RESISTANCE 345 



force diminishes. Later, in Section 288, we learned that the 

 strength of the current decreases as the resistance in circuit 

 increases. 



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 r61e 

 in electricity, it becomes necessary to have a unit of resist- 

 ance. 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 3OO-foot length of common iron telegraph 

 wire has a resistance of I ohm. An approximate -ohm for 1 

 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. 



In Section 299 we learned that substances differ very 

 greatly in the resistance which they offer to electricity, and 

 so it will not surprise us to learn that while it takes 300 feet 

 of iron telegraph wire to give I ohm of resistance, it takes 

 but 39 feet of number 24 copper wire, and but 2.2 feet of 

 number 24 German silver wire, to give the same resistance. 



NOTE. The number of a wire indicates its diameter ; number 30, for 

 example, being always of a definite fixed diameter, no matter what the 

 material of the wire. 



If we wish to avoid loss of current by heating, we use a 

 wire of low resistance ; while if we wish to transform elec- 

 tricity into heat, as in the electric stove, we choose wire of 

 high resistance, as German silver wire. 



