The present volume consists mainly of the earlier investiga tions which Heinrich Hertz carried out before his great electrical researches. Hitherto they have been difficult of access, being scattered amongst various journals, and some his inaugural dissertation) could scarcely be obtained at all. Of later date are the last experimental investigation, the Heidelberg lecture (published by the firm of Emil Strauss in Bonn, by whose kind permission it is included in the present volume), and the closing paper, which is a further proof of the gratitude and admiration which Hertz cherished towards his great master, who has now followed him. The papers are for the most part arranged in the order of their publication. By the kindness of Senator Dr. Gustav Hertz I have been able to include in the Introduction extracts from Hertz's letters to his parents, which give us an insight into the course of his scientific development, and the way in which he was led to attack the problems herein discussed.
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