32 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



metals, and all acids and salt solutions decrease in resistance with 

 rise of temperature. 



A rod of a substance like glass or porcelain has, at ordinary 

 room temperature, a resistance which is expressed in millions of 

 millions of ohms, but the resistance decreases rapidly with rise of 

 temperature. Both glass and porcelain become fairly good con- 

 ductors at a low red heat. This is strikingly shown by the fol- 

 lowing experiment : Fine copper wire is wound around the ends 

 of a thin-walled glass tube about 20 centimeters long, and these 

 copper wires are connected through a fairly high metallic resist- 

 ance to the terminals of a i,ooo-volt transformer. The side of 

 the tube is then heated with a blast lamp. At a low red heat a 

 sufficient amount of current begins to flow to develop a very con- 

 siderable amount of heat, and the glass tube becomes still hotter, 

 which permits still more current to flow, which makes the glass 

 tube still hotter, and so on, until the glass tube melts down be- 

 cause of the heat which is generated in it by the flow of current. 



Alloys which change but little in resistance with change of 

 temperature are especially suitable for resistance standards and 

 resistance boxes. * Wires of manganin f are now almost uni- 

 versally employed for this purpose. Figure 1 5 J shows the 

 change of resistance of a manganin wire with temperature. A 

 manganin wire which has a resistance of 100 ohms at 15 C. has 

 a resistance of 100.01 ohms at 20 C. ; a german-silver wire 

 which has a resistance of 100 ohms at 15 C., has about 100.2 

 ohms resistance at 20 C.; and 3 copper wire which has resistance 

 of 100 ohms at 15 C., has about 102 ohms resistance at 20 C.; 

 that is, for the specified rise of temperature the change of resist- 

 ance of the manganin wire is only o.oi per cent, the change of 

 resistance of the german-silver wire is 0.2 per cent, and the 

 change of resistance of the copper wire is 2 per cent. 



* See Chapter X. 



f Manganin is an alloy of 84 parts by weight of copper, 12 parts by weight of 

 tiickel, and 4 parts by weight of manganese. 



J From the results of Dr. Lindeck. See the Proceedings of the International 

 Electrical Congress, Chicago, 1893, P a E e l6 S- 



