Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A room temperature 19-channel magnetic field mapping device for cardiac
  signals   We present a multichannel cardiac magnetic field imaging system built in
Fribourg from optical double-resonance Cs vapor magnetometers. It consists of
25 individual sensors designed to record magnetic field maps of the beating
human heart by simultaneous measurements on a grid of 19 points over the chest.
The system is operated as an array of second order gradiometers using
sophisticated digitally controlled feedback loops.

5
The categories of this paper are: gr-qc
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The categories of this paper are: astro-ph
13
The categories of this paper are: quant-ph
35
The categories of this paper are: mtrl-th
29
The categories of this paper are: funct-an
8
The categories of this paper are: hep-ex
32
The categories of this paper are: atom-ph
17
The categories of this paper are: q-fin
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The categories of this paper are: q-bio
25
The categories of this paper are: patt-sol
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The categories of this paper are: nlin
24
The categories of this paper are: dg-ga
11
The categories of this paper are: hep-lat
21
The categories of this paper are: adap-org
1
The categories of this paper are: cs
22
The categories of this paper are: chao-dyn