After several internships in Leipzig and Offenbach he became an assistant of Carl Ludwig Moll at the Technical University of Riga.

In 1888 Arnold founded together with Heinrich Dettmann the Russian-baltic electrotechnical factory (german “Russisch-baltische Elektrotechnische Fabrik”) to build electric generators.

Afterwards, in 1891, he replaced Charles Eugene Lancelot Brown as leading electrical engineer at Oerlikon.

In cooperation with his assistant Jens Lassen La Cour, he published his book on alternating current machines (german „Die Wechselstromtechnik“), which includes five volumes.

In 1905 he was appointed Geheimer Hofrat and he received the honorary doctor degree from Leibniz University Hannover.