Synthesized answer
The passages indicate that a question remains regarding the essentiality of very high frequency for the production of the presented results. Even with the author's current experience, answering this question would be difficult [Passage 1, Passage 2].
While some experiments might be performed with low frequencies, very high frequencies are noted as desirable due to the numerous effects they secure and their convenience in obtaining the high potentials necessary for most of the contemplated experiments [Passage 1]. However, the exact degree to which very high frequency is essential is explicitly stated as an unanswered question [Passage 1, Passage 2].
Synthesized from the book passages below. Chat with the book on Feynman for follow-up.
From the book
frequency is essential for the production of the results presented is a question which, even with my present experience, would embarrass me to answer. Some of the experiments may be performed with low frequencies; but very high frequencies are desirable, not only on account of the many effects secured by their use, but also as a convenient means of obtaining, in the induction apparatus employed, the high potentials, which in their turn are necessary to the demonstration of most of the experiments here contemplated.
Title: Experiments with alternate currents of high potential and high frequency by Nikola Tesla Description: A lecture delivered before the Institution of Electrical Engineers, London, February 1892. Complete with illustrations.A short time ago I had the honor to bring before our American Institute of Electrical Engineers some results then arrived at by me in a novel line of work. I need not assure you that the many evidences which I have received that English scientific men and engineers were interested in this work have been for me a great reward and encouragement. I will not dwell upon…