PHYSICAL INVESTIGATIONS OFF THE NEW ENGLAND COAST. 409 
negatively by the use of a rubber electrophorus excited with guu cotton, so that a 
positive charge on the attracting plate made it necessary to raise it to bring the 
aluminium balance into the sighted position. A higher reading of the micrometer 
thus indicates a positive potential of the attracting plate. The constant of the instru- 
ment was determined iu the physical laboratory of Princeton College by the use of a 
storage battery of 48 cells, of which the electromotive force was accurately known. 
It was taken as 2.5 volts for each division of the micrometer head. 
The collector was constructed on the plan recommended by Exner. A short 
metal chimney was made, mounted on an ebonite rod. It held a caudle, in the flame 
of which stood the end of a short copper wire, joined to the attracting plate of the 
electrometer, when desired, by a flue silk-covered copper wire. When an observation 
was made, this collector was held at heights above the deck of two, three, and four 
metres, and the respective settings of the micrometer were taken. Ground readings 
were made at the beginning and end of the set. It was found to be indifferent whether 
the ground contact was made by the Anger of the observer or by a copper wire making 
connection with the water. The collector was held as far out over the rail as possible 
on the weather quarter of the vessel. 
In the following table are collected the observations thus made and an approxi- 
mate value of the rate of change of potential with the vertical height of the colic dor. 
dv 
This change is expressed in volts per metre and appears in the column headed y - . It 
(Xlv 
was always positive. 
Date. 
Time. 
Potentials at — 
2 metres. 
3 metres. 
4 metres. 
dv 
dn 
Aug. 4 
13 0 
107 
172 
275’ 
100 
6 
10 50 
8 
22 
7 
16 0 
60 
no 
25 
7 
9 10 
85 
107 
135 
28 
14 0 
30 
70 
145 
75 
15 20 
47 
102 
162 
60 
19 15 
67 
107 
20 
8 
8 45 
97 
155 
200 
45 
13 20 
120 
130 
142 
12 
18 
11 30 
50 
70 
97 
27 
16 30 
82 
117 
150 
33 
20 
13 45 
120 
222 
50 
21 
16 30 
100 
Sept. 4 
13 45 
92 
130 
150 
20 
5 
10 0 
62 
112 
25 
The values of ^ were taken, when practicable, from the potentials at the dis 
tances 3 and 4 metres above the deck, since it was thought that the disturbing elfect 
of the vessel would be less for them than for those taken at the lower points. 
In one or two cases there appeared to be no change of potential. This was 
believed to be due to defective insulation, and the observations are not included in 
the table. No negative potentials were observed. 
In connection with the electrometer observations the readings of the dry and wet 
bulb thermometers were taken, and from them were obtained the relative humidity 
