Queen Self-Regulating Tube. Fig. 200, page 493 'Dental Electricity' by Levitt E Custer, 1901.
Licence: Attribution 4.0 International (CC BY 4.0)
Credit: Queen Self-Regulating Tube. Fig. 200, page 493 'Dental Electricity' by Levitt E Custer, 1901. Wellcome Collection. Source: Wellcome Collection.