(sometimes simultaneously with salinity). For 
long-period waves, reversing thermometers and 
water bottles were used. Repeated bathythermo- 
graph lowerings were made, and more recently 
the fast-responding thermistor beads have been 
utilized. 
THERMISTOR BEADS 
The most common method of measuring sea 
temperatures at the present time is with fast- 
responding thermistor beads. These come in 
various types; the most common is a small, 
glass-encapsulated, high-thermal-resistant 
material with connecting leads. A current pas- 
sing through the beads is greatly influenced by 
temperature. Thus the temperature of the sea 
water is proportional to the electrical current. 
Since the beads are not precisely matched, 
resistance-wise, to each other, and temperature- 
resistance relations are not exactly linear, 
several schemes have been devised to increase 
accuracy by using compensating resistors in with 
the bead or by amplifying short stretches of the 
most linear part of the resistance-temperature 
curve. Another problem is their fragility and 
the watertight integrity between the glass beads 
and electrical connections (fig. 3). 
One effective thermistor device to deter- 
mine sea temperatures and identify internal 
waves is the 16-channel temperature-sensing unit 
developed y the U. S. Navy Electronics Labora- 
tory (NEL).? It includes 16 thermistor beads 
cast in plastic and attached to electric leads. 
The leads and beads are part of a bridge circuit 
that feeds a recording-type potentiometer. The 
recorder prints numbered points consecutively 
from 1 to 16 on a power-driven strip chart, the 
location of each number indicating a particular 
temperature. In normal operation, a full cycle 
of 16 recordings requires approximately half a 
minute (fig. 4). The temperature at each depth 
is indicated on the chart to an accuracy of 
better than 0.02°F. 
The beads are suspended in one or more ver- 
tical series from a ship, anchored buoy, or 
fixed platform (fig. 5). Temperature can thus 
be recorded at up to 16 depths for any desired 
period of time. However, to determine the 
character of internal waves, the depth of the 
isotherms must be scaled from the measured tem- 
peratures at fixed depths. 
THERMISTOR CHAIN 
Strings of thermistor beads may be towed 
from a ship and their sensing elements scanned 
electronically. This procedure allows the iso- 
therms to be identified and printed directlyl° 
with reference to time or distance. 
NEL thermistor chain, from which are sus- 
pended the strings of thermistor beads, makes 
it possible to sense vertical sections of tem- 
perature structure from the surface down to 800 
feet when the ocean-going NEL research vessel 
138 
USS MARYSVILLE cruises in waters of greater 
depth (fig. 6). 
The thermistor chain, together with the 
drum on which it is wound, is large and rugged, 
weighing 37,500 pounds. The chain is composed 
of large, flat links about a foot long, 9 inches 
wide, and an inch thick. At its end is a 2300- 
pound, streamlined torpedo-shaped weight to 
hold it down. The weight and its connected 
chain are lowered from the stern as the oceano- 
graphic vessel proceeds through the water. 
About 80 pairs of insulated wires fit 
through grooves inside the flat links. Every 
27 feet the wires connect with the thermistor 
beads, which sense the sea temperature. 
The upper ends of the electrical leads 
connect to a recorder located in the vessel's 
laboratory. Signals from the beads are scanned 
electronically every 10 seconds, and lines 
showing the depths of constant temperatures are 
printed on 19-inch-wide tape. This is equiva- 
lent to taking a bathythermograph lowering 
every 100 feet if the ship proceeds at 6 knots. 
Recorded on the same tape are the depth of the 
weight (or maximum depth of observation) at the 
end of the chain and, in addition, the sea sur- 
face temperature (the uppermost bead in the 
chain). This instrument thus gives essentially 
a two-dimensional picture of oceanic thermal 
structure and internal waves from the surface 
to 800 feet. 
ISOTHERM FOLLOWER 
Another device for directly measuring the 
temperature of internal waves is the isotherm 
follower’, which automatically traces isother- 
mal vertical oscillations with reference to 
time. This instrument is comprised of (1) a 
sea sensing unit; (2) an electric winch con- 
taining a cable to which the sea sensing unit 
is attached; (3) electronic components (servo- 
mechanism, amplifiers, power supply, controls, 
etc.); and (4) two recorders (depths and tem- 
perature) (fig. 7). 
The sea sensing unit contains a thermistor 
bead balanced in a bridge circuit with a re- 
sistance corresponding to the desired isother- 
mal temperature. If the bridge becomes unbal- 
anced, a thyratron tube is fired. This 
activates a winch and causes it either to wind 
in or let out the sea unit, "locking it" onto 
the desired isotherm. The isotherm depth is 
continuously recorded on the ship by means of a 
pressure sensor in the sea unit. The net re- 
sult is a trace of the given isotherm depth, 
effective to 600 feet, with reference to time. 
The isotherm followers have been employed 
singly, or in triangular arrangements of three, 
to eons the speed and direction of internal 
waves by time of arrival and plwse shiftsl3, 
Virtually continuous operations throughout the 
summer months up to periods of one week have 
been successfully conducted from the NEL Oceano- 
graphic Research Tower. Here they are suspended 
from the structure by 40-foot boams (fig. 8). 
