What is the etymology of "la magneto"?

Question

La magneto is rare among -o terminal words in that it is feminine (at least in some dialects). The DLE gives its ultimate etymology as:

Del lat. magnes, -ētis 'imán¹'.

Given the word has clearly not been directly inherited from Latin^{1 2}, what is its etymology? Does it explain why it is feminine?

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<sup>1. The only occurrence of magneto before 1881 in the RAE's CORDE is one example from the 14th century: "... turniano magneto leboreto..." (Gran crónica de España, Juan Fernández de Heredia (1385)).
2. The only pre-19th century occurrences of it in Google ngrams are references to Ancient Magnetum (Magneto in Portuguese).</sup>

Answer 1

If we look at its first modern use in Spanish, the earliest appearances of magneto in Spanish are as a compound adjective in the phrases "máquina magneto-eléctrica" and "aparato magneto-eléctrico". It appears the former term was more common, and the abbreviated form magneto eventually superseded it in popularity.

This etymology is confirmed in the following dictionary:

magneto, abrev. de máquina magneto-eléctrica [< lat. magnēs, -ētis, 'iman'], f.

• Lexicón etimológico y semántico del Latín y de las voces actuales que proceden de raíces latinas o griegas, Santiago Segura Munguía (p.342)

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The similar word dínamo appears to be feminine for analogous reasons:

L'Electrical Review de Londres propone dos medios de utilizar la fuerza del viento conmbinada con la del agua para obtener una fuerza regular y constante con aplicación á las máquinas dínamo-eléctricas.

• Revista agustiniana dedicada al Santo Obispo de Hipona en su admirable conversión á la fe, Volume 6 (1883)