362 PROTOPLASM 



potential, i.e., part of the total potential difference between the 

 particle and its surrounding medium. If this value is desired, 

 it may be calculated by a formula. It cannot be directly, 

 electrometrically, measured. In addition to the potential on 

 the surface of the particle, the field strength is a factor: it is a 

 definite part (determined by the type of apparatus) of the total 

 potential across the field. It is expressed in volts per centimeter 

 and may be roughly measured by connecting a voltmeter to two 

 platinum electrodes fused into the glass chamber. Voltmeters 

 are shunted and are of high resistance so as not to receive the 

 full flow of current. As the solution in the cataphoresis chamber 

 is of equally high resistance, the current will be divided ; the part 

 going through the voltmeter will reduce the recorded potential. 



A more exact calibration of the cataphoretic chamber involves 

 measuring the cross-section area of the cell; then, by measuring 

 the current during experiments and the specific resistances of 

 the solutions used, the field strength in the cell may be calculated 

 by Ohm's law for each experiment (method of Abramson and 

 Moyer). 



The experimental difficulties attendant on precise work in 

 cataphoresis are greater than here indicated. One problem to be 

 solved is the correct level in the chamber at which the rate of 

 migration of the particles should be measured. The rate is 

 different at different levels, because the aqueous medium carries 

 on an independent electroendosmotic flow, while the particles 

 migrate cataphoretically. More than this, the electroendosmotic 

 flow of the water is in one direction (to the cathode) close to the 

 walls of the chamber, while in the center of the chamber there is a 

 hydrodynamic flow of the water in the opposite direction due to a 

 return current. This is true because the chamber is a closed one, 

 and the water must follow a circuit. One of these currents 

 opposes and the other aids the movement of the particles. 

 Figure 158, in which the thickness of the chamber is greatly 

 magnified in proportion to its length, illustrates the situation. 

 Either the average of a number of readings at different levels 

 from top to bottom must be taken, or all readings must be made 

 at one of the two stationary levels which lie midway between the 

 opposing currents. (Even in this latter case, it is well to average 

 the rates of travel of a particle in both directions, by reversing 

 the polarity of the electric circuit.) The two stationary levels 



