What is the last number of electrodes that operate on all currents?

Electrodes that are designed to operate on all currents typically refer to those that can be used with alternating current (AC), direct current (DC), or have specific characteristics that allow for versatility in welding applications. In this context, the correct response identifies electrode classifications that cater to a broad range of electric current types.

The numbers mentioned, specifically 3, 4, and 7, correspond to certain electrode classifications under the American Welding Society (AWS) standards that are recognized for their adaptability. For example, an electrode classified with these numbers can be used in multiple welding processes, accommodating various electrical current types, making them suitable for a wide array of applications in structural welding.

Understanding this area is crucial, especially in a practical setting where flexibility with equipment can greatly enhance a welder's ability to perform in different environments and materials. These are fundamental competencies according to the Los Angeles City Structural Welding Code, which requires knowledge about the equipment utilized, including the types of electrodes compatible with various current forms.