Art. xi.j Cole, Experience with Capillary Electrometer. 271 
on the result of the size of the other limb of the U-tube and 
also that of the anode surface. To examine the first point a 
set of three U-tubes was prepared, whose capillary limbs were 
made from the same piece of uniform thermometer tubing of 
1.05 mm. inside diameter. The other limbs measured 3.7, 8 
and 12.2 mm. respectively in diameter. They were found to 
compare insensitiveness in the ratio of 10: 18: 18.4. This 
indicates that the size of the larger limb is of some importance. 
It should not be much less than 10 mm., but there is little ad- 
vantage in making it larger than that. In Paschen’s original 
instrument it measured 24 mm. The large surface of the mer- 
cury anode was another peculiarity of his instrument. Follow- 
ing him I at first used an anode surface 62 mm. in diameter. 
Replacing this by one of 39 mm. made no measurable difference 
in the sensitiveness. 
Practical experience emphasizes the importance of some 
cautions given by Ostwald and by Burch in the references cited. 
In particular : 
a) . Good insulation must be provided. 
b) . The terminals must be kept short-circuited through a 
conductor except during the momentary and occasional pressure 
of a key in taking a reading. Ostwald’s short-circuiting key is 
satisfactory. 
c) . The acid strength may be considerably varied, say 
from I to 3 up to I to 20 by volume. Results are somewhat 
irregular if the acid is as weak as i to 25. 
d) . Hydrochloric acid is not satisfactory. 
e) . A new instrument will usually be irregular, and will 
“crawl” with the key between stops until its terminals have 
been short-circuited for a day or so. 
f) . In using the instrument, avoid anodic polarization of 
the capillary as much as possible. Try to keep it the kathode. 
The ill effects of too much anodic polarization can be partly 
remedied by electrolysis. 
Several novel forms of capillary electrometer were con- 
structed in connection with the present study. One was sug- 
gested by the extraordinary electro-capillary engine described 
