Currents of electricity as utilized by the electrical engineer fall naturally into three classes, viz.: those produced by chemical action in batteries; those produced by magnetic induction in dynamos; and those produced by the direct action of heat in thermo-cells.
©1931 by Iowa State Teachers College
"Practical Methods of Producing a Current of Electricity,"
Science Bulletin: Vol. 3:
7, Article 4.
Available at: https://scholarworks.uni.edu/science_bulletin/vol3/iss7/4