IMPOR TANT DEFINITIONS. 1 1 



degree contributed to the progress of the science of electricity. 

 Thus the unit of resistance has been called the Ohm; the 

 unit of electric-motive force, the Volt ; the unit of intensity 

 of current, the Weber; the unit of electro-static capacity, the 

 Farad. The multiples and sub-multiples of these units are 

 designated by terms analagous to those of our metric system,* 

 but under certain other conditions adapted to the uses oftenest 

 made of them in their electrical applications. These terms 

 are mega and micro, which respectively express a million of 

 units and the millionth part of a unit; thus a million of 

 ohms is called a megohm, and the millionth part of an ohm 

 a microhm. The most generally used, however, of all these 

 multiples and sub-multiples are the megohm and the micro- 

 farad. 



We must now learn what these units represent. 



The ohm practically represents the resistance of a column 

 of pure mercury i square millimetre in section, and 1*0486 

 metre long, at the temperature of o Centigrade. * This is 

 about the resistance of an iron telegraph wire 4 millimetres 

 in diameter and 100 metres long. 



The volt represents nearly the electro-motive force of a 

 Daniell's cell, with sulphate of copper and water slightly 

 acidulated, for if this be expressed by i, the volt will be 

 represented by o'9268. 



The webef represents the volt divided by the ohm. 



The farad represents the capacity of an electric condenser 

 placed under such conditions that the quantity of electricity 

 carried by a volt through the resistance of one ohm would 

 charge it to the tension of a volt. This kind of unit is little 

 used except in the study of submarine cables. All the 

 calculations relating to these units will be found in my Expose 

 dcs applications de Velectricite, tome /., p. 432. 



Measuring instruments graduated to these units can now be 

 had at Gaiffe the Paris instrument maker's, Rue Saint Andre 



* See the Translator's Appendix No. i. (TR). 



