Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On a theory of electricity and gravitation   This is the second paper by Nordstr{\"o}m on his five dimensional theory. It
aims at generalizing the relations $\mathbf{D} = \epsilon \mathbf{E}$,
$\mathbf{B} = \mu \mathbf{H}$ of electromagnetism to the five dimensional
gravito-electric theory and to show that this is consistent with the principle
of equivalence. Some steps are taken toward providing a foundation for a theory
of matter. Nordstr{\"o}m hopes, as stated in the conclusion, to show ''that the
five dimensional perspective, while still having only a formal content, may
however provide footholds for further developments of the theory of
gravitation, which otherwise would have been hard to find''.

13
The categories of this paper are: quant-ph
4
The categories of this paper are: cond-mat
27
The categories of this paper are: comp-gas
33
The categories of this paper are: plasm-ph
32
The categories of this paper are: atom-ph
11
The categories of this paper are: hep-lat
9
The categories of this paper are: nlin
14
The categories of this paper are: math-ph
17
The categories of this paper are: q-fin
31
The categories of this paper are: ao-sci
10
The categories of this paper are: q-bio
16
The categories of this paper are: stat
35
The categories of this paper are: mtrl-th
7
The categories of this paper are: hep-th
34
The categories of this paper are: chem-ph
28
The categories of this paper are: alg-geom