RESISTANCE 



213 



arranged as shown in Figure 79. To the physician electric 

 stoves are valuable, since his instruments can be sterilized in 

 water heated by the stove, and without coal or odor of gas. 



A convenient device is seen in the heating pad (Fig. 80), 

 a substitute for a hot-water bag. Embedded in some soft thick 

 substance are insulated wires in which heat is developed, and 

 over this is placed a covering of felt. 



Electric lights. The incandescent bulbs which illuminate 

 our buildings consist of a fine, hairlike thread inclosed in a glass 

 bulb from which the air has been removed 

 (Fig. 81). When an electric current is sent 

 through the delicate filament, it meets a strong 

 resistance. The heat developed in overcoming 

 the resistance is so great that it makes the 

 filament a glowing mass. The absence of air 

 prevents the filament from burning, and it 

 merely glows and radiates the light. 



Resistance. Since resistance plays so im- 

 portant a role in electricity, it becomes neces- 

 sary 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 p IG gi. An in- 



we remember that a 3OO-foot length of common 



iron telegraph wire has a resistance of I ohm. 



An approximate ohm for rough work may be made by winding 



9 feet 5 inches of number 30 copper wire 1 on a spool or arranging 



it in any other convenient form. 



Substances differ very greatly in the resistance which they 

 offer to electricity. It takes 300 feet of iron telegraph wire to 

 give i ohm of resistance; 39 feet of number 24 copper wire, 

 and but 2.2 feet of number 24 German silver wire. 



1 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. 



candescent electric 

 bulb. 



