Whom do we have to thank for this clever stuff? First, Nobel-Prize winning German chemist Fritz Haber (1868–1934) and his student Zygmunt Klemensiewicz (1886–1963) developed the glass electrode idea in 1909. The modern, electronic pH meter was invented about a quarter century later, in 1934, when American chemist Arnold Beckman (1900–2004) figured out how to hook up a glass electrode to an amplifier and voltmeter to make a much more sensitive instrument. He was a granted a patent in October 1936.
Artwork: Arnold Beckman's original amplifier pH meter from his 1936 patent. When a pair of electrodes (a glass electrode, 14, on the left, and a second electrode, 15, on the right) are suspended in the test solution (blue), a voltage (potential difference) is generated between them proportional to the pH of the test solution. This device is wired into a vacuum-tube amplifier circuit (not shown) with a simple ammeter showing the pH. From US Patent 2,058,761: Apparatus for testing acidity by Arnold Beckman et al, National Technical Laboratories, courtesy of US Patent and Trademark Office.