The string galvanometer was one of the earliest instruments capable of detecting and recording the very small electrical currents produced by the human heart and provided the first practical Electrocardiogram (ECG). The original machines achieved "such amazing technical perfection that many modern day electrocardiographs do not attain equally reliable and undistorted recordings".
Previous to the string galvanometer, scientists were using a machine called the capillary electrometer to measure the heart’s electrical activity, but this device was unable to produce results of a diagnostic level. Willem Einthoven invented at Leiden University the string galvanometer in the early 20th century, publishing the first registration of its use to record an electrocardiogram in a Festschrift book in 1902. The first human electrocardiogram was recorded in 1887, however it was not until 1901 that a quantifiable result was obtained from the string galvanometer. He was awarded the Nobel prize in Physiology or Medicine in 1924 for his work.
Einthoven's invention consisted of a very long (several meters) silver-coated quartz filament of negligible mass that conducted the electrical currents from the heart. This filament was acted upon by powerful electromagnets positioned either side of it, which caused sideways displacement of the filament in proportion to the current carried due to the electromagnetic field. The movement in the filament was heavily magnified and projected through a thin slot onto a moving photographic plate.
The filament was originally made by drawing out a filament of glass from a crucible of molten glass. To produce a sufficiently thin and long filament an arrow was fired across the room so that it dragged the filament from the molten glass. The filament so produced was then coated with silver to provide the conductive pathway for the current. By tightening or loosening the filament it is possible to very accurately regulate the sensitivity of the galvanometer.
Patients are sat with both arms and left leg in separate buckets of saline solution. These buckets act as electrodes to conduct the current from the skin's surface to the filament. The three points of electrode contact on these limbs produces what is known as Einthoven's triangle, a principle still used in modern day ECG recording.
- 'Einthoven's String GalvanometerThe First Electrocardiograph', Moises Rivera-Ruiz et al, Tex Heart Inst J. © 2008 by the Texas Heart Institute 
- ‘ECG Complete’, S. Bowbrick and A.N. Borg 2006, Elsevier Limited Page 2.
- ‘ECG Complete’, S. Bowbrick and A.N. Borg 2006, Elsevier Limited Page 10.
- Einthoven (1901)
- A History of Electrocardiography pg 112-113
- ‘ECG Complete’, S. Bowbrick and A.N. Borg 2006, Elsevier Limited Page 9-10.