40 ELEMENTARY LESSONS ON [CHAP. I. 



to electric quantities is given in Lesson XX., Art. 246. 

 We may, however, make the remark, that two insulated 

 conductors of the same form, but of different sizes, differ 

 in their electrical capacity ; for the larger one must 

 have a larger amount of electricity imparted to it in 

 order to electrify its surface to the same degree. The 

 term potential is employed to signify the degree to 

 which a body is electrified. A given quantity of elec- 

 tricity will electrify a body up to a certain " potential " 

 (or degree of electrification) depending on the size of the 

 body. A large quantity of electricity imparted to a 

 conductor of small capacity will electrify it up to a 

 very high potential; just as a large quantity of water 

 poured into a vessel of narrow capacity will raise the 

 surface of the water to a high level in the vessel. The 

 exact definition of Potential, in terms of energy spent 

 against the electrical forces, is given in the Lesson on 

 Electrostatics (Art. 237). 



It will be found convenient to refer to a positively 

 electrified body as one electrified to a positive or high 

 potential; while a negatively electrified body may be 

 looked upon as one electrified to a low or negative 

 potential. And just as we take the level of the sea 

 as a zero level, and measure the heights of mountains 

 above it, and the depths of mines below it, using the 

 sea level as 'a convenient point of reference for differ- 

 ences of level, so we take the potential of the earth's 

 surface (for the surface of the earth is always electrified 

 to a certain degree) as zero potential, and use it as a 

 convenient point of reference from which to measure 

 differences of electric potential. 



LESSON V. Electrical Machines. 



38. For the purpose of procuring larger supplies of 

 electricity than can be obtained by the rubbing of a rod 

 of glass or shellac, electrical machines have been 



