1. The problem statement, all variables and given/known data When the internal resistance is large in comparison to the external resistance, the terminal potential difference falls to a small value. This is used to make high voltage supplies safe for use in a laboratory. Explain how this makes the supply safe. 2. Relevant equations Vterminal = EMF - Vinternal 3. The attempt at a solution If the internal resistance is high, since Vinternal = I r, where r is the internal resistance, the Vinternal will be high. This means the Vterminal will be low. And because of the internal resistance, the current will also be low. So then what is the point of having a high voltage power supply?