
String galvanometer

In the late 1800s, telegraphy was developing as a way for distant communication. Messages were converted to dots and dashes that were sent as electric pulses and could be converted to sound or visual signals at the distant site. That conversion was done by a coil in a galvanometer, which had a limited frequency. Clément Adair, a French engineer, replaced the coil with a much faster wire or "string" producing the first string galvanometer. Augustus Waller had discovered electrical activity from the heart and produced the first electrocardiogram in 1887. But his equipment was slow. Physiologists worked to find a better instrument. In 1901, Willem Einthoven described the science background and potential utility of a string galvanometer, stating "Mr. Adair as already built an instrument with a wires stretched between poles of a magnet. It was a telegraph receiver." Although Einthoven is sometimes credited with inventing it, he did not invent the string galvanometer. He was a leader in applying the string galvanometer to physiology and medicine, leading to today's electrocardiography. Einthoven was awarded the 1924 Nobel prize in Physiology or Medicine for his work.
History.
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 adapted the string galvanometer at Leiden University 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. In 1908, the physicians Arthur MacNalty, M.D. Oxon, and Thomas Lewis teamed to become the first of their profession to apply electrocardiography in medical diagnosis.
Mechanics.
Einthoven's galvanometer consisted of a silver-coated quartz filament of a few centimeters (see picture on the right) and 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 shot 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.
The original machine required water cooling for the powerful electromagnets, required 5 operators and weighed some 600 lb.
Procedure.
Patients are seated 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.