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



from them by air only. It is not improbable that this view of 

 induction may be hereafter modified. 



52. Heat produced in a Conductor by a Current. — The work 

 done in driving a current, C, for a unit of time through a con- 

 ductor whose resistance is R, by an electromotive force E, is 

 EC = RC 2 (§ 17). This work is lost as electrical energy, and is 

 transformed into heat. As Dr. Joule has ascertained the quan- 

 tity of mechanical work equivalent to one unit of heat, we can 

 calculate the quantity of heat produced in a conductor in a given 

 time, if we know C and R in absolute measure. In the metrical 

 series of units founded on the metre, gramme, and second, if we 

 call the total heat 9, taking as unit the quantity required to 

 raise one gramme of water one degree Centigrade, we have 



*=s • • ( 28 > 



In the British system, founded on feet, grains, and seconds, 

 with a unit of heat equal to the quantity required to raise one 

 grain of water one degree Fahrenheit, we must substitute the 

 number 24861 for 4157 in the above equation. 



53. Electrochemical Equivalents. — Dr. Faraday has shown* 

 that when an electric current passes through certain substances 

 and decomposes them, the quantity of each substance decom- 

 posed is proportional to the quantity of electricity which passes. 

 Hence we may call that quantity of a substance which is decom- 

 posed by unit current in unit time the electrochemical equiva- 

 lent of that substance. 



This equivalent is a certain number of grammes of the sub- 

 stance. The equivalents of different substances are in the pro- 

 portion of their combining-numbers ; and if all chemical com- 

 pounds were electrolytes, we should be able to construct experi- 

 mentally a table of equivalents, in which the weight of each sub- 

 stance decomposed by a unit of electricity would be given. The 

 electrochemical equivalent of water, in electromagnetic measure, 

 is about 0*02 in British, 0*0092 f in the metrical system. The 

 electrochemical equivalents of all other electrolytes can be de- 

 duced from this measurement with the aid of their combining- 

 numbers. 



54. Electromotive Force of Chemical Affinity. — When two sub- 

 stances having a tendency to combine are brought together and 

 enter into combination, they enter into a new state, in which the 

 intrinsic energy of the system is generally less than it was before ; 

 that is, the substances are less able to effect chemical changes, or 

 to produce heat or mechanical action, than before. 



* Experimental Researches, series vii. 

 t -0093/5 by Weber and Kohlrausch. 



