Signedbelow scale: VOLTS / JAMES G. BIDDLE, PHILADELPHIA / 30 VOLTS ca 101 OHMS
Inscribedto the left of scale: No. 443760
diagram on lid marked: ORDRE 1256/13 INSTR 69244 No. 13355
on dial in German: Mit vertikaler Skalenstellung zu benutzen [translation: To be used with the scale placed vertically]
FunctionThis device can measure direct and alternate voltage of any frequency. It does so by determining the power consumption through a fine wire of known resistance by measuring the increase in its length cause by the heat produced. This is translated to a voltage reading on the scale. Below is a detailed description from the original instrument's card in the museum.
"Fine wire stretched between two posts: one post under the scale. The other post at the bottom of the instrument. About the center of the wire, a thread is taken off perpendicularly, making one turn around a pulley which is on the spindle carrying the pointer. The thread is then attached to a flat spring which takes up the slack in the wire due to heating by the current. A D-shaped aluminum disc is attached to a spindle which passes between the poles of a permanent magnet for damping. All mounted on a circular bakelite disc".
ProvenanceFrom the Harvard Engineering School. Removed to the collection in 12/1956.