A Shielded Facility for Low-Level Magnetic Measurements

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Abstract
A facility has been constructed for the detection and measurement of time‐varying magnetic fields of ∼10−9 G amplitude, in a bandwidth of 0.5 to 100 000 cps. The facility includes a shielded room and magnetic detector, which now consists of a copper coil, eventually to be cooled in liquid helium, feeding a parametric amplifier; it will initially be used to investigate bioelectric current flow in human heart muscles by mapping the time‐varying fields outside the body. The shielded room now has two layers of 0.060 in. Moly‐Permalloy, and a third inner layer of 0.19 in. aluminum enclosing a 86 in.×88 in.×88 in. volume will soon be installed. Measured shielding factors are given; at present magnetic ``shaking'' is used to produce a shielding factor of ∼400 in the 0–5 cps range, which rises rapidly with increasing frequency.

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