Wikipedia
The string galvanometer, also known as the Einthoven galvanometer, invented around 1901 by Dutch physician Willem Einthoven was the first practical electrocardiograph (ECG); it was one of the earliest instruments capable of detecting and recording the very small electric currents produced by the human heart and produced the first reliable electrocardiograms. The original machines achieved "such amazing technical perfection that many modern day electrocardiographs do not attain equally reliable and undistorted recordings". Einthoven was awarded the 1924 Nobel prize in Physiology or Medicine for his work.
Usage examples of "string galvanometer".
The person with murder in his heart cannot hide it from the string galvanometer, nor can that person who wrote the false note in which the very lines of the letters betray a diseased heart hide that disease.