Industries, Inc., Model E-1 with Monel fittings, 
which is mounted on the side of the carrier. 
This pump gives a no-flow pressure of about 
20 psi, which is adequate for moderate lengths 
of tubing. However, the filling time of a 
piece of tubing goes up as the square of the 
length; it took us an hour and forty minutes to 
pump through 1 kilometer of our polyethylene 
tubing. 
In our apparatus the "vacuum" indicated in 
Figure I is provided by the experimentalist. 
With a 12 or 15 foot drop to water level, the 
necessary suction may be too much for anyone 
who is not an experienced pipe smoker. Mechani- 
cal alternatives are available. 
BUBBLE TRAPS 
Bubbles in the line can be a serious 
problem. Even when they do not break the 
electrical continuity entirely, they can raise 
the impedance to a point where leakage voltages 
dominate the signal. Fortunately, no bubbles 
can arise or persist in tubing that is well 
submerged. Even quite long bubbles in a line 
filled on deck will disappear completely if 
the line is dropped 20 or 30 meters below the 
surface. Nor do bubbles seem to form or collect 
in floating lines being towed at the surface of 
the water. Most of the bubble trouble occurs 
in the lines on deck (or on shore) which carry 
a negative hydraulic head because of their 
height above sea level, and which may be 
exposed to the heat of the sun. The high 
permeability of polyethylene to atmospheric 
gases may also contribute to bubble formation. 
We have found it convenient to introduce 
glass bubble traps into the lines at points of 
greatest elevation. These permit quantities 
of gas to collect without breaking the circuit. 
A valve in each trap allows the gas to be with- 
drawn periodically. These traps also prevent 
accidental bubbles from being pumped into lines 
during the initial filling. Because these 
bubble traps require breaking into the poly- 
ethylene line, they are potential sources of 
leakage voltages and should be carefully insu- 
lated from accidental electrical contacts. 
It is useful to have an auxiliary 
electrode in each line so that one can test for 
continuity to the sea with an ohm-meter, with- 
out passing current through the operating 
electrodes. A platinum wire electrode sealed 
directly into a pyrex bubble trap, and plated 
with silver/silver chloride serves this 
purpose very nicely. 
With towed lines one has to worry about 
bubbles which may enter the open end of the 
line which is dragging at the surface. A 
177 
narrow piece of torn bed-sheet, wrapped several 
times around the end of the line so as to form a 
sort of tubular wick two or three feet long 
dragging behind the line, is apparently enough 
to solve this problem. 
GROUNDING 
With the Keithly 603, the ground connection 
serves as a common input for both high impedance 
leads. Between the ship and the sea there may 
be large fluctuating potential differences due 
to corrosion, or electrical leakages. Even 
with the large in-phase rejection of the Keithly 
instrument, such a ground signal is unacceptable. 
To get around this, we isolate all of our 
electronics from the ship, except for AC power, 
and use a towed G.E.K. electrode for a single 
common ground directly to the sea. 
This arrangement produces considerable AC 
pickup, but the AC can easily be kept out of 
the electrometer by connecting the inputs to 
ground with by-pass capacitors. 
OPERATING PROCEDURE 
The salt bridges must first be filled with 
sea water. It is safer to start witha 
completely empty line; otherwise a small amount 
of water will tend to collect in the low spots 
in a line and one may discover that one has 
inadvertently created enough U-tube manometers 
connected in series to defeat the pump's best 
efforts. If the line is on a reel, the reel 
can be turned on its end; if the line is 
weighted to the bottom of a channel, and is 
looping gently from weight to weight, one has 
a nasty problem. 
After the lines are full the pump is 
turned off and disconnected, and air is admitted 
into the Y to drain the shunt. The lines are 
separately tested for continuity. The line 
resistances ought to be perfectly steady and 
proportional to the line lengths. Otherwise 
one looks for a bubble or a leak. 
If the lines are all right, the water levels 
are drawn back up in the Y to form the shunt, 
and the electrode zero is determined. The 
adjustable zero on the Keithly 603 permits one 
to make this initial electrode zero coincide 
with the zero on a chart recorder. Since the 
electrometer zero will drift slowly with time, 
even more than the electrode zero, it is also 
desirable to record the signal produced by 
shorting the electrometer inputs together. The 
difference between this signal and the electrode 
zero is due to the mismatch between the two 
electrodes. It will be small and will change 
slowly and steadily with time, unless the 
