SOUKCES OF ILLUMINATION 



93 



other than that of the ordinary arc, for with most of them their 

 efficiency tends to decrease rapidly after they have run a few 

 hundred hours. 



Fig. 27 indicates clearly the construction of the lamp as 

 now supplied by Messrs. C. Baker of High Holborn. It consists 

 essentially of two parts : the tube, containing the mercury, 



FIG. 27. Mercury-vapour Lamp. (C. Baker.) 



and a resistance, which has to be suited to the voltage of the 

 electric supply. These are enclosed in a metal cover, and the 

 whole is carried on a heavy base, with a square upright, on 

 which it is adjustable for height. 



To start the lamp it requires to be tilted and then quickly 

 brought back to a horizontal position, so that the mercury 

 flows from the positive to the negative pole and completes the 

 circuit ; for this purpose the metal cover is pivoted in a suit- 

 able manner. It sometimes happens that the lamp does not 



