1 68 ELECTRIC LIGHTING. 



The apparatus is moreover provided with two other systems 

 of regulating screws, one of which moves the two carbons and 

 the luminous point without extinguishing it, and the other 

 allows one of the carbons to be displaced. Finally, screws 

 attached to the upper carbon-holder give a means of easily 

 fixing the carbons with regard to each other so as to supply 

 a diffused or a condensed light. Two small bull's-eyes in 

 the side of the lamp allow the action of the delicate parts of 

 the mechanism to be observed, and the effects of the regu- 

 lator to be noted. 



In order that the generator of light may work always under 

 the same conditions, whatever may be the variations in the 

 circuit external to the lamp, Siemens has interposed in the 

 circuit a regulator of resistance which we are going to 

 describe, as it has more importance than at first sight would 

 be supposed ; for these variations, by changing the conditions 

 of velocity in the motor, oa one hand, and on the other by 

 causing many sparks, may injure the machine and the col- 

 lector. By 1856 Lacassagne and Thiers saw the necessity 

 of a regulator of this kind, and had invented one which I 

 have described in my Expose des applications de V electricity 

 t. V., p. 506, and which was an accessory of their electric 

 lamp ; but these systems had been little used before Siemens. 



Siemens' arrangement consists of an electro-magnet with a 

 thick wire, the armature of which acts in the manner of\ relay 

 on a contact which, when the armature is not attracted, has 

 the effect of introducing into the circuit a derivation with a 

 resistance nearly equal to that of the voltaic arc. With a 

 suitable regulation of the antagonistic spring, the derivation 

 is therefore substituted for the voltaic arc whenever the re- 

 sistance of the latter becomes so great that the armature is 

 no longer retained. This is what happens not only when 

 the lamp is put out or withdrawn from the circuit, but also 

 when very great variations occur in the working of the lamp. 

 The helix forming the derivation is placed in a tin vessel 

 filled with water to prevent the wire from becoming too hot 



