In the late 1700s medical researchers learned that muscles produce tiny electric impulses now known as "action potentials." Italian biophysicist, Carlo Matteucci (1811-1868), identified action potentials in a pigeon's heart in 1843 and, in 1856, German scientists Rudolf Albert von Kölliker (1817-1905) and Heinrich Müller (1820-1864) recorded these electric currents from a frog's heart. Reasoning that these recordings could reveal irregularities and, hence, heart disease, researchers attempted to develop accurate measuring devices. The French physiologist, Augustus Waller (1856-1922), found that cardiac currents could be recorded by placing surface electrodes on the body. In 1887, Waller developed a capillary electrometer--tubes of mercury that rose and fell with the changes in heart muscle current--which, however, was imprecise and difficult to use. So Dutch physiologist Willem Einthoven set out to design an improved apparatus. In 1903 he described the result, his string galvanometer consisting of a thin, silver-coated quartz stretched between...