(p. 174) In Edison’s view, . . . , Westinghouse did not pose a serious threat in the power-and-light business because he used the relatively more dangerous alternating current, certain to kill one of his own customers within six months.
Edison’s conviction that direct current was less dangerous than alternating current was based on hunch, however, not empirical scientific research. He, like others at the time, focused solely on voltage (the force that pushes electricity through a wire) without paying attention to amperage (the rate of flow of electricity), and thought it would be best to stay at 1,200 volts or less. Even he was not certain that his own system was completely safe–after all, he had elected to place wires in underground conduits, which was more expensive than stringing wires overhead but reduced the likelihood of electrical current touching a passerby. Burying the wires could not give him complete peace of mind, however. Privately, he told Edward Johnson that “we must look out for crosses [i.e., short-circuited wires] for if we ever kill a customer it would be a bad blow to the business.”
Source:
Stross, Randall E. The Wizard of Menlo Park: How Thomas Alva Edison Invented the Modern World. New York: Crown Publishers, 2007.
(Note: ellipsis added, bracketed words in original.)