1 2 ELECTROLYSIS. 



end of the first second. A body, falling in vacuum, acquiring 

 a speed of 981 centimetres at the end of the first second, the 



value of a dyne will be _ &' amme _ _^ gramme, or about 



g 



one milligramme. 



ERG. The erg is the unit of work. It is the product of the 

 unit of force by the unit of length, its value being, therefore, 



x o-01-= JL grammetre, or ~ kilogram- 



metre. One kilogrammetre is, consequently, equal to about 

 100 millions of ergs. From the foregoing, it is easy to estab- 

 lish the relations existing between the mechanical and electrical 

 units. The mechanical equivalent of heat is 425, which means 

 that one calorie, or the quantity of heat necessary for raising 

 one degree centigrade one litre of water, is equal to 425 

 kilogrammetres. One erg will consequently be equal to 



U8.100.000 x 425 



One calorie, therefore, is equivalent to 41,692,500,000 ergs. 



As will be seen, the dyne and the erg are extremely small 

 quantities which cannot be utilised for industrial purposes. 

 The kilogramme and kilogrammetre are, on the contrary, of 

 ready use, whether in connection with mechanical, chemical, 

 calorific, or electrical energy. 



WATT. The late Sir William Siemens proposed to call 

 watt the product of one volt by one ampere; this renders 

 possible the measuring the rate of production of an electric 

 machine. Wires of varying diameters can be wound on the 

 frame of a dynamo machine. When the wire is thin, the 

 electromotive force obtained is great, and the intensity small ; 

 when, on the contrary, the wire is thick, the electromotive force 

 is small and the intensity great ; but in all the combinations, 

 the product of the volts by the amperes is sensibly the same. 

 The watt may therefore be used as a comparison between 

 various forms and sizes of electric machines. 



OHM'S LAW. The law of Ohm gives the relation which 

 exists between the electromotive force, the intensity, and the 

 resistance of a current. Calling E the electromotive force; 



