The strength of a steady current depends upon these two factors only,
the electromotive force which causes it and the resistance which it
has to overcome.
317. Resistance. Since resistance plays so important a role in
electricity, it becomes necessary to have a unit of resistance. The
practical unit of resistance is called an ohm, and some idea of the
value of an ohm can be obtained if we remember that a 300-foot length
of common iron telegraph wire has a resistance of 1 ohm. An
approximate ohm for rough work in the laboratory may be made by
winding 9 feet 5 inches of number 30 copper wire on a spool or
arranging it in any other convenient form.
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