Magnetic influence on portable instruments

The performance of electric instruments in magnetic fields has been studied as related to portable voltmeters, ammeters, and wattmeters and, a brief survey of certain items of laboratory equipment whose external fields might affect nearby instruments has been made. The influence of magnetic fields on two common types of portable instruments is shown in Figures 1 and 2. The instrument under test was placed in a given magnetic field whose direction was adjusted to produce maximum effect. When a-c fields were used, their phase angles also had to be adjusted. Test instrument readings were held constant by means of a reference instrument so located as to be unaffected by the magnetic field. Readings taken were repeated with the field reversed in direction. Half the difference between the two readings, expressed as a percentage of full-scale deflection, was taken as the influence of the field. Manufacturer´s catalogue statement of accuracy, and the approximate length of the scale are shown in the curves to indicate the general instrument quality. It is emphasized that accuracy specified by the manufacturer does not cover such conditions as the presence of magnetic fields.