120 Mr. P. E. Nipher on the Electric Light. 



in passing through the lens; and by interposing thin plates of 

 glass between the standard candle and its screen to produce 

 there a similar absorption, we have no doubt that measure- 

 ments may be made with this instrument with much greater 

 accuracy than is possible by the ordinary method. 



The power of the light is practically, therefore, proportional 

 to the square of the product of the measurable distances. It 

 will be, of course, unnecessary to measure the variations both 

 in D and in d, as the scale can be so graduated as to give both 

 distances at one reading. 



XV. The Electric Light. By P. E. Nipher*. 



IN the Philosophical Magazine for January 1879, p. 30, 

 Mr. W. H. Preece gives a discussion, in which he shows 

 the condition to be supplied in electric lighting, in order to 

 obtain a maximum effect. In equation (2), p. 31, he gives, 

 for the heat distributed to the incandescent material, 



(p + r + lf 



where p represents the battery-resistance, and r and I repre- 

 sent the resistances of the connecting wires and an incan- 

 descent lamp respectively. 



For n lamps joined up in series, we must substitute nl for I ; 



while if joined in multiple arc, we must put - for L In either 



case the value of H is found to be a maximum when the resist- 

 ance of the lamp system is equal to that of the rest of the cir- 

 cuit. 



Mr. Preece then proceeds on the assumption that this con- 

 dition cannot be complied with, if n is large, reaching the con- 

 clusion that the amount of heat liberated in each lamp varies 

 inversely as the square of the number of lamps. This is true 

 in either of the two cases discussed by him. 



If, however, we have n lamps arranged in n f parallel circuits, 

 in each of which we have n" lamps, the previous equation 

 becomes 



W^l 



H w = 





With this arrangement it is always possible to supply the 

 condition which makes W f a maximum, entirely irrespective 

 * Communicated by the Author. 



