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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). Moving coil galvanometers were not at the time sufficiently sensitive to record the electrical currents involved.

Willem Einthoven solved the problem by producing a very long filament of negligible mass that conducted the electrical currents from the heart. This filament was acted upon by powerful electromagnets, which caused sideways displacement of the filament in proportion to the current carried. The movement in the filament was measured using sensitive optical devices.

The filament was originally made by drawing out a filament of glass from a crucible of molten glass.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.

The original machine required water cooling for the powerful electromagnets, required 5 people to operate it and weighed some 600 lb. Because the machine was too large to be brought to the patients bedside, the current from the heart was transmitted to the machine using a telephone cable.

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