202 GENERAL SCIENCE 



Effects of Electric Currents : Heating Effects. Con- 

 nect two or three dry cells in series with a short piece of 

 fine iron or German silver wire. The wire will become 

 red hot and may be burned. Try this experiment with 

 different lengths of fine iron and copper wire with differ- 

 ent-sized batteries. 



A current in passing through a wire must overcome 

 the resistance of the wire. This resistance differs with 

 the kind of material of which the wire is made, the size 

 of the wire, and its length. Some sub- 

 stances are much better conductors of 

 an electric current than others. For 

 example, copper is a better conductor 

 than iron, and silver is a slightly better 

 conductor than copper. For a given 

 kind of material the resistance varies 

 with the size and length of the wire. 

 The smaller the wire the greater the 

 FIG 172 ACommer- res i s tance, and the longer the wire of a 

 ciai Tungsten Lamp, given size the greater the resistance. 



Table of Resistances of a Few Common Metals 



Silver . . 1.00 Iron ,, . . 6.00 German silver . 15.00 

 Copper . . 1.11 Platinum . . 7.20 Spring steel . 13.5 



German silver wire is commonly used in electric 

 heaters, electric toasters, electric stoves, electric flat- 

 irons, and similar appliances. The size of the wire is 

 carefully graded for the current on which it is to be 

 used in order that the temperature will be right for the 

 different uses. 



Electric Lighting. - - The same principle as that in- 

 volved in electric heating is involved in the electric light 

 bulb. A fine wire or filament of carbon is inclosed in a 



