Electrical doubler. P.79.

Photograph: David Remes (2003)

Some of Darwin’s inventions were inspired by his medical practice. He experimented with various therapeutic methods including electrotherapy, which was coming into fashion in the late 18th century. For electrical treatment he needed a special generating machine. Among his designs of 1778 is an ‘electrical doubler’ which was quite successful and became well known at that time. Darwin’s friend Abraham Bennet (1750-1799) to whom Darwin passed his improvements, later described Darwin’s work in the following way:

Dr Darwin made the first attempt with two plates mowing between two others by a lever, so as to bring them exactly to the same position in each operation. This contrivance he soon improved by another instrument in which the plates stood vertically and moved by rack work in a direction exactly parallel to each other.

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