686 Mr. C. T. R. Wilson on the Condensation Method 
the cylinder through the unblackened portions of the 
glass*. 
The apparatus gave a value for the least expansion required 
in order that rainlike condensation might result, which agreed 
well with that obtained in previous experiments with much 
smaller apparatus. 
The results of one series of measurements, those of May 
15th, 1903, are given below. 
p =gauge-reading when piston is at bottom. 
w =pressure required to support weight of piston. 
B=barometer-reading. 
7 =maximum vapour-pressure of water at temperature of 
experiment. 
Then B-—w-—a7 
U9/0, = B-p—t 
In the experiment temperature = 15° C. Barometer=766 mm. 
T= som W— oO tan, 
In the Table which follows the result of the expansion cor- 
responding to various values of the gauge-reading is given 
(a) when the upper and lower plates were metallically con- 
nected; (6) when a difference of potential of 160 volts was 
maintained between them. 




nin cia P.D.=0. P.D.=160 volts. 
millimetres. 
1S) ee erate Shower. 
por Rater seer Very few drops. 
LOIS dean excenet No drops. | 
TGS Ging ele No drops. | 
Tey oases ator Very few drops. No drops. | 
DDS aetna Shower. No drops. 
LGODiy haat Shower. No drops. 
(Al eSiue site ee Shower. Very few drops. 
17 PS sr Dense shower. Very few drops. 



Value of v/v, when rainlike condensation begins (in the 
absence of an electrical field) 
ne era 
4 766 —153—13 
In the early experimentst the value found for the least 
expansion required for rainlike condensation was v,/vj;=1°252. 

* The apparatus was exhibited at the British Association meeting at 
Southport last September, and the removal of the ions by an electric field 
demonstrated. 
+ Phil. Trans. vo]. clxxxix. p. 265. 
