Portable electrocardiograph, 1946.

Electrocardiograph made by the Cambridge Instrument Company, England, shown here in its leather carrying case, with the case open. The electrocardiograph detects and records the tiny electrical signals that coordinate the heart's beats, and which can indicate heart disorders. The signals ripple outwards and are picked up by metal sensors stuck to the skin. Willem Einthoven (1860-1927) developed the electrocardiograph during the early 1900s. Portable versions date from 1928.