The existence of electromotive phenomena in living tissue has been know since the 1794 experiments of Aloysio Luigi Galvani, Professor of Anatomy at Bologna University. However it was not until the development in 1901 of the String Galvanometer by William Einthoven of Leiden University that it was possible to accurately record the electrical activity of the heart. This functioned by using optical devices to record the displacement of a string of conductive material, the displacement caused by surrounding electromagnets reacting to the hearts electrical current passing along the string.

Einthoven eventually entered into an agreement with the Cambridge Scientific Instrument Company, a company founded by Charles Darwin’s nephew Horace, to produce electrocardiographs. The first table model was developed by the company in 1911 but these were large and often required a number of people to operate, so there was a requirement for more portable machines.

In 1926 the company developed a portable electrocardiograph which could be placed on a trolley and weighed approximately 80 pounds. The example shown is a later but still early model from circa 1930. It is made primarily of metal and synthetics and comes with a metal, trunk casing for easy portability. In the picture opposite the String Galvanometer is the central horizontal cylinder; the light beam transmitting the deflections would pass from the box on the right through the narrow slit in the box on the left  and onto a roll of photographic film inserted behind. The three black switches visible on the left operated shutters that could be opened or closed over the slit as needed.