30 Mr. W. H. Preece on the Electric Light. 



of a circuit, which is the exact equivalent of another form of 

 energy expended in another point of the circuit. Thus, if we 

 produce light by a galvanic battery, it is the equivalent of 

 chemical work done in the battery. If it be produced by a 

 dynamo-machine driven by a steam-engine, it is the equiva- 

 lent of coal consumed in the furnace. The object to be at- 

 tained in any economical utilization of this energy is to con- 

 vert the greatest possible portion of it into light. 



3. Now the relations that exist between the work done, the 

 current flowing, the resistances present, and the heat deve- 

 loped are easily demonstrated. The work done (W) in any 

 circuit varies directly with the electromotive force (E) in that 

 circuit, and with the quantity of electricity (Q) that passes 

 through it, or 



W=EQ; 



but by Ohm's law the electromotive force is equal to the pro- 

 duct of the resistance (R) of the circuit into the current (C) 

 flowing, or 



E = CR; 



and by Faraday's law the quantity of electricity passing de- 

 pends upon the strength of current (C) and the time it flows 



Therefore, substituting these two values in the above equation, 

 we get 



W = C 2 E*; 



in which we have what is known as Joule's law, which gives 

 us the work done (W), or its. equivalent, the heat generated 

 (H) in any circuit. By regardiog the time as constant, we 

 can put the equation 



H=0 2 R . . . (1) 



4. Now let us take the case of a battery whose electromo- 

 tive force is E and whose internal resistance is p. Let the 

 resistance of the connecting wires be r. Let us also have a 

 particular resistance I, which may be a wire to be heated to 

 incandescence, or a lmpp to be lit by the arc ; then by Joule's 

 law (1), 



but by Ohm's law, 



r- E 

 .-. H= E2 



p + r + l 



