Images(5)

AC mil-ammeter

Datecirca 1900

Inventory Number0317

ClassificationAmmeter

Subject

Maker Weston Electrical Instrument Corporation founded 1888

User Department of Physics, Harvard University founded 1884

Cultural Region

Place of Origin

Place of Use

Dimensions20.1 × 19 × 8.7 cm (7 15/16 × 7 1/2 × 3 7/16 in.)

Material

DescriptionAn ammeter consisting of a wooden box with an arc-shaped window containing a scale that goes from 0 to 750 (milliamperes). Over the scale moves a pointer held by two aluminum wires that originate at a moving vane at the center of a mechanism composed of a metal tape coil and a hair spring. (Normally hidden from view by the box). There is a reflecting band under the scale.

The box has two electrical connectors on top, and there are two metal binding posts on its right side. These used to hold a leather strap, of which now only some residues remain.

Painted on the box is a blue triangle.

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)

Curatorial RemarksThe blue triangle indicates use in Physics 3 (Harvard) in the late 1920s.

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.