1012 
For the registration of the movement I had at my disposal a Morse 
registering instrument, the paper ribbon of which was moved by a 
three-phase motor of + H.P. A Morsr-key served as signal instrument. 
This arrangement appeared very accurate when tested by means of 
a chronometer. *) 
The mercury particles were obtained by ExRENHAFT’s method. For 
the other substances | made use of an oil spray which is sold for 
medical purposes. 
4. After the condenser had been carefully adjusted horizontal, 
I proceeded to the measurement of ty, ts, -t'y, and. t, ofa definite 
particle. This was brought above in the field of vision, and at the 
moment that it, falling, crossed one of the 2 horizontal lines, which 
served as marks, the Morse-key was pressed down. Then the field 
was excited, and the time of rising ¢; was measured in the same 
way. This happened several times in succession, the time of falling 
t', also being noted down, when the electric field was reversed. Then 
gravity was cancelled by an electric force, and the indicator was 
pressed down when the particle in a horizontal direction passed a 
following vertical dividing line on the left or on the right of the 
preceding one. The sense of the displacement was indicated by 
different signs. 
When for a particle the observation was over, the distances 
between the dots on the paper were measured. These distances, expres- 
sed in cm., which are proportional to the times of falling and 
rising, are indicated by vt. The factor of reduction of t to ¢ was 
determined repeatedly with an accurate chronometer. 
Equation (6) only holds for spherical particles. 1 used the following 
criterion to test the spherical shape. When the particle has different 
dimensions in different directions, it will be orientated under 
influence of an electric force, and experience another resistance than 
in falling. Now follows from equations (4a, 6, c): 
2vy 
! 
Vy Vs 
Et hao, Ee one mtn 
1 
If, however, the factor of resistance in the falling is a in the 
? 
movement under an electric force = , equation (13) holds no longer, 
but instead : 
1) This had already appeared before in experiments by Prof. Zeeman in an 
optical determination of the current velocity in a cylindrical tube. These Proc. 18, 
1916, p., 1240. 
