scholar.archive.org (Chrome): "Annoying anti-scrape countermeasure" page with an ALTCHA "I'm not a robot" checkbox. Blocked (CAPTCHA), not attempted. Query tried: "unipolar induction" year:[1900 TO 1925]. Curl searches for "Kennard unipolar induction", "effect of dielectrics on unipolar induction" and "Pegram unipolar induction electron theory" returned no parsable results.
scispace.com: the Pegram 1917 PDF link (https://scispace.com/pdf/unipolar-induction-and-electron-theory-4vzw6ooe9q.pdf) rendered blank in Chrome and gave HTML to curl. Blocked.
kirkmcd.princeton.edu/examples/EM/: hosts copies of Kennard 1912 (kennard_pm_23_937_12.pdf), Kennard 1917 (kennard_pm_33_179_17.pdf), Pegram 1917 (pegram_pr_10_591_17.pdf), Barnett, Howe 1915 (Electrician 76, 169), etc. They are listed in McDonald's bibliography. Direct fetch returned "401 Unauthorized" (password-protected). The main PDF http://kirkmcd.princeton.edu/examples/faradaydisk.pdf downloaded fine. Not retried with credentials (never log in).
semanticscholar.org website: JS page, no text. The API worked until it hit the rate limit (429).
archive.org in Chrome: page-read permission denied. The API was used via curl and worked.
HathiTrust full-text search: see hathitrust note (partial).