In a system of arc lighting, however, we have to deal with entirely
different conditions; for, while in the incandescent system the adding
of a lamp, which diminishes the resistance, requires no increase of
electromotive force, the contrary is the case in the arc light system.
Here every additional lamp added to the circuit means an increase in
resistance, and consequent increase in electromotive force or potential.
Taking for example a well known system of arc lighting, we find that the
lamps require individually an electromotive force of 40 volts with a
current of 10 amperes. In other words, the difference in potential at
the two terminals of every such lamp is 40 volts. Consequently, if the
circuit were touched in two places, including between them only one
lamp, no injurious effects would ensue. If we touch the circuit so as
to include two lamps between us, the effect would be greater, since the
potential between those two points is 2 x 40 volts. We might continue
in this manner touching the circuit until we had included about 7 or
8 lamps, when the shock would become fatal, since the point would be
reached at which the difference of potential is great enough to send a
dangerous current through the body.
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