Any kind of electrolyte solution could, in 
principle, be used in such a salt bridge. How- 
ever, the boundary between two different 
electrolyte solutions has a contact potential, 
called a liquid junction potential, somewhat 
analogous to that between an electrode and a 
solution, though much less sensitive. While 
the traditional use of concentrated potassium 
chloride in laboratory salt bridges is supposed 
to minimize such liquid junction potentials, we 
preferred to eliminate them entirely, if we 
could, using sea water throughout. 
CHOICE OF TUBING 
of standard sea water is 
)"*. The resistance of a 
long, thin column of sea water is large and in- 
creases with length. The electrical leakage 
path from such a column through the walls of an 
insulating tubing to sea water outside should 
be slight, but will increase with length. The 
net result, barring pronounced local leakages, 
will be that a sea-grounded DC signal fed into 
an infinite length of tubing filled with, and 
surrounded by, sea water - or any other con- 
ductor, for that matter - will damp out expo- 
nentially according to the formula 
Vela 4 
The conductance 
only .03-.05 (ohm-cm. 
(1) 
where S is the leakage conductance per unit 
length through the walls of the tubing, R is 
the resistance per unit length of the sea water 
column, x is the linear distance along the 
tubing. 
The damping length (sr), thus defined, 
depends on the specific bulk resistivity ( , 
the thickness 7 , and the internal diameter of 
of the tubing, as well as on the specific 
conductivity oO of the sea water column ac- 
cording to the following approximate relation: 
gaye (a 
We have determined the damping lengths 
with sea water of a number of assorted samples 
of readily available tubing, with the results 
given in Table I. It is evident from the 
Table that polyethylene tubing is the most 
suitable for long distances. Since the poly- 
ethylene, at 2.7 cents/ft., was also the 
cheapest of the tubings we tested, and since 
it is quite inert chemically and very tough 
physically, the choice was not difficult. The 
polyethylene tubing as supplied by the manu- 
facturer (Crystal-X Corporation, Lenni Mills, 
Pa.), came in assorted lengths, some 500, 
1000, and 2000 lengths, but mostly 100 ft. 
lengths. We found that new and unused lengths 
of this tubing could be easily welded together 
(2) 
174 
by heating the ends over a gentle flame until 
they became transparent and sticky; and then 
joining them together with a little pressure 
causing them to flare out slightly. Quenching 
with cold water speeds the hardening. A well- 
formed joint of this kind does not appear to 
weaken the tubing either physically or electri- 
cally. 
Once the tubing has been exposed to sea 
water, however, these welds do not take very 
well, even when the two ends have been well 
cleaned in all the different ways we could think 
of. Some improvement can be had by sloshing the 
two ends in mineral oil before heating, but the 
joints are still likely to be bad and should be 
tested carefully before they are used. 
The 5/16" o.d. polyethylene tubing which we 
have used proved to be very well suited for 
towing. It is buoyant, even when filled with 
water, and snakes along very freely on the sea 
surface. The small diameter and smooth surface 
help keep the towing resistance very low. At 
10 knots in a moderate sea, a 200 meter length 
of tubing can be held with one hand without too 
much difficulty. 
The breaking strength of this tubing is 
about 200 lbs. 
To connect the polyethylene tubing to glass 
tubing, or to make temporary connections between 
two lengths of the polyethylene, we have used 
short lengths of snug-fitting (%"' i.d.) tygon 
tubing as couplings. Because polyethylene, like 
most insulators, develops a markedly conducting 
surface film after sea water has dried on it, 
such connections tend to be very leaky electri- 
cally unless made with clean tubing. A_ liber- 
al application of mineral oil to all surfaces 
prior to making the connection seems to help a 
little. In any case, electrical leakage can be 
expected if the connection has borne the 20 lbs. 
pumping pressure for any length of time: some 
seepage evidently occurs. For this reason, the 
welded connections are decidedly preferable for 
connecting polyethylene pieces to one another. 
Glass-to-polyethylene connections should be 
kept well insulated in dry surroundings. 
CIRCUITRY REQUIREMENTS 
Sea water is not a very good conductor. A 
3/16" i.d. tubing filled with sea water offers 
about a megohm of resistance for every 100 meters 
of length. This high impedance of a long salt 
bridge is the biggest single drawback to the 
salt bridge technique at sea. While high im- 
pedance measurements and shipboard working 
conditions are not totally incompatible, neither 
are they very congenial. Most insulating 
surfaces seem to develop leakage resistances of 
