Final answer:
The voltage measured when the welding machine current is zero is called the open circuit voltage, which essentially is the voltage potential present without any current flow.
Step-by-step explanation:
The voltage measured when the welding machine current is zero is known as the open circuit voltage. This is the potential difference across the terminals of the welding machine when it is not connected to a complete circuit and thus no current is flowing. The open circuit voltage is important in welding, as it represents the voltage available to start an arc when the electrode touches the workpiece.
The terminal voltage is the potential difference across the terminals when the device is connected in a complete circuit and current is flowing. However, if we make the current I zero, then the terminal voltage equals the electromotive force (emf), which can be considered the inherent voltage capacity of the welding machine. Standard voltmeters operate by measuring current, and a different technique is required to measure emf directly because making current zero is not practical for standard voltmeter operation.