SECT. 5] INTERNAL WAVES 733 



to establisli the existence of density gradients. In the sea, measurements are 

 commonly of temperature since they are easily accomplished, and the tempera- 

 ture and salinity gradients usually coincide. In addition, the salinity gradients 

 are normally small. 



On one occasion, however, internal waves were directly measured by their 

 vertical oscillations of density. Kullenberg (1935) floated a large, buoyant 

 container on a given density boundary and recorded its depth. A drum was 

 filled with glass balls and guided vertically by a cable. A recording manometer 

 on the drum successfully furnished a 7 -day record of its depth when it was 

 weighted sufficiently to float on the maximum density gradient found in the 

 southern Kattegat ^ in summer. 



Various instruments have been employed to measure vertical oscillation of 

 temperature (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. 



B. Thermistor Beads 



An eff'ective device (LaFond, 1959b) to determine temperature is the 16- 

 channel temperature-sensing unit developed at the United States Navy 

 Electronics Laboratory (NEL). It consists of 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. 3). 



The NEL temperature-sensing unit is designed to measure temperatures in 

 the range from 28°F to 90°F. The strip chart employed is 1 1 inches in width, 

 divided into 100 units. On the face of the unit are mounted 16 jacks to accom- 

 modate the leads of the 16 sensing elements, 16 range switches, a voltmeter and 

 a rheostat for maintaining constant current during operation. 



The sensing elements and thermistor beads, mounted at the ends of cables, 

 can be lowered in a vertical string to various depths of the thermocline. The 

 cables are plugged into the 16 jacks on the recorder, which places their re- 

 sistance across one arm of the bridge circuit for the temperature range under 

 measurement. The beads are suspended in one or more vertical series from a 

 ship or fixed platform. Temperature can thus be recorded at up to 16 depths 

 for any desired period of time. However, for the character of internal waves, the 

 depth of the isotherms must be scaled from the measured temperatures at 

 fixed depths. Similar strings may be towed and the sensing elements scanned 

 electronically, which allows the isotherms to be printed directly (Richardson, 

 1958) with reference to time or distance. 



1 An arm of the North Sea, between Sweden and Denmark. 



