280 DAVID R. BRIGGS 



at a depth within the cell where the motion of the particle with respect 

 to the liquid is the only motion observed. Usually, in order to elimi- 

 nate movement of the fluid in the cell from chance vibrational dis- 

 placements, one end of the cell is tightly closed at the time of measure- 

 ment. Whether this is the case or not, when an electric field is ap- 

 plied along the lumen of the cell there will occur an electrosmotic 

 displacement of the fluid along the interface between the cell wall and 

 the fluid and, in order to maintain hydrostatic equilibrium in the ves- 

 sel, a counterflow of fluid must take place in the center of the cell 

 hrnien. Particles present will be carried along with these layers of 

 moving fluid as well as be themselves displaced by electrophoresis in 

 the electric field. The result is that at only two regions within the 

 cell, and, along the line of focus of the microscope, will the observed 

 movement of particles be due only to electrophoretic displacement. 

 In a flat cell these regions of static fluid wih be found at 0.211 and 

 0.789 of the depths of the cell. In a cylindrical cell the static layer 

 occurs at 0.147 of. the diameter from the wall of the cylinder. Meas- 

 urements of particle velocity must be made at these depths in the cell. 

 Because of the lenslike action that occurs at the wall of a cylindrical 

 cell it is difficult to know accurately at what depth in the cell the 

 microscope may be focused (but see alternative method on page 282). 

 This is not so with the flat cell. For this reason, the flat cell gen- 

 erally is preferred to other cell shapes. The depths given as the ones 

 at which measurements should be made in this shape of cell, however, 

 are correct only if the width/depth ratio in the cell is greater than 20. 

 Even then measurements should be made near the center of the width 

 axis and not near the edges of "the cell. The movement of the fluid 

 in the flat cell due to electrosmosis will follow a parabolic curve where 

 displacement is a maximum in one direction at the walls and in the 

 other direction at the center of the cell. If the parabolic displace- 

 ment curve is to be symmetrical in the cell and the 0.21 and 0.79 

 levels are to be correct, the electrosmotic displacement of fluid at each 

 wall (top and bottom) of the cell must be equal. Hence it is neces- 

 sary, for the sake of simplicity in making measurements, that the top 

 and bottom sheets of glass be of identical electrokinetic properties, 

 i.e., be constructed of glass of identical composition. 



The thinner the cell the greater will be the rate of change in veloc- 

 ity of liquid flow due to electrosmosis with depth in the cell and the 

 thinner will be the lamellae of liquid in which a minimum of motion of 

 the fluid in either direction occurs. It is necessary to be able to focus 



