World War I interrupted his research in England and he was called to serve in the Austro-Hungarian army as a dispensing chemist. Nonetheless, he continued his experiments in the hospital pharmacy and was able to prepare a Ph.D. dissertation while still a soldier.

During his thesis defense at Prague University (1918), he met Prof. Kucera, who invited Heyrovský to join his research group to study the dropping mercury electrode (DME) for electrocapillary measurements. This was tedious work. A voltage was applied to a DME and a reference electrode was immersed in a test solution. After 50 drops of mercury were collected, they were dried and weighed. The applied voltage was varied and the experiment repeated. Measured weight was plotted vs. applied voltage to obtain the curve. Heyrovský continued this work in his own laboratory and eliminated the weighing step by monitoring drop-time. In 1921, he had the idea of measuring the current flowing through the cell instead of just studying drop-time. Using an amperometer, he made his first experiment on New Year's Day, 1922. It didn't work. Heyrovský was undaunted and borrowed a sensitive galvanometer to measure currents flowing from an electrolytic cell with a potentiometer as the voltage source.

On February 10, 1922, the “polarograph” was born as Heyrovský recorded the current-voltage curve for a solution of 1 mol dm-3 NaOH. Heyrovský correctly interpreted the current increase between -1.9 and -2.0 V as being due to deposition of Na+ ions, forming an amalgam. From this beginning, the measure-ment of polarographic current was extended to fundamental and theoretical studies of electrode processes, accompanying chemical reactions and analysis. Heyrovský was possibly the most powerful influence on the development of electroanalytical science in the twentieth century.

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