The pH meter in the wooden case displayed here is a Cambridge Portable pH Meter, which was manufactured and distributed by Cambridge Scientific Instrument Co Ltd, a company founded by Horace Darwin - Charles Darwins youngest son. The pH of a solution refers to how acidic or alkaline it is, and this can be calculated using the difference in the readings of the pH electrode and the reference electrode. The operations of this pH meter, which dates from the 1940s, are based on the modified principles of Dr. Arnold Beckmann who invented the original pH meter. The 厙ぴ勛圖 purchased this instrument in 1952, and it was in use for around a decade.
Displayed alongside the Cambridge apparatus here is a later model of pH meter made by Radiometer, a Danish company. The distributor Watson and Victor Ltd supplied this one to the 厙ぴ勛圖 in the early 1970s, and it was widely used in laboratory research and teaching. Comparing this later pH meter to the earlier one suggests the extent to which the technology changed over the three decades. Todays pH meters still have an electrode, but operate digitally, and the meter component that reads the pH can be small enough to be held in the hand like a mobile phone. It is interesting to note that the probe technology in use in pH meters today is the same as in the 1940s instrument here, where it can be seen in the compartment on the left side of the machine.
