Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Extracting Energy from an External Magnetic Field   In this paper we describe the theory of a device that is able to extract
energy from an external magnetic field. The device is a cylindrical magnetic
insulator that once put in rotation makes electromagnetic angular momentum to
be stored in the electromagnetic field in contrary direction to the mechanical
angular momentum of the device. As a consequence due to total angular momentum
conservation the device increases its angular velocity (when {\epsilon}{\mu}>1)
Natural units are used in the paper and all conservation laws are rigorously
satisfied. The voltage generated by the device is found solving explicitly
Maxwell equations for rotating magnetic insulators in external fields, a
subject that have provoked lots of polemics in the literature and which we hope
to be here clarified due to our pedagogical presentation.

26
The categories of this paper are: bayes-an
7
The categories of this paper are: hep-th
25
The categories of this paper are: patt-sol
9
The categories of this paper are: nlin
23
The categories of this paper are: solv-int
10
The categories of this paper are: q-bio
19
The categories of this paper are: eess
37
The categories of this paper are: supr-con
33
The categories of this paper are: plasm-ph
21
The categories of this paper are: adap-org
15
The categories of this paper are: nucl-ex
36
The categories of this paper are: cmp-lg
22
The categories of this paper are: chao-dyn
6
The categories of this paper are: astro-ph
24
The categories of this paper are: dg-ga
29
The categories of this paper are: funct-an