EKGs and EEGs
Electricity is useful in sending signals, since electrical impulses can travel extremely rapidly. Communication tools, such as the telegraph and the telephone, both invented in the 19th century, take advantage of this. But electrical signaling is much, much older. Animals with nervous systems use electrical impulses for sensing and responding to their environments. It is electrical impulses traveling along nerve fibers that convey sensations, such as heat, or cause muscles to contract.
Scientists and doctors learned how to use the human body’s electrical activity to understand and diagnose medical problems. If a nerve fiber is exposed, one can place electrical contacts on the nerve fiber and record a nerve impulse. Fortunately, it turns out that the electrical activity can often be detected from the surface of the body. In 1887 the English researcher Augustus Waller showed that one could record the changing heart voltages from the surface of the chest. The technique of recording the electrical activity of the heart, known as electrocardiography, was greatly advanced by the Dutch researcher Willem Einthoven, who invented a new means of measuring current (the string galvanometer). Two new electronic devices, which became widely available in the 1920s, were even bigger improvements. These were the oscilloscope (a type of cathode-ray tube, similar to a computer monitor or TV receiver) and the electron-tube amplifier (used to make an extremely feeble current stronger so that it could be recorded).
The recorded signal of the heart is called an electrocardiogram (ECG) or EKG (from the German spelling of the word). A related technique, recording the electrical activity of skeletal muscle, produces an electromyograph or EMG. A recording of the electrical activity of the brain, obtained from the surface of the head, is called an electroencephalogram or EEG. Electroencephalography was developed by the German psychiatrist Hans Berger in the 1920s; it underwent important improvements in the 1930s and became widely used in the 1940s.