THE HENRY. 147 



But every branch of the subject was euriclied by Faraday, and 

 among his most brilliant investigations are those relating to the 

 " capacity of condensers," and especially the influence of the dielectric. 



If an insulated, conductor is charged with electricity, the quantity 

 which exists upon it will depend on the potential and the capacity of 

 the conductor. It is exactly as if one spoke of the quantity of water 

 in a lake or pond as depending on the depth (pressure, or ''poten- 

 tial") and tlie area of the bottom (capacity). If two conductors are 

 near each other, but sei)arated by a comparatively thin layer of air, 

 glass, sliellac, or other dielectric, the "capacity" of the combination is 

 much greater than that of either of the conductors, and it is known as 

 a "condenser." The well-known Ley den jar is a common type. In 

 speaking of the "potential" to which a condenser is charged, the word 

 is used very much in the same sense as "electro-motive force" in what 

 has gone before. Potential, therefore, may be, and constantly is, 

 expressed in volts. The unit of capacity, the farad, is the capacity of 

 a condenser which is charged to a potential of 1 volt by 1 coulomb of 

 electricity. 



To continue the analogy already used, the unit of capacity (area of 

 bottom) for vessels holding water might be defined as that which would 

 require unit depth to hold unit quantity. 



The joule is the unit of work. The name of James Prescott Joule 

 will forever be associated with the most splendid generalization of the 

 present age, namely, the principle of the conservation of energy. 

 Through his interest in electro-magnetism, and especially by his inves- 

 tigation of the efficiency of electric motors, he was led to the consider- 

 ation of the correlation of the various forces of nature, and associated 

 with Prof. William Thomson, now Lord Kelvin, he executed a remark- 

 able series of exi^eriments aftbrding cumulative proof of the indestruc- 

 tibility of energy. With great appropriateness his name has been 

 given to the unit of work. It is related directly and simply to the 

 "erg," wiiich is the unit of work of the centimeter gram-second sys- 

 tem. Ileference has already been made to the fact that when a current 

 of electricity is passed through a conductor heat is generated, the 

 amount depending on the resistance of the conductor and the strength 

 of the current. This heat is the equivalent of the energy electrically 

 expended. The joule is the energy expended in one second by a cur- 

 rent of 1 ampere passing through a resistance of 1 ohm. In the com- 

 mon incandescent or glow lamp the energy ex})ended as heat in the 

 carbon filament is about 63 joules in every second. 



In addition to the unit of work, it is also extremely desirable to have 

 a unit of rate of work, or, as it has been called by many writers, 

 "activity," but whicliis more commonly expressed by the word "power." 

 It is only natural that the name of one who was the first to recognize 

 the necessity for a quantitative evaluation of the rate at which energy 

 was absorbed, and to give numerical expression to it in the definition 



