period of time is aided by placing a disc of fil- 
ter paper over the end of the electrode under the 
plastic. However, this increased the time constant, 
at least several-fold. 
It has been found empirically that the 
reference electrode must "see" the solution 
directly through the plastic film and so should 
not be covered by the retaining ring. This obser- 
vation is inconsistent with the view that the OH™ 
ions diffuse only through the layer of electrolyte, 
and it is not yet understood. 
Such electrodes are easy to make. The silver 
and platinum parts are cast in some plastic such 
as an epoxy. Electrical connections are made to 
the back of the electrodes. Care must be taken 
that only silver and platinum are exposed on the 
outside. Any polyethylene can be used. Thick- 
nesses of .001 to .002 inches give convenient 
diffusion rates and response times. 
This polarographic technique is a valuable 
tool in experimental physiology. I have used 
electrodes to follow oxygen changes ina 1 ml. 
chamber containing a bit of seaweed or a single 
copepod. It has also been just as readily applied 
in a 55 gallon oil drum, following the respiration 
of a 30 kg. shark. In one instance it was used to 
measure the oxygen in the expired breath of a 
whale. These indicate to some degree the versa- 
tility of such electrodes. Here, however, I will 
only describe their use for in situ recording in 
the ocean. 
This is probably the most difficult and also 
the most desired oceanographic task to which they 
might be applied. To accomplish it one must over- 
come several inherent shortcomings in the electrode. 
These will be discussed in turn. 
A. Stirring 
The electrode current represents an actual 
consumption of molecular 0, from the solution 
being measured. This depletes the thin layer of 
liquid immediately in front of the plastic mem- 
brane. Stirring the solution renews this depleted 
surface layer. If the solution is not stirred 
this layer will progressively lose more of its 0 
and a gradient will be set up in the liquid. In 
such a situation the flux of 0, reaching the 
platinum will be limited by a combination of the 
diffusion resistance through the solution as well 
as through the plastic. Since anything that 
changes this resistance will alter the electrode 
current it is necessary to maintain constant dif- 
fusion conditions. The most direct way of realiz- 
ing this is to mix the solution so strongly that 
it contributes a negligible fraction to the over- 
all 0, diffusion resistance. This can be accom- 
plished in over-the-side work by running the winch 
fast or accentuating the current past the electrode 
by funnel arrangements or small stirring motors. 
In laboratory applications the stirring may 
335 
be below saturation as long as it is constant. 
A synchronous motor rotating a large magnet (from 
a magnetron) can be used to induce rotation in a 
small teflon corrected magnet over a considerable 
distance. I have used such a method to stir small 
containers deep inside a water bath. The constant 
speed of the motor insures a steady record. 
Figure 1B shows an electrode holder for line 
monitoring. The stream is directed against the 
plastic film directly over the Pt electrode. This 
creates a maximum of effective stirring with a 
minimum of flow. If the incoming tube is brought 
very close to the electrode a flow of 10 ml/min. 
may give a saturation (maximum) electrode current. 
Thinner and more diffusible membranes (i.e. 
teflon as compared to polyethylene) present less 
diffusion resistance. This produces a larger 
electrode current as well as making its response 
faster. But it accentuates stirring difficulties 
since the diffusion resistance in the liquid must 
be likewise reduced. Thus a compromise must be 
struck between sensitivity, speed, and stirring 
difficulties. 
B. Speed of response 
An electrode with a .001" thick polyethylene 
layer will show a 90 per cent response in about 15 
seconds. If the silver area is large (more than 
10 times) compared to the platinum, the response 
to a step function will be very closely exponen- 
tial. With much less silver area, there is a long 
slow component in the current. This presumably 
represents some polarization phenomena at the 
silver surface. 
One would like a faster electrode for vertical 
recording in the sea since with a fast winch the 
electrode can cover more than 100 meters in the 90 
per cent response time. 
From diffusion theory it can be shown that the 
time to reach diffusion equilibrium across a bar- 
rier of thickness t is proportional to 
t2 
diffusibility 
This suggests the dual approach 
of thinner and more diffusible films to lessen the 
time. Teflon is better than polyethylene by this 
criterion since 0, diffuses through it about 3 
times faster. It is also tougher in thin films. 
I have recently used electrodes in the laboratory 
with .00025" teflon which showed a 90 per cent 
response in 3 seconds. As already indicated, how- 
ever, the stirring problem then is more severe. 
C. Pressure response 
When the electrode is exposed to a constant 0, 
tension, its current decreases with increasing 
hydrostatic pressure. This dependence is shown in 
Figure 2 for pressures up to 15000 1b/in2. (which 
equals about 30,000 ft. in depth). Over the first 
5000 pounds the curve is approximately linear. For 
