A Suitable Current Stabilizer for the Tiselius Apparatus. ∗

Abstract

It has become evident that present methods of current control for the electrophoretic apparatus of Tiselius can be improved. It is common practice to use a voltage-stabilized power supply to furnish the current. While this power supply gives a constant voltage across the whole cell, it does not furnish a constant current because of fluctuations of resistance within the cell. Most of these fluctuations probably arise within the electrode vessels. Inasmuch as the resistance of the central part of the cell is not a constant fraction of the total resistance of the cell, the voltage drop across this part and the field strength within it do not remain constant. Because of this it is customary 1 to measure the current and to calculate the field strength by means of the equation: X = I/qk₈.

X = field strength in volts per cm.

I = current in amperes.

q = cross-sectional area of the central part of the cell.

k₈ = specific conductance of sample in reciprocal ohms.

In order to apply this equation to apparatus employing a voltage-stabilized power supply it is necessary for the operator to make frequent observations of the current and to compensate for fluctuations by altering the applied voltage. This frequent adjustment is tedious as well as time-consuming.

It is preferable that current-stabilization rather than voltage-stabilization be used. The simple circuit which has been found suitable for the Tiselius apparatus is herein given. It is based on the principle that the plate current of a pentode vacuum tube remains constant despite changes in plate voltage. Further stabilization is obtained by the degenerative effect of the resistance in the cathode circuit.

In operation the switch S₂ is connected to the dummy load R₃ and then the switch S₁ is closed.