86 ELECTRIC LIGHTING. 



sumption of the carbons, for these continually tend to ap- 

 proach the block of magnesia by their own weight. A lamp 

 of this kind had previously been invented by Staite. 



Wallace Farmer's Lamp. This system is so primitive 

 that we are surprised that it has received any attention. It 

 was invented with a view to obtaining a longer duration of 

 the carbons, and with this object thin plates of carbon are 

 used instead of carbon pencils. These plates are placed so 

 as to form a very small angle with each other, so that the arc 

 may be progressively displaced as the carbons are hollowed 

 out. An electro-magnetic mechanism analagous to those of 

 the other lamps brings the plates nearer together as the action 

 proceeds, and this mechanism acts by variations in the inten- 

 sity of the current. It is certain that a lamp on this system 

 will remain lighted for a long time ; but we very much doubt 

 whether the electric intensity it produces can be compared 

 with that of other lamps, for the incandescent part of the 

 carbons is then considerably reduced in extent and calorific 

 intensity. 



Houston and Thomson's Lamp. As regards arrange- 

 ment, Houston and Thomson's Lamp resembles the ordinary 

 apparatus ; only the lower carbon, instead of being connected 

 with a clockwork mechanism, is supported by an arm fixed 

 to a strong flat spring. This arm carries the armature of an 

 electro-magnet placed below it, and is insulated metallically 

 from the upper carbon-holder, which is placed in communi- 

 cation with the positive pole of the electric generator. The 

 electro-magnet is in metallic communication with the lower 

 carbon and the negative pole of the generator, so that the 

 circuit is completed through the elastic arm of the lower 

 carbon-holder and the two carbons. The electro-magnetic 

 armature forms therefore a trembler, as in an electric bell, and 

 the effect is a very rapid series of interruptions of the current 

 between the two carbons; and this occasions repeated sparks, 



