[ 87 ] 
VIIL—ON AN EXPERIMENT CONNECTING ELECTRO- 
MOTIVE FORCE AND SURFACE TENSION, sy W. F. 
BARRETT, F.z.5.E. 
[Read March 18, 1878.] 
THE surface of every liquid is known to be in a state of tension, 
similar to that of a membrane stretched equally on all sides.— 
This tension arises from the fact that each liquid has a certain 
intrinsic energy depending on the extent of its surface, and 
hence to increase the area of the surface of a liquid requires the 
expenditure of work. The surface of a liquid, therefore, resists 
extension and, further, tends to contract. This is seen in the 
force required to blow a soap bubble, and in the immediate 
shrinkage of the bubble when the blowing ceases.; it is seen in 
the breaking up of a liquid jet into elongated fragments and 
ultimately into spherical drops: in fine, wherever a liquid surface 
exists, this state of surface tension comes into play. 
The amount of this superficial tension per unit of surface 
varies not only with the nature of the liquid, but also with that 
of the medium which adjoins the bounding surface of the liquid. 
Every two unmixable liquids in contact with each other have 
therefore a special coefficient of surface tension. Now the 
value of this coefficient may, among other causes, be made. to 
vary in a remarkable degree by the electrification of the liquid 
surface. If, for example, mercury—which has the highest surface 
tension of any liquid—be in contact with dilute sulphuric acid, 
the surface tension of the mercury is increased when an electro- 
motive force is directed from the acid to the mercury: on the 
other hand it is diminished when the contrary electrification is 
set up. Owing to this fact, the following phenomenon, first 
noticed by Faraday, occurs. When the positive pole ofa battery is 
immersed in a drop of acid resting on a large drop of meréury, to 
which the negative pole is connected, the mercury tends to gather 
itself up and change its form in a curious way ; the current here 
passes from the acid to the mercury : when, however, the direction 
of the current is reversed, the mercury becomes dull and fiattened 
out. 
If, now, we imagine a platinum wire, immersed in the acid 
and brought very near to but not touching the mercury, the 
