The objective of the voltage clamp is to measure currents in the membrane generated by an event that impinges upon the membrane, as during the generation of the action potential. We can't reach into the membrane to measure the current directly, but current can be measured using a trick. If we monitor the voltage of the membrane during, the action potential, and keep the potential from changing by injecting an equal and opposite amount of current through our electrode as is being injected through the sodium and potassium channels, we have an indirect but accurate measure of the action potential currents. Here's a cartoon illustrating the principle. The voltage is monitored by the sharp electrode, the current is injected by the tube electrode. When sodium current is inward, the tube current is equal and opposite outward. When the potassium current is outward, the tube current is equal and opposite inward.