272 



THE ELECTRIC TELEGRAPH. 



the unit, and to multiply the result with the conducting 

 power of the metal required. 



A great point in favour of mercury is the ease with 

 which it may be procured in a chemically pure state. 



M-H-H-H 



rrrn MM 



LMLLL 

 LI 



M M M U_ 



~~rrr 



MTFFFffl 



LUJJJJJJ 

 i M \ 



JJ-VU 



MINI 



Fig. 131. 



There are two ways which are highly recommended : one 

 followed by Dr. Werner Siemens ; the other followed by 

 Dr. A. Matthiessen with equally good results. By the 

 former method the mercury is placed in an evaporating 

 dish, with about three-quarters of an inch thickness of con- 

 centrated sulphuric acid over it. It is then carefully boiled 



