On a conventional T/S diagram, such a 
thermocouple would appear as a rectangle paral- 
lel to the axes. The EMF is proportional to the 
area of the rectangle. The corresponding EMF 
in our sea water salt bridge circuit will be 
proportional to the net area, if any, enclosed 
by the T/S diagram of the circuit. 
From this it appears that the largest 
error to be expected in the open ocean would 
occur on a deep vertical measurement, if the 
long line carried water of surface salinity to 
great depths. This error would be about 10 
microvolts, which would be negligible. 
Strangely enough, the other two possible 
second order errors will be identically zero. 
The pressure and temperature will be the same 
inside a submerged tube as outside, so all 
cross effects between these two variables will 
cancel out. The pressure and salinity would 
seem a more likely pair. However, it is clear 
that if the salt consisted of a single thermo- 
dynamic component, say sodium chloride, the 
influence of both variables would be exercised 
through changes in the one thermodynamic chemi- 
cal potential, and no cross effects would be 
possible. Any cross effects arising from the 
presence of several components in the sea salt, 
would have to be considered as third order 
errors. 
OTHER CONSIDERATIONS 
We have received several suggestions that 
we avoid our bubble problems by continuous 
pumping of sea water through our lines. Even 
if this could be done without electrical 
leakages through a pump, we would expect 
undesirable electrokinetic potentials to be set 
up in the line by the water flow. 
Because of the traditional use of concen- 
trated KCl in laboratory salt bridges, we have 
also considered its use in our system. While 
it would greatly enhance the thermocouple effect, 
KCl solution might be desirable for horizontal 
measurements in rivers and estuaries where the 
local water is not suitable anyhow. It would 
be most undesirable for deep vertical measure- 
ments at sea: the added weight of the salt in 
a kilometer of tubing would create a prohibi- 
tive hydrostatic pressure difference on a 
vertical sounding. 
Similar, though less severe, hydrostatic 
pressure problems will arise if tubing filled 
with high salinity surface water is lowered to 
great depths. This can be prevented by an 
appropriate initial dilution of the water used 
to fill the tubing. 
184 
OTHER USES 
Another potential use of the salt bridge 
technique at sea may be of interest to biologists. 
If two salt bridges are lowered together, and 
one of them is closed at the end by a membrane 
permeable to only one kind of ion, the signal 
produced will be a direct measure of the ratio 
of chemical "activities" of the ion inside and 
outside the membrane on the two sides of the 
membrane. Since the temperatures and pressures 
will be the same on both sides, and the salini- 
ties nearly the same, this ratio will vary 
essentially only as the concentration of the ion 
varies with depth. 
Suitable glass membranes for H', Nat, Kt 
and Cat now exist. Moreover, any simple coated 
metallic electrode, such as the Ag/AgCl electrode, 
can be made thin with two identical faces to 
serve the same purpose. Thus the possibility 
exists of making simple, direct, in situ measure- 
ments of individual ionic concentrations at depth. 
REFERENCES 
1. Michael Faraday, Phil. Trans. Roy. Soc. 1832: 
Part c,, 263% 
2. W. S. von Arx, Pap. Phys. Oceanogr. Meteor. 
11(3) (1950). 
3. W.V.R. Malkus and M. E. Stern, J. Mar. Res. 
LC) 7m G1952)) 
4. M. S. Longuet-Higgins, M. E. Stern and H. 
Stommel, Pap. Phys. Oceangr. Meteor. 13(1 
(1954). 
5. G. J. Janz, "Reference Electrodes", Academic 
Press, New York, 1950. Chapter 4, 220-222. 
6. A. C. Redfield, J. Mar. Res. 12(1), 121 
(1953); 
ACKNOWLEDGMENT 
The author is especially indebted to Dr. 
Willem Malkus for first suggesting this investi- 
gation, to Dr. W. S. von Arx for indispensable 
advice, and to Dr. Vaughan T. Bowen for generous 
encouragement. He is also most grateful to the 
Allyn Vines for the use of their basement during 
the current measurements in Woods Hole. Finally, 
special mention must be made of Phil Ballard and 
Ellen Langenheim, the other two-thirds of the 
"we'' so often mentioned. 
This work was supported through a grant from 
the National Science Foundation. 
Contribution No. 1225 from the Woods Hole 
Oceanographic Institution. 
