ARC LAMPS 89 



positive carbon moves forward at a greater rate than the 

 vertical negative, the speeds being adjusted to accurately 

 counterbalance the rates of burning. In the Liliput a 

 considerably narrower negative carbon is used, 6 mm. in 

 diameter, as against an 8 mm. positive. These burn 

 away at the same speed and are correspondingly moved 

 forward by the adjustment. 



Small lamps of this kind are connected directly to a 

 wall-plug or lamp-holder, with a suitable resistance 

 placed in the circuit, and it is important that the wires 

 be connected to the proper lamp-terminals. Fig. 25 

 shows the method of connection. A flexible lead is fitted 

 to a lamp-holder and its positive and negative wires 

 determined by means of pole paper paper coated with 

 potassium iodide and starch, for instance. The paper is 

 moistened and the ends of the two wires placed on it 

 near together ; the positive wire gives a blue patch. This 

 wire is then connected to one terminal of the resistance, 

 and the other terminal joined to the positive pole of the 

 lamp by means of a length of wire. The circuit is com- 

 pleted by joining the other wire of the supply cable 

 direct to the negative lamp -terminal. 



The lamps are adjustable for height on a suitable pillar, 

 and can be tilted if required. The carbons have to be 

 carefully adjusted until their points just touch when 

 moved forward, and so that the negative carbon does not 

 obstruct the crater which forms on the horizontal positive 

 carbon. The arc is struck by momentarily bringing the 

 two carbons into contact, by means of the milled head, 

 and then withdrawing them slightly ; a good crater 

 forms in a minute or so, and then a steady light is given. 



Enclosed Arcs Alternating Current. Enclosed arc 

 lamps are very popular for projection purposes, and can 

 be used for photomicrography. They have the dis- 

 advantages, in the forms at present made, that disturbing 

 reflections are given by the enclosing glass cylinder, and 



