A portable meter for measuring low frequency currents in the human body.

A portable meter has been developed for measuring low frequency currents that flow in the human body. Although the present version of the meter was specifically designed to measure 50/60 Hz "contact currents," the principles involved can be used with other low frequency body currents. Contact currents flow when the human body provides a conductive path between objects in the environment with different electrical potentials. The range of currents the meter detects is approximately 0.4-800 microA. This provides measurements of currents from the threshold of human perception (approximately 500 microA(RMS)) down to single microampere levels. The meter has a unique design, which utilizes the human subject's body impedance as the sensing element. Some of the advantages of this approach are high sensitivity, the ability to measure current flow in the majority of the body, and relative insensitivity to the current path connection points. Current measurement accuracy varies with the accuracy of the body impedance (resistance) measurement and different techniques can be used to obtain a desired level of accuracy. Techniques are available to achieve an estimated +/-20% accuracy.

Rate of occurrence of transient magnetic field events in U.S. residences.

Recent interest in the transient magnetic field events produced by electrical switching events in residential and occupational environments has been kindled by the possibility that these fields may explain observed associations between childhood cancer and wire codes. This paper reports the results of a study in which the rate of occurrence of magnetic field events with 2-200 kHz frequency content were measured over 24 h or longer periods in 156 U.S. residences. A dual-channel meter was developed for the study that, during 20 s contiguous intervals of time, counted the number of events with peak 2-200 kHz magnetic fields exceeding thresholds of 3. 3 nT and 33 nT. Transient activity exhibited a distinct diurnal rhythm similar to that followed by power frequency magnetic fields in residences. Homes that were electrically grounded to a conductive water system that extended into the street and beyond, had higher levels of 33 nT channel transient activity. Homes located in rural surroundings had less 33 nT transient activity than homes in suburban/urban areas. Finally, while transient activity was perhaps somewhat elevated in homes with OLCC, OHCC, and VHCC wire codes relative to homes with underground (UG) and VLCC codes, the elevation was the smallest in VHCC and the largest in OLCC homes. This result does not provide much support for the hypothesis that transient magnetic fields are the underlying exposure that explains the associations, observed in several epidemiologic studies, between childhood cancer and residence in homes with VHCC, but not OLCC and OHCC, wire codes.

Small integrating meter for assessing long-term exposure to magnetic fields.

A small, lightweight meter has been developed for magnetic-field measurements, particularly those needed for exposure-assessment purposes. This meter, known as the AMEX-3D, continuously measures all three axes of magnetic-flux density and electronically combines the data into a single estimate of cumulative exposure to the root-mean-square (rms) resultant flux density. The AMEX-3D weighs about 120 g, measures 2.7 cm x 5.1 cm x 10.2 cm, and is battery powered. Two panel-mounted jacks are provided for measuring battery voltage and for reading cumulative exposure data from the unit. The instrument has, within 3 dB, a flat response to magnetic flux densities at all frequencies in its 30-1,000 Hz bandwidth. A detailed analysis of error sources in the AMEX-3D leads to an estimate of +/- 20% as the accuracy of the instrument over its dynamic range, which extends from 0.02 to 15 microT. The AMEX-3D was tested in the field by asking electric-utility distribution linemen to wear AMEX-3D and EMDEX meters simultaneously while working. Agreement between the two measures of exposure was excellent.