170 Introduction to the Study of Science 



ing wire, the greater is its resistance to the current. It resembles 

 the effect of the size of a pipe upon the flow of water ; the larger the 

 pipe the greater the flow, and the smaller its area of cross section the 

 less the flow. It is readily seen that this is an important factor in 

 the selection of the size of wire for transmission of electricity. Ex- 

 amine a Brown and Sharp wire gauge and become familiar with sev- 

 eral diameters of copper or other wire and their numbers according to 

 the " B & S " gauge. 



Application of these facts to the tungsten. What we have 

 learned as to the relation of temperature and dimensions of 

 the conducting material to its resistance to an electric current 

 has to be applied to the tungsten filament. This is found to 

 be thinner and much longer than the carbon filament of a lamp 

 of corresponding candle power. It is at once evident that the 

 smaller diameter and the greater length must increase the 

 resistance to the electric current. It is plainly intended to 

 make the electric current do the largest amount of useful work 

 in its passage from entrance to exit. This is precisely what 

 every electric appliance in the home is designed to do, to 

 make the electric current entering the house by one wire do the 

 required amount of useful work, heating or lighting, in passing 

 through the appliance to the wire by which it leaves the house. 



The resistance of tungsten increases with rise in tempera- 

 ture. The higher the resistance, the greater will be the heat 

 produced by the electric current in passing. The vaporizing 

 temperature of tungsten is very much higher than that of 

 carbon, as used in the vacuum of the incandescent bulb ; it is, 

 therefore, at once evident that it can be made more nearly 

 " white hot " or incandescent, thus giving a brighter and 

 whiter light than the carbon lamp. 



To summarize : the greater the resistance of any conducting 

 medium to an electric current, the medium remaining* a con- 

 ductor, the hotter the medium will become. And the longer 

 the filament used as a radiating area, with its thickness and 

 other conditions remaining the same, the greater will be the 

 amount of light it will produce. 



