Mr. W. H. Preece on the Electric Light, 33 



be produced by a magneto- or dynamo -machine worked by a 

 steam-engine consuming a given amount of coal per unit time, 

 E is no longer constant, for it varies with the resistances in 

 the circuit. The constant in this case is the work done in the 

 steam-engine in unit time. Calling this W 1? the total heat 

 generated in the circuit when the lamps are joined up in series 

 will be 



=»-*«* m£m3' (6 > 



and since the light varies inversely as n (§ 10), the light 

 emitted 



L 1 = W lX "* .. ; (7) 



n{p + r + nl) 



and when joined up in multiple arc, 



I 



L 2 =W lX n (8) 



n(p + r+-) 



Or by putting p + r=0 in equation (7), and — = in the de- 

 nominator of equation (8), we get 



n 

 and 



So that beyond certain limits, when the current is produced 

 by a dynamo-machine, if n lamps be joined up in series, the 



total light becomes diminished by — , and the light emitted by 



each lamp becomes diminished by —% 



If they are joined up in multiple arc, the total light is dimi- 

 nished by — , and the light emitted by each lamp -g- In the 



latter case the rapid diminution in the light emitted is due to 

 the fact that the heat is developed in the machine itself instead 

 of in the resistances external to it. 



13. We have assumed Wi to be constant ; but this is only 

 the case when a certain limit is reached, and when the velocity 

 of the rotating coils in the dynamo-machine has attained a 



Phil. Mag. S. 5. Vol. 7. No. 40. Jan. 1879. D 



