suitable to be used with the Duboscq Lantern. 335 



either of the trains, or neither of them, shall he released is 

 determined by the position of a double -toothed detent which, 

 placed between the final spur-wheels of the two trains, locks 

 both of them when in its mean position, but releases one or 

 other when shifted to right or left. The position of this de- 

 tent is determined by the current through the lamp, it being 

 attached to one end of a three-arm lever, the two other ends 

 of which are respectively attached to the armature of the con- 

 trolling electromagnet and to an opposing spiral spring. 

 When the moment of pull of the electromagnet upon its 

 armature is greater than that of the opposing spring, the 

 detent is pulled over one way, releasing the approximating 

 train of wheels while retaining locked the parting train. 

 When the moment of the pull of the opposing spring exceeds 

 that of the electromagnet on its armature, the detent is pulled 

 over the other way, locking the approximating train and 

 releasing the parting train. When the pull of the electro- 

 magnet exactly balances that of the opposing spring, both 

 trains are locked. Now when the current is at first turned on, 

 there is a sudden pull upon the armature of the electromagnet; 

 but the carbons are not instantly parted, partly because of 

 the inertia of the train of wheels, and partly because of the 

 backlash of the mechanism. Two or three seconds may 

 elapse before the arc is struck. This delay is serious, either 

 when working with dynamo or with accumulators. If the 

 dynamo is shunt-wound, the shortcircuiting even for this short 

 period demagnetizes the field-magnets. If the dynamo is 

 series-wound or compound-wound, or if accumulators are 

 being used, there is overheating during the period of delay. 

 Supposing, however, the arc to be struck, then the inertia of 

 the train of wheels makes itself evident in another way ; for 

 it parts the carbons too far, producing a long arc of consider- 

 able resistance ; and as the current then drops below its normal 

 value, the armature goes over the other way, and the other 

 train of wheels is momentarily released. This alternation 

 between the two trains, which often lasts for some time, pro- 

 duces a disagreeable instability. 



The feeding mechanism of arc-lamps next comes in for con- 

 sideration. The object of the feeding mechanism is to supply 

 carbon as fast as it is consumed, and so keep the light constant. 

 But the light cannot be kept constant unless the consumption 

 of electric energy in the arc is constant. The electric energy 

 is the product of two factors — the current through the arc, and 

 the difference of potential between the electrodes. Calling 

 the current i and the potential difference e, it is the product 

 ei which is to be kept constant. Now, as remarked at the 



