A comparison of American direct-current switchboard voltmeters and ammeters
A 1911 technical report from the National Bureau of Standards comparing direct-current switchboard voltmeters and ammeters, written for instrument makers, testing laboratories and electrical engineers. Halftone photographs show stripped instrument movements from above and in profile — horseshoe magnets, coil assemblies, terminal screws and pointer springs — giving designers heavy dark silhouettes and small mechanical details against flat grey grounds.
