Artificial Lighting 169 



the carbon filament in operation. Tungsten increases in re- 

 sistance with the rise and loses with the fall of temperature. 

 Carbon decreases with the rise and increases with the fall of 

 temperature. Copper, aluminum, mercury, platinum, and 

 silver behave as tungsten does, with change in temperature. 



An interesting experimental result of unusually low tem- 

 peratures in modifying the resistance of certain metals to the 

 electric current may be mentioned. Metals at the temperature 

 of liquid air show a marked decrease in electrical resistance. 

 What is the temperature of liquid air? Recently it has been 

 discovered that in the neighborhood of 270 C., 454 F., or 

 3 Absolute, certain metals, as tin, lead, mercury, and platinum, 

 show a most remarkable drop from a measurable resistance 

 to practically no resistance to the electric current. An electric 

 current, started in any one of these metals at this temperature 

 or lower, continues indefinitely without the slightest sign of 

 diminution. 



Resistance and dimensions. Length and thickness of the 

 substance determine the amount of resistance a given material 

 may offer to an electric current. As regards length it is gen- 

 erally true that the longer the wire of a given thickness, the 

 greater is the resistance it offers. This may be demonstrated 

 experimentally. 



Exercise: Resistance and length. Connect one "dry cell" with 

 one hundred feet of No. 26 copper wire and this with an ammeter. 

 What is the strength of the current shown by the ammeter? Record 

 it. Connect cell and ammeter with fifty feet of the same wire. What 

 does the ammeter show? Compare the readings. This strength of 

 the current from the shorter ^wire will be about twice that delivered by 

 the longer wire. Hence it generally holds that with a constant pres- 

 sure the current is inversely proportional to the length of the conducting 

 wire, the thickness remaining the same. 



Exercise : Resistance and thickness. Tests may be made with 

 wires of different thicknesses or areas of cross section. One will find 

 that the thicker the wire, the less is the resistance it offers, or the 

 greater is the strength of the current which it delivers, as shown by 

 the ammeter. The smaller the area of the cross section of a conduct- 



