IX. ELECTROPHORESIS 285 



supplying its electromotive force. Batteries of the "B" type supply- 

 ing 45-90 V. can be used as the source of applied voltage. The field 

 strength, E (v. /cm.), acting on the particle will be given by the rela- 

 tionship, E = I /AX, where / is in amperes, A is in s(iuare centimeters, 

 and X is in mhos. The mobility of the particle will then be m = 

 {d/t)/E, where d is the distance (cm.) the particle travels in time t 

 (sec), m will have the dimensions s(iuare centimeters pei' volt sec- 

 ond. 



The value of X, the specific conductivity, must be obtained for 

 each suspension to be studied. This may be measured with a stand- 

 ard 1000 cycle Wheatstone bridge conductivity apparatus. The area 

 of cross section of the cell, A, must be determined carefully for each 

 new cell at the point along the length of the cell at which subsequent 

 observations of mobility will be made. The average cell lumen depth 

 is obtained by measuring the distance between top and bottom of the 

 lumen at several points along the wide axis of the cell (at the correct 

 position along its length) with the microscope micrometer focusing 

 adjustment. The width of this axis is obtained by measuring with 

 a microscope cathetometer. 



When a new cell is put into use, it is desirable to test it for uni- 

 formity of construction by introducing into the cell a test suspension 

 of quartz or collodion particles in a dilute solution (about 0.01%) of 

 some surface-active substance such as a soluble protein (at some pH 

 away from its isoelectric point) and measuring the velocity of dis- 

 placement of the particles (under constant field strength) at a number 

 of depths in the cell. When the observed particle velocity is plotted 

 against the fractional depth in the cell at which the observation is 

 made, a parabolic curve, symmetrical about the center, should be ob- 

 tained. Because the walls of the cell will be coated with the protein 

 just as the particles are coated, the electrosmotic displacement of 

 fluid along the wall should equal the electrophoretic displacement of 

 the particles there (but in opposite directions) and it should be found 

 that u, the observed velocity of the particles, at the wall will be zero. 

 Also under these conditions u at the 0.21 and 0.79 levels in the cell 

 should be equal to % u observed at the center of the cell (at the 0.5 

 level). If these relationships are found, the construction of the cell 

 (width to depth ratio, parallelism of the top and bottom plates, etc.) 

 is correct and the cell should yield dependable mobility measurements. 

 As a further check on the dependability of the mobility measurement 

 (which will be dependent upon the accuracy with which \, A,I and u 



