the order of 1011 ohms or less when exposed to 
damp sea air. Such leakage paths are electro- 
lytic; when they connect two different metals, 
they form electrolytic cells with EMFs of the 
order of several tenths of a volt. If one of 
these leakage cells happens to discharge across 
a circuit element, such as a salt bridge, with 
a resistance of the order of 10° ohms, a spuri- 
ous signal of the order of several tenths of a 
millivolt may be developed in the circuit. If 
by some mischance, such a leakage path makes 
connections with the shipboard DC power supply, 
the situation can be many times worse. Clearly 
extraordinary precautions are called for. 
Fortunately the greater part of our 
circuitry problems have been solved for us in 
recent years by the electronics industry with 
the development of reliable high-impedance 
electrometers. We have found that the Keithly 
603 Electrometer Amplifier is very satisfactory 
in taking a few DC millivolts from a pair of 
high impedance salt bridges and converting 
this signal to a low impedance output of several 
volts. The Keithly performance seems to be 
quite immune to the damp salt air and power 
supply fluctuations of shipboard operation. 
THE ELECTRODE SYSTEM 
As the whole advantage of the salt bridge 
system arises from the thermal and chemical 
isolation of the electrodes, it follows that 
the actual electrode arrangement is very 
important. The system we have employed is 
shown in Figure I. 
The electrodes are sealed into pyrex 
electrode chambers immersed in an oil-filled 
thermos flask. Each electrode chamber connects 
to a 3-way stopcock, permitting it to connect 
down to the sea or up to an inverted-Y shunt, 
or to be cut off from the circuit entirely. 
The inverted Y leads up to a third 3-way stop- 
cock by means of which the sea water in the 
system may be admitted and manipulated. The 
temperature in the thermos flask is not regu- 
lated, but is allowed to drift in a gradual 
pursuit of the ambient room temperature. The 
fishhook shape of the electrode chambers is 
intended to protect the electrodes from sudden 
exposure to colder or saltier (i.e. denser) 
water that may make its way down from the 
stopcocks by convection. As long as no sudden 
changes occur at the electrodes, the long term 
drifts can be followed easily by means of the 
inverted-Y shunt. 
THE SEA WATER SHUNT 
This inverted-Y arrangement provides the 
one unique feature of the salt bridge system 
175 
which promises to give it greatest usefulness. 
By raising the water levels on both sides until 
they join at the Y, a low impedance sea water 
shunt can be established between the electrode 
compartments, permitting the electrode zero to 
be determined quickly and directly. By lowering 
the water level the shunt is broken and the 
measurements of sea voltages may be resumed. 
this manner the unknown, and slowly drifting, 
contact potentials of the electrodes can be 
determined as often as may be needed during any 
set of measurements. In towing experiments, the 
burdensome procedure of reversing the ship's 
course can be dispensed with, although right- 
angle legs will still be necessary if both 
components of the surface potential gradient are 
to be determined. 
In 
DETAILS OF CONSTRUCTION 
We have used the conventional G.E.K. 
silver/silver chloride electrodes developed by 
Dr. von Arx, several of which were kindly pro- 
vided for us by his assistant Mrs. Nellie 
Anderson. These are sealed into the pyrex 
electrode chambers with epoxy cement. 
The 1-liter thermos flask filled with 
mineral oil is closed with a wooden lid satu- 
rated with oil and coated outside with paraffin. 
The flask is mounted in a wooden carrier which 
is also liberally smeared with paraffin. 
The 4 mm. standard taper pyrex stopcocks 
are used only as hydraulic switches; they do 
not provide effective electrical switching 
because their leakage resistance between 
branches is usually less than a megohm. They 
do protect the electrodes from the hydraulic 
pressure variations which occur when the lines 
are drained or filled. These stopcocks are 
wired to a sheet of paraffin-coated masonite 
mounted on the frame of the wooden carrier. 
All the glass-to-glass connections, as well 
as the glass-to-polyethylene connections are 
made with tygon couplings for flexibility and 
shock resistance. 
The tygon leaders in the branches of the Y 
are especially important, since they provide 
the necessary high impedance when the Y is 
drained and the shunt broken. Most materials, 
including glass and polyethylene, tend to form 
moist, juicy, highly conducting surface films 
when repeatedly exposed to sea water. Fortu- 
nately, tygon seems to be entirely immune to 
this. We have found that a ec Y will drain 
quickly to something like 10°~ ohms after 
having been filled with sea water for 60 hours. 
Our high pressure sea water supply is pro- 
vided by a little centrifugal pump, Eastern 
