A zero current vacuum tube galvanometer

The Early Development of Electronic pH Meters

“…The papers that followed up on Goode’s pH meters fell into two broad categories: those that embraced its continuous-reading aspect and described improvements by using newly available tubes and/or alternative circuits − , and those that merely incorporated one or more vacuum tubes with a milliammeter instead of the usual null detector (until then: a more sensitive galvanometer, a capillary electrometer, or a four-quadrant electrometer) in otherwise traditional potentiometer designs ,− . For example, Treadwell reported that he had reproduced Goode’s results with a vacuum triode of different manufacture, and had likewise obtained a wide range of grid voltages for which the output voltage was, to within 1%, a linear function of the grid potential.…”

“…These papers − also highlighted some of the difficulties with such early applications of vacuum tubes to high-impedance sources. The sockets in which those tubes were mounted sometimes leaked so much electricity (thereby reducing the input resistance of the pH meter) that Elder and Wright found it necessary to remove them, and solder their wires directly to the pins of the vacuum tube.…”

“…Even though many of these authors − merely used the vacuum tube as a high-impedance null-detector for comparing the cell potential with that of a manually operated reference potentiometer in an otherwise by then conventional circuit, they clearly established that the vacuum tube could be used directly with glass electrodes. Of course, the developments of new insulating materials and solid-state electronics have since made it possible to build much smaller, less power-hungry, and less expensive pH meters, which have greatly contributed to their widespread use.…”

“…References − were restricted arbitrarily to those papers that were published between 1925 and 1933, and no completeness is claimed for even that short period. There were many subsequent papers based on and referring to these earlier studies, but a cutoff had to be made somewhere.…”

“…Shortly after Beckman started working on his pH meter, Sørensen asked Radiometer, then a new Danish maker of vacuum tubes established in 1935 by Børge Aagaard Nielsen and Carl Schrøder, to produce pH meters, and by the end of 1937, Radiometer brought out its first commercial pH meter, their PHM1. Like the Lautenschläger, Ströhlein, Cambridge, and Beckman instruments and many of their noncommercial predecessors − , it was a potentiometer-based instrument with vacuum tubes as input for its null detector. Like Cambridge and Beckman, Radiometer soon also produced glass electrodes.…”