SCIENCE AND PRACTICE. 



271 



This is shown graphically by the curve a, b, c, d, Fig. 131, 

 the curve ef showing the corresponding conducting powers 

 of mercury at the different temperatures. 



Mattecci first observed that bismuth behaved differently 

 at its point of solidification to the other metals. At the freez- 

 ing point of water the conducting power of this metal is about 

 0-74 times that of mercury ; between this point and 250 C. 

 the melting point its conducting power follows the law 

 common to pure metals ; at its fusing point its conducting 

 power increases suddenly until it equals that of pure mer- 

 cury at the same temperature, very nearly; from which 

 point it decreases again in conducting power as the tem- 

 perature increases. This behaviour is probably due to its 

 crystalline structure. 



Under the head of fused metals we come to mercury, 

 which is always in this state at ordinary temperatures. 



Dr. Werner Siemens proposed to employ a body of this 

 metal as unit of resistance. He considered it of the greatest 

 importance to have the unit of resistance expressed as a 

 geometrical body of that material, which is commonly 

 referred to as unit when speaking of conducting powers, 

 by which all practical problems are facilitated. As an 

 instance, if it be required to know the resistance of a cer- 

 tain length and section of any metal, it is only necessary 

 to calculate what it would be in mercury, knowing the 

 length and section of a body of the latter representing 



