C. GENERAL PHYSICS. 165 



satisfied himself that a magneto-electric machine could be 

 constructed which would give induced currents, all in the 

 same direction. The ring armature employed by him was 

 not essentially difierent from that invented by Gramme, al- 

 though, according to Professor Andrews, through a serious 

 blunder in the construction of his machine, Pacinotti could 

 not possibly have attained M. Gramme's results. 



THE GRAMME ELECTEIC MACHINE. 



The Gramme mairiieto-electric machine has for some time 

 been employed for supplying light at the Houses of Parlia- 

 ment in London, and an idea of its power may be gathered 

 from the fact that it is w^orked by a steam-engine at a dis- 

 tance of 480 feet from the point at which the light is pro- 

 duced, the current being conducted thither by coj^per wires 

 one sixth of an inch in diameter. The maQ:nets in the ma- 

 chine are arranged upon three massive rectangular blocks, 

 and make 389 revolutions per minute, at an expense of 2f 

 horse-power, and give a light equal to over 3000 of the En- 

 glish standard candles. 



A NEW THEORY OF GALVANIC RESISTANCE. 



According to Herwig, the resistance to a galvanic current 

 offered by any metallic conductor is a subject that has as yet 

 been but little understood. The phenomena themselves con- 

 sist principally in the following : (1) The nearly uniform rate 

 at which the resistance increases with the temperature ; (2) 

 metals in a liquid condition have a greater resistance than 

 as solids at the same temperature ; (3) for liquid metals the 

 resistance increases with the temperature, but to a much less 

 degree than for solids ; (4) the transition from solid to fluid, 

 as well as that from fluid to vaporous condition, is accompa- 

 nied with an increase of resistance ; (5) in the case of satu- 

 rated metallic vapors a diminution of the resistance is pro- 

 duced by an elevation of temperature ; (6) a greater resist- 

 ance is found at the boundary surface between fluid and 

 solid portions of the same metal than within the vaporous 

 atmosphere itself. 19 (7, VIII., Tl. 



