Signedbelow scale: WESTON A.C. MIL-AMMETER
on old, darkened, metal plate: MADE BY THE / WESTON ELECTRICAL INSTRUMENT Co. / NEWARK, N.J., U.S.A. / PATENTED [several numbers follow] / OTHER PATENTS PENDING / MODEL 155 No. 378.
Inscribedsticker on box: L= 2.24mh
written on box: 750 mil Amp
written on box: AC M .750 R=1.11 (ohm)
FunctionThis is an ammeter for AC current. It most likely works by deflection of its needle by a magnetic field generated at a coil through which the current to be measured passes. The sticker marked with "L=2.24 mh" indicates its inductance (AC resistance) when connected to a circuit.
For more information on this instrument, see the patent number 427,022 granted in 1898 to E. Weston, here.
ProvenanceFrom the Department of Physics, Harvard University.