Signedon oval plaque: WESTON / MODEL 433 No. 413 / WESTON ELECTRICAL INSTRUMENT CO., / NEWARK, N.J., U.S.A.
Inscribedaround screw in middle of oval plaque: ADJUST FOR ZERO
below scale: VOLTS / A.C. / RES.FOR 125V [illegible follows]
FunctionThis is an alternating current voltmeter, most likely of a moving iron type in which the magnetic field produced by a coil makes a piece or iron rotate, which has the needle attached.