The testing of brushes for life and performance under various altitude conditions

IN brush-life testing on machines, many confusing, conflicting, and variable results are observed even under normal atmospheric conditions. These uncertainties are increased greatly in high-altitude tests. Many tests by various laboratories have shown that most brushes are affected greatly by the changes in humidity, oxygen content, or pressure of the surrounding air. It is a common experience for carbon brushes to wear away at extreme rates up to half an inch or more per hour when run on high-speed commutators or slip rings in extremely dry atmospheres.