A Signal Too Small to See
The heart’s rhythm is controlled by electrical impulses. These produce tiny voltages that spread to the skin, but the signal is only a few thousandths of a volt at best, far too small for ordinary meters of the nineteenth century. Early physiologists nonetheless showed that electrical activity could be detected in animal hearts. The challenge was to record it quickly enough to follow a heartbeat, which lasts well under a second.
Waller and the Capillary Electrometer
In 1887 the British physiologist Augustus Waller presented what is usually described as the first recording of a human heart’s electrical activity. He used a capillary electrometer, a glass tube containing mercury and acid in which the boundary moved in response to a voltage, and he photographed the movement. His human subjects had their hands and feet in jars of salt water, which served as electrodes. The instrument was sluggish and distorted the signal, so the traces were rough, and Waller himself did not see a major medical use for the technique.
Einthoven and the String Galvanometer
The Dutch physiologist Willem Einthoven, working in Leiden, took the idea further. He knew that the capillary electrometer was too slow, and he developed a much more sensitive device: the string galvanometer. At its core was a very thin, conductive thread, originally made of fine quartz coated with metal, stretched between the poles of a powerful electromagnet. When a tiny current from the patient flowed through the thread, it moved sideways in the magnetic field. A strong light shone on it and the shadow of its movement was projected onto a moving strip of photographic paper, leaving a continuous curve.
Einthoven described the method in the early 1900s, with key publications between about 1901 and 1903, and by 1908 a commercial version was being made in England. He also named the main deflections of the trace using letters, including P, QRS and T, and these labels are still used. He proposed that readings be taken between pairs of limbs, which became three standard leads (I, II and III), sometimes described using the idea now known as Einthoven’s triangle. He received the Nobel Prize in Physiology or Medicine in 1924.
What Early Machines Were Like
The first practical machines were large:
- They are often described as weighing hundreds of pounds, with water-cooling for the electromagnet and a team of people needed to operate them.
- The patient sat with both hands and one foot in buckets of salt water, which acted as electrodes.
- Photographic paper had to be developed before a trace could be read.
In a well-known experiment around 1905, Einthoven’s laboratory was linked by cable to the hospital about a mile or so away, so that a patient’s heart signal could be transmitted and recorded in the lab. This is sometimes called an early form of telecardiology. The principle relied on the same kind of line signaling that had been developed for the electric telegraph in the previous century.
From Laboratory to Clinic
Early on, the ECG was a research instrument. Physicians such as Thomas Lewis in Britain used it to study irregular rhythms and showed that it could identify specific disorders, including atrial fibrillation. Other clinicians in Europe and North America added to the interpretation of the traces. During the 1910s and 1920s hospitals bought machines, and by the 1920s and 1930s the ECG was becoming a routine part of the diagnosis of heart disease, with the recognition that the changes seen in a heart attack have a characteristic pattern.
Further changes made the device smaller and cheaper:
- Vacuum tube amplifiers replaced the very sensitive string with an electronic signal and a stylus writing on heat-sensitive or ink paper.
- Chest leads, added in the 1930s, allowed more detailed views of the heart.
- Portable machines allowed recording at the bedside or in an ambulance.
- Long-term recorders, developed in the mid-twentieth century, allowed a record over a day or more.
- Modern machines are digital and can include automated measurements and stored records.
Why the ECG Mattered
The electrocardiogram gave doctors a record, not just an impression. It could be stored, compared and shared, which suited a growing scientific approach to diagnosis. It also supported later technology that depends on understanding heart rhythm, including cardiac pacing and devices to deliver a shock to correct a dangerous rhythm.
Reading an ECG remains a skill that needs training, and a trace alone does not say whether someone is healthy. This article is history, not a guide to interpreting results or choosing tests.