ELECTRIC LIGHT. 



271 



tions of the armature and anchor are small, and 

 very slight changes in the strength of the cur- 

 rent and brilliancy of the light are immediately 

 corrected. 



The Serrin lamp (Fig. 2) is a favorite though 

 complicated device, that has been used in both 

 France and England where only a single light 

 is required ; and lately some ingenious improve- 

 ments have been added to it by M. Lontin. In 

 it the carbons are held vertically one over the 

 other. The upper carbon is made to travel by 



means of a rack and pinion attached to the bot- 

 tom of its holder and driven by a spring which 

 is under the control of an electro-magnet, in the 

 base of the lamp. According to the force of 

 the current passing into it from the main cur- 

 rent, this electro-magnet attracts or releases 

 an armature, and the effect of this oscillating 

 movement is to cause the lower carbon-holder 

 to rise or fall, with the irregularities in the 

 strength of the main current, which is itself 

 producing the voltaic arc. The separation of 



Fio. 1. 



the carbon points to suit any required length 

 of arc is effected by raising or depressing the 

 upper carbon-holder, by means of a screw placed 

 at the top of its upright, where the horizontal 

 arm is hinged. 



A novel form of lamp of more recent inven- 

 tion, and possessing certain decided advantages, 

 has been devised by a Russian engineer named 

 Rapieff, and is now in use in the office of the 

 u London Times." In this apparatus (Fig. 3) 

 each carbon is as it were split lengthwise, and 

 the halves placed relatively to each other in 

 the form of a V, approaching each other only 

 at the point of illumination. In proportion as 

 the carbons are consumed, they are caused by 

 an ingenious arrangement of cords and pul- 

 leys to approach each other; and thus the 



FIG. 2. 



FIG. 3. 



voltaic arc is always produced through a con- 

 stant distance. 



With rods measuring twenty inches in length, 

 and about six millimetres in diameter, a light 

 may be uninterruptedly maintained for nine or 

 ten hours. In this apparatus the current does 

 not pass through the entire length of the car- 

 bon pencils, but enters by means of curved 

 metallic arms at points within two inches of 

 the luminous focus; hence the resistance of- 

 fered to the current is kept constant, whatever 

 may be the length of the carbons. This se- 

 cures a uniformity of illumination that can not 

 be obtained in lamps where the current has to 

 pass through the entire length of the carbon 

 pencil, for as this shortens the resistance de- 

 creases ; hence the intensity of the current, and 



