Signedplaque on inner surface of lid: MADE BY THE / WESTON ELECTRICAL INSTRUMENT Co / NEWARK, N.J., U.S.A. / PATENTED / NOV 8, 1888, FEB. 17, 1891, MAY 14, 1889, DEC 19, 1893, JUL 16, 1901, APR 29, 1890, OCT 4, 1891 / MODEL 45 No. 12357
below scale: WESTON D.C. AMMETER MODEL 45
Inscribedover scale: USE ONLY WITH EXTERNAL SHUNT
FunctionAn ammeter for direct current. It is most likely a galvanometer. Seems to be designed for portability.