This facility powered a lot of Boston's surface and elevated transit system in the early days. These very heavy copper cables carried approximately 600 volts direct current. The insulators in view were composite ones. Those I have seen were marked J.M. Anderson Co. Boston around the inside (near the pinhole area). They had one-inch pinholes and were common throughout Boston's overhead transit system. Some are still in use; others have been replaced by standard light gray cabletop insulators. Click "Previous" for an overall look at the plant and more of this line extending from it.