EXPERIMENTAL VALIDATION 



The experiment checks the results of the theory for a special case. If the 

 agreement between theory and experiment is good, the theory and the assumptions 

 on which it is based can then be applied with confidence to a range of similar 

 problems. 



The basic problem is to get experimental curves. A sequence of measure- 

 ments of concentration at several points along a vertical line is required, repeated 

 several times, preferably at times varying exponentially. It is clear that the instru- 

 ment used in making the measurement or extracting a sample should disturb the layer 

 system as little as possible, both in moving into the fluid and in extracting a sample. 

 The measurement of the chlorinity of very small samples of NaCl solution was con- 

 sidered. However, as this method would have required many insertions of the sam- 

 pling probe, and as adequate microchemical techniques were not available, it was 

 decided instead to make conductivity measurements with a special microcell that 

 allows a given sequence of measurements to be made with a single insertion. 



Since both the diffusion coefficient and the conductivity of the salt solution 

 depend on temperature, a temperature bath was required. 



Apparatus 



MICROCELL AND BRIDGE 



The microcell was built into a capillary tube with an enlarged lenticular 

 chamber. The microcell consists of two platinized platinum electrodes firmly 

 spaced within a glass chamber with a volume of 0.01115 ml between its tip and a 

 level '/•> inch above the upper electrode. 



The bridge is basically an ac Wheatstone bridge excited with a low- 

 voltage, single-stage vacuum-tube amplifier and equipped with a null-indicator 

 tube. The bridge is provided with a vacuum-tube oscillator circuit supplying 1000- 

 c/s bridge current. The operator balances the bridge by selecting a ratio and 

 adjusting a resistance in the fourth arm of the bridge. A capacitor in parallel with 

 this resistance can be varied to nullify the effect of the capacitance of the leads 

 from the bridge to the cell. 



A Hamilton Model 0010 Micro-Syringe was used to withdraw the liquid 

 through the conductivity cell. It consisted of a 1-cc syringe with a plunger in 

 which a lead screw was mounted to withdraw minute amounts of liquid; as the lead 

 screw was turned, the inner part of the plunger moved in or out, changing the vol- 

 ume of the liquid in the syringe. The change in volume per turn of the lead screw 

 was 0.0076 ml ±1.3 percent. 



The lead screw could be withdrawn a total of 33 turns. In practice, it was 

 turned three turns at each new position to insure that the sample was first thoroughly 

 flushed from the chamber. Thus, 11 samples could be obtained for each insertion 

 of the probe assembly. The assembly was securely attached to a feeding mecha- 

 nism, a 12-cm, 28-threads-to-the-inch screw on which was attached a turn knob 

 divided into 100 equal parts. The probe assembly could be advanced to each posi- 

 tion with a least count of 0.01 mm. 



A cathetometer was used to determine depth (refer to Procedure). 



