pH meter: A simple and speedy device to measure the acidity and alkalinity of a fluid. A pH meter acts as a volt meter that measures the electrical potential difference between a pH electrode and a reference electrode and displays the result in terms of the pH value of the solution in which they are immersed.
The pH meter was invented in 1934 by the American chemist Arnold O. Beckman (1900-2004) to measure the sourness of lemons. Beckman's original device was housed in a walnut box 12 inches wide, 8 inches deep and 9 inches high. It measured electrical current flowing into a glass electrode immersed in a solution. The amount of current indicated the solution's acidity.