THE WORLD'S LONGEST SALT BRIDGE 
by DR. PAUL C. MANGELSDORF, Jr. 
Woods Hole Oceanographic Institution 
Woods Hole, Massachusetts 
ABSTRACT 
A variation of the Geomagnetic Electro- 
kinetograph has been developed in which the 
electrodes remain aboard ship immersed in sea 
water at constant composition and temperature. 
The electrical connection to the sea is made 
with lengths of polyethylene tubing filled with 
sea water. This arrangement greatly reduces 
the sensitivity of the system to gradients of 
salinity and temperature in the sea. It also 
permits a direct electrode zero by means of a 
sea water shunt between the two electrodes. 
INTRODUCTION 
It was Faraday, in 1832, who first pointed 
out that the motions of flowing water masses in 
the earth's magnetic field might be detected by 
the measurable EMFs which such motions ought to 
induce. Although Faraday was himself unable 
to verify the effect, his predictions were 
amply confirmed by later observers. Ultimately, 
in 1950, Von Arx reported the successful de- 
velopment of a practical instrument, which he 
named "Geomagnetic Electrokinetograph", for 
measuring fhe motionally induced potentials in 
the ocean. The theoretical interpretation of 
these potentials was also developed by Von Arx 
in the same paper; by Malkus and Stgrn®; and by 
Longuet-Higgins, Stern, and Stommel”. 
The biggest problem in measuring electro- 
magnetic potentials in the ocean arises from 
the fact that most measuring circuits are 
metallic and conduct electricity by means of 
free electrons. Sea water is an electrolyte 
solution which conducts by means of assorted 
ions, both positive and negative. Any me- 
tallic electrode inserted into the ocean as a 
probe can only function with the occurrence of 
one or more electrode reactions involving both 
ions and electrons. Such electrode reactions 
will be sensitive to temperature, to pressure, 
to ionic concentrations (i.e. salinity), to 
contamination (z.b. oxygen tension). Moreover, 
if any of the participating reactions are at 
all irreversible, the electrode will be polar- 
izable and its contact potential will change 
as current is passed. 
To overcome these difficulties, Von Arx 
used massive silver/silver chloride electrodes 
which are reasonably reversible to chloride ion 
in sea water (though they are probably poisoned 
somewhat by sulfate and bromide ions which are 
also present~). By taking great care to match 
each pair of electrodes as exactly as possible, 
Von Arx was able to minimize the differential 
responses to temperature, salinity, and oxygen 
tension which would cause spurious signals. But 
even the best electrode pairs displayed small, 
slow, residual drifts in contact potential, 
which could only be evaluated by interchanging 
the positions of the electrodes in the water. 
With towed electrodes, the simplest way to do 
this, though time-consuming, has been to reverse 
the ship's course. 
This system of towed silver/silver chloride 
electrodes has proven quite successful and has 
been widely used during the past decade for 
determining the horizontal potential gradients 
at the surface of the ocean. However, it is 
quite unsuitable for measuring vertical po- 
tential gradients. Two electrodes at different 
depths will automatically sense the corresponding 
differences in pressure, salinity, temperature, 
and oxygen tension, and will report these as 
large spurious potential differences. But, 
according to Malkus and Stern, the vertical po- 
tential distribution in the ocean would also be 
extremely valuable, if it could be measured, 
since it would yield directly the total east- 
west transport at the point of measurement. 
THE SALT BRIDGE PRINCIPLE 
After these difficulties had been drawn to 
our attention by Dr. Malkus, it occurred to us 
that there ought to be some advantage in moving 
the electrodes with their very sensitive 
metal/electrolyte junctions up out of the ocean 
entirely, and keeping them in a controlled en- 
vironment on shipboard. The electrical contacts 
with the ocean could then be made with columns 
of salt solution contained in insulating tubing 
of some sort. This is exactly the principle of 
the salt bridge - a familiar device frequently 
used by electrochemists to avoid dipping an 
electrode directly into a solution, if either 
the electrode or the solution would suffer 
thereby. 
Superior numbers refer to similarly numbered references at the end of this paper. 
