chamber would quickly follow the expansions it was frequently possible to 
take as many as six or eight observations before any diminution of the super- 
saturation in the fog chamber became apparent. 
About one-half of the observations were taken by grouping alternate read- 
ings. After expansions to get rid of persistent nuclei a series of four to six 
alternate readings were taken in succession. The results of the two methods 
were practically the same as indicated by the data. 
On examining the data it will be found that the velocity of the cloud with 
the field off is practically the same in every case. It will also be noted that 
the difference between the velocities with the field off and field on varies di- 
rectly as the voltage. When the voltage is double the difference is approximate- 
ly double. We should expect this since the velocity due to gravity is always 
the same. 
During all observations the distance between the electrodes in the fog cham- 
ber was invariably 5 millimeters. The timing of the clouds, however, was 
taken through only a distance of 2 millimeters. This was necessary for several 
reasons. By timing through a short distance the error due to evaporation 
would be small. Again, it was found on expansions that the upper electrode, 
the terminal of which passed through a rubber stopper, would project the 
surface of the cloud, giving it an accelerated motion which it would maintain 
for a distance of approximately two millimeters. All observations were made 
with a micrometer microscope adjusted to a cathetometer support. Owing to 
the projection due to the electrodes it was found necessary to bring the first 
cross hair of the microscope near the middle of the space between the 
electrodes so as to get the time of the cloud when moving at a uniform speed. 
One of the greatest difficulties encountered in timing the clouds, especially 
when high voltages were used, was the breaking up of the sharply defined 
surface of the cloud by the action of field. Under the circumstances it was fre- 
quently impossible to time the surface cloud for the second cross hair of 
the microscope. When 3000 volts were used the surface of the cloud on ap- 
proaching the positively charged electrode would exhibit a phenomena some- 
what analogous to the scintillations that take place inside of the spinthari- 
scope. Particles would be projected upward in all directions, giving the cloud 
a hazy appearance. As a rule the main body of the cloud could be seen descend- 
ing toward the positively charged electrode, while the scintillations were taking 
place. The best observations for the velocity determinations of the clouds were 
obtained at voltages varying from 2000 to 2400 volts. The electrodes in every 
case were charged with a storage battery of 1600 cells, which enabled a varia- 
tion of the static field. All time observations were taken with a stop watch. 
The temperatures given with some of the determination have reference to the 
room in which the experiment was conducted. For two or three days in No- 
vember the heating was not good in the laboratory, the temperature maintained 
being aboutl8®C. At that temperature it was found impossible to get well de- 
fined clouds for observation. An electric heater was brought into service and 
