Self-administered transcranial electrical stimulation (TES) is not new (see Nature 498, 271–272; 2013). Between the 1740s and 1930s, there was widespread and unregulated commercial availability of TES machines for private and domestic use (see, for example, I. R. Morus in Frankenstein's Children, Princeton University Press; 1998).

During Victorian and Edwardian times, TES machines that dispensed static, frictional, faradic or battery electrical current could be bought everywhere. Some physicians, therapists and patients claimed that TES could generate feelings of euphoria and even improve mental performance.

Physicians of the period recommended that currents of no more than 5–10 milliamps should be applied to the head because of the risks of burning and shock, although many users chose to experiment. Side effects included headaches, flashes of light, dizziness and nausea, especially when connections were imperfect or broken.

The consequences could be even more serious. The Dutch physician Jan Ingenhousz knocked himself unconscious when performing electrical experiments in 1783, and Benjamin Franklin suffered retrograde amnesia after accidentally administering an electric shock to his head.

As today, some medical practitioners of the time believed the benefits of TES to be exaggerated. Footnote 1