A. C. commutation is much more difficult than D. C. commutation owing to the transformer e. m. f. Nowadays, 3-divided brushes are used for A. C. commutator machine for the purpose of decreasing the short circuit current. In this paper, the contact resistance between brushes and contact resitance between brush and commutator due to the short circuit current during operatioh are measured and discussed.
Investigation of tensile strength measurement of electrographite electrode are described. Specimens of various cross section are tested. The two kinds of chuck are used, one is standard 20° chuck for metallic pieces and the other is 90° chuck for cement tests. The tensile strength tester is Amsler type. Some important results are reported.