ELECTRIC FURNACES 



141 



157. Changes Produced by the Current. The difficulty 

 which a current meets in passing through a conductor 

 is called resistance. Some substances are much better 

 conductors than others. Thus, a silver 



wire is a better conductor, that is, has a 

 smaller resistance, than a platinum wire 

 of the same diameter. For any given ma- 

 terial, the smaller the diameter of the wire 

 the greater the resistance. The heating 

 effect of German silver wire of high resist- 

 ance is used in the electric heater, electric 

 pad, electric flat-iron, and electric stove. 

 In passing through a fine wire of platinum 

 or carbon, the current finds so much 

 resistance that it heats the wire 

 white hot, forming the incandes- 

 cent electric light. If carbon 

 wire is used, it must be placed in a bulb free 

 from air, or the carbon will burn up (cf. 51). 

 The "arc" electric light (Fig. 131) consists of 

 two pencils of gas carbon (cf. 119) placed such 

 a distance apart that there is great resistance 

 at the gap between them. The heat produced 

 changes some of the carbon to the vapor state, 

 and makes the vapor white hot. The temper- 

 ature produced is about 3,800 C. 



158. Electric Furnaces. An electric arc furnace (Fig. 

 132) is an arc enclosed in a box made of high-melting 

 materials, such as fire clay, lime, or magnesia (magnesium 

 oxide; cf. 48). 



FIG. 130. 

 Electric Bell. 



