VOLTAIC ARC LAMPS. 169 



during long interruptions of the current, such as those re- 

 quired by replacing the carbons. 



The lamp just described is not, however, the only one 

 made by Siemens. He has already patented eight forms. 



Lout ill's Lamp. From a published account of the Lontin 

 machines we extract the following description of this lamp : 



" The first and principal advantage of this regulator is that its 

 moving and regulating parts are such that it can work in any 

 position, upright, horizontal, or upside-down, and without being 

 stopped or changed in its action by even the strongest shocks or 

 oscillations. 



" In these regulators a quite new application has been made 

 of the heating produced by the current in a metallic wire to cause 

 the separation of the carbons and keep it perfectly constant. 

 Thus the use of electro-magnets is dispensed with, and the con- 

 sequent cost of the additional electric power required to over- 

 come their resistance in the circuit, while the length of the arc 

 remains absolutely fixed, so that a more regular light may be 

 obtained. 



" The approach of the carbons in proportion to the combustion 

 is obtained by another not less happy application of a derived 

 current taken from the light current itself, and acting as follows: 



" There is a solenoid in the apparatus formed of a coil of fine 

 wire in quantity sufficient to offer a very great resistance to the 

 current. This coil encloses a movable iron rod, which when at 

 rest keeps back the moving power that brings the carbons nearer 

 together. So long as the carbons are at a distance adjusted to 

 the amount required for a good light, the whole current passes 

 through them, on account of the great resistance of the coil; 

 but when the separation increases, a small portion of the current 

 passes through the coil and excites it so that the movable iron 

 rod is attractive, and the moving power released from its stop 

 brings the carbons nearer by the amount required for maintain- 

 ing the length of the arc : at this moment the solenoid ceases to 

 act, and the irod rod again stops the motor, which has merely to 

 bring the carbons nearer together and is extremely simple." 



This employment of a derivation from the light current 



