b'Choosing the electrophoresis chamber and power supplyIn electrical terms, the process of electrophoresis is closely associated with the following equations derived from Ohms law: Voltage = currentresistance (V = IR)Wattage = currentvoltage (W = IV) ResistanceCurrent The electrical resistance of the assembled electrophoresisFor a given gel/buffer system, at a given temperature, cell is dependent on buffer conductivity, gel thickness,current varies in proportion to the field strength (voltage) temperature, and the number of gels being run. Althoughand cross-sectional area (thickness and number of gels). the resistance is determined by the gel system, theWhen using a constant current setting, migration starts resistance varies over the course of the run.slow, and accelerates over time, thus favoring stacking in discontinuous gels.In discontinuous buffer systems (and to a lesser extent in continuous buffer systems) resistance increases over theWhen running under constant current, set a voltage limit course of electrophoresis. This occurs in the Tris-glycineon the power supply at or slightly above the maximum buffer system as highly conductive chloride ions in theexpected voltage to avoid unsafe conditions. At constant gel are replaced by less conductive glycine ions from thecurrent, voltage increases as resistance increases. If a local running buffer.fault condition occurs (e.g., a bad connection), high localResistance decreases as the temperature increases.resistance may cause the voltage to reach the maximum for the power supply, leading to overheating and damage of Voltagethe electrophoresis cell. The velocity of an ion in an electric field varies in proportion to the field strength (volts per unit distance). The higher thePower voltage, the faster an ion moves. For most applications, weWattage measures the rate of energy conversion, which recommend a constant voltage setting.is manifested as heat generated by the system. Using constant power ensures that the total amount of heatA constant voltage setting allows the current and powergenerated by the system remains constant throughout the to decrease over the course of electrophoresis, providingrun, but results in variable mobility since voltage increases a safety margin in case of a break in the system.and current decreases over the course of the run. ConstantThe constant voltage setting does not need adjustmentpower is typically used when running IEF strips. When to account for differences in number or thickness of gelsusing constant power, set the voltage limit slightly above being electrophoresed.the maximum expected for the run. High local resistance can cause a large amount of heat to be generated over a small distance, damaging the electrophoresis cell and gels. 52'