﻿in the Moisture condensed on Glass Surfaces. 341 



Variation in Value of Initial Current dependent on 

 Hygrometric State. 



Experiments were made to determine how the initial value 

 of the current varied with the hygrometric state o£ the 

 atmosphere. For this purpose a tin box was constructed in 

 which the hygrometric state could be varied by the intro- 

 duction of certain quantities of water. In this the grid was 

 placed, and the whole was made air-tight. At a fixed tempe- 

 rature the hygrometric state, H, inside the box assumes a 

 definite value: the initial current for this value of H was 

 determined. On introducing a small quantity of water the 

 value of H changes, and after allowing a sufficient time to 

 elapse it becomes steady, when the value of C, the current, is 

 again determined. 



To measure H inside the box the dew-point method was 

 used. Soldered in the top of the box was a thin hollow 

 cylinder of polished brass through which water at varying 

 temperatures could be passed. The temperature at which 

 dew was formed and the temperature of the air in the box 

 determines the dew-point. The detection of moisture on the 

 surface of the cylinder was facilitated by means of a feather 

 mounted on a spindle which could be manipulated from outside 

 the box. A window was introduced for viewing this. 



The introduction of moisture was effected by means of a 

 long glass tube running from the top to the bottom of the 

 box. A small rubber cork closed the top of this, being re- 

 moved rapidly when water was introduced, and as rapidly 

 replaced. A definite amount of water was used which ran 

 down the glass tube into a dish inside the box. 



The experiment was carried out as follows : — 



A definite quantity of water was inserted in the box over- 

 night. The next morning the dew-point was carefully deter- 

 mined and the initial current measured. Water was again 

 placed in the box, and so on. 



Special difficulties presented themselves owing to the deter- 

 mination of the dew-point having to be made in a limited 

 space practically devoid of air-cur rents, so that great care 

 had to be taken. The mode of procedure adopted was to 

 determine the dew-point roughly with ice-cooled water, then 

 to take water at a temperature rather greater than this dew- 

 point and pass it through the cylinder. If no dew appeared 

 a further quantity of water T 2 o °f a degree cooler was taken, 

 and the process repeated, and so on. 



The galvanometer used was adjusted to read to 10~ 7 amp. 

 and an E.M.F. of 110 volts was used ^throughout the de- 

 terminations. 



