This advertisement for the Model G pH meter highlights the different electrodes that could be used with it. The Model G was Beckman Instruments's first commercial pH meter.

Arnold Beckman invented his first pH meter in 1934 at the request of a chemist from the California citrus industry, who needed an accurate way to measure the acidity of his product. The resulting “acidimeter” with its glass electrode was renamed the Model G pH meter in 1937 and produced on a larger scale by Beckman’s company, National Technical Laboratories. This instrument kicked off the rapid development not only of NTL and Beckman Instruments, but also of the electronic scientific instrument industry.