JOHN H. NORTHROP AND GLENN E. CULLEN 637 



Since the cell is of uniform diameter the drop in potential is deter- 

 mined simply by dividing the total voltage by the distance between 

 the three-way stop-cocks. (In solutions of less than 0.1 n the drop 

 in potential in the saturated zinc sulfate may be considered as negli- 

 gible; if more concentrated solutions are used it is necessary to apply 

 a small correction for the resistance of the zinc sulfate.) 



Influence of the Voltage. — It was found that the rate of migration 

 was directly proportional to the voltage between the limits of 1 to 

 4 volts per cm., provided the experiment was not allowed to run too 

 long. At low voltages, the rate of migration remained constant 

 until the boundary approached the zinc sulfate, but if the potential 

 drop was increased beyond 2 volts per cm., it was found that the 

 boundary moved at the proper rate for the first 4 to 5 mm., but then 

 became much too slow on one or both sides. There was also a tend- 

 ency for the boundary to become convex on one side, showing that 

 the migration of the water was interfering with the measurement. If 

 the solution is of high conductivity, the voltage must be still further 

 decreased to prevent heating effects and subsequent convection 

 currents. 



Influence of the Sugar Concentration. — The presence of sugar greatly 

 facilitates the adjustment and maintenance of a sharp boundary line. 

 No effect on the velocity of migration could be observed up to 0.5 m . 

 Higher concentrations than this decrease the velocity presumably on 

 account of the viscosity. 



Calculation of the Potential from the Velocity of Migration. — The 

 value for the potential difference between the particle and the sur- 

 rounding solution is calculated by means of the Helmholtz-Lamb 

 equation^ 



in which 



n = viscosity of the solution. 



V = velocity of particle in cm. per second. 



K = dielectric constant of the solution. 



X = potential gradient; i.e., the drop in potential in E.s.u. per cm. 



All electrical units are electrostatic. 



