suitable to be used with the Duboscq Lantern, 337 



the other. Neither will two work in parallel with one another; 

 for the weakening of current in one throws more current 

 through the other, and the instability before alluded to — called 

 "hunting" by electric engineers — becomes yet more pro- 

 nounced. 



The lamp that I have adapted to the Duboscq lantern is one 

 known in commerce as the " Belfast " arc-lamp, its principles 

 of construction being due to Mr. F. M. Newton ; but I have 

 had the design altered to suit the special work. In this lamp, 

 as previously mentioned, the striking and feeding mechanisms 

 are separate. The arc is struck by means of an electromagnet 

 E of the tubular pattern, having as its armature an iron disk 

 A, which, when no current is passing, is held up by a short 

 spiral spring at about 3 millim. from the end of the electro- 

 magnet. The lower carbon-holder is mounted upon this disk, 

 so that the arc is struck by the downward movement of 

 the lower carbon. The feeding mechanism is both simple 

 and effective. The upper carbon-holder is a long straight 

 tube of brass : it passes through a collar in the frame of the 

 lamp, and also through a metal box B above. This metal box 

 contains a piece of curry-comb with the steel bristles of the 

 comb set to point obliquely inwards and downwards. They 

 grip the carbon-holder and allow it to be pushed downwards, 

 but not upwards. The box itself is mounted upon a strong 

 brass lever, L, close to the point of the lever. One end of this 

 lever is drawn downwards by an adjustable spiral spring S, 

 whilst the other carries an iron armature which stands imme- 

 diately above the poles of an electromagnet, which is wound 

 with fine wire and placed as a shunt to the lamp. Above the 

 lever there is a contact-screw, platinum-tipped, making con- 

 tact with the lever, exactly as in the ordinary trembling electric 

 bell, and the lever and contact-screw are included in the shunt- 

 circuit. The attraction of the shunt-magnet for its armature 

 is opposed by the pull of the spiral spring. Whenever, by 

 reason of the resistance of the arc, a sufficient current flows 

 through the shunt-circuit, the opposing spring is overcome, 

 and the lever is set into vibration like the lever of an electric 

 bell, but more rapidly. The vibratory motion is thus com- 

 municated to the box containing the steel wire comb, which 

 at once, by an action well known in mechanism, wriggles 

 the carbon-holder downwards by innumerable small successive 

 impulses. So soon as the motion of the carbon has reduced 

 the resistance of the arc, the shunt-current diminishes and 

 the feeding action ceases, to recommence when required. It 

 is found best for lantern-purposes to send the current upwards 

 through the lamp, the lower carbon being the positive one. 



Phil. Mag. S. 5. Vol. 23. No. 143. April im. 2 A 



