| 324 / | HD | ELECTRICITY: MEASURING AND TESTING |
| 76.11 | DF | MEASURING, TESTING, OR SENSING ELECTRICITY, PER SE {46} |
| 76.39 | DF | .~ Frequency of cyclic current or voltage (e.g., cyclic counting etc.) {6} |
| 76.41 | DF | .~.~ Frequency comparison, (e.g., heterodyne, etc.) {4} |
| 76.44 | DF | .~.~.~ With filtering {2} |
| 76.46 | | .~.~.~.~ Plural |