444 Prof. Maxwell and Mr. F. Jenkin on the Elementary 



but in all cases the calculation is simple, and in many cases 

 the lines and surfaces can be graphically constructed without 

 any calculation. 



Part III. — Measurement of Electric Phenomena by their Electro- 

 magnetic Effects. 



13. Preliminary. — Before treating of electrical measurements, 

 the exact meaning in which the words " quantity/' " current," 

 "electromotive force," and "resistance" are used will be ex- 

 plained. But, in giving these explanations, we shall assume the 

 reader to be acquainted with the meaning of such expressions as 

 conductor, insulator, voltaic battery, &c. 



14. Meaning of the words " Electric Quantity." — When two 

 light conducting bodies are connected with the same pole of a 

 voltaic battery, while the other pole is connected with the earth, 

 they may be observed to repel one another. The two poles pro- 

 duce equal and similar effects. When the two bodies are con- 

 nected with opposite poles, they attract one another. Bodies, 

 when in a condition to exert this peculiar force one on the other, 

 are said to be electrified, or charged with electricity. These 

 words are mere names given to a peculiar condition of matter. 

 If a piece of glass and a piece of resin are rubbed together, the 

 glass will be found to be in the same condition as an insulated 

 body connected with the copper pole of the battery, and the resin 

 in the same condition as the body connected with the zinc pole 

 of the battery. The former is said to be positively, and the latter 

 negatively electrified. The propriety of this antithesis will soon 

 appear. The force with which one electrified body acts on an- 

 other, even at a constant distance, varies with different circum- 

 stances. When the force between the two bodies at a constant 

 distance, and separated by air, is observed to increase, it is said 

 to be due to an increase in the quantity of electricity ; and the 

 quantity at any spot is defined as proportional to the force with 

 which it acts, through air, on some other constant quantity at a 

 distance. If two bodies, charged each with a given quantity of 

 electricity, are incorporated, the single body thus composed will 

 be charged with the sum of the two quantities. It is this fact 

 which justifies the use of the word " quantity." 



Thus the quality in virtue of which a body exerts the peculiar 

 force described is called electricity, and its quantity is measured 

 (cateris paribus) by measuring force. 



The quantity thus defined produced on two similar balls simi- 

 larly circumstanced, but connected with opposite poles of a vol- 

 taic battery, is equal, but opposite; so that the sum of these two 

 equal and opposite quantities is zero ; hence the conception of 

 positive and negative quantities. 



