CH. II] ALTERNATING AND DIRECT CURRENT LANTERNS 69 



That is, if 10 to 12 amperes of direct current give satisfactory 

 illumination in a given case, it would require from 25 to 30 

 amperes of alternating current to give the same brilliancy of 

 screen image. Naturally also the heating with the larger alter- 

 nating current is greater than with the smaller direct current 

 (see also 768). 



103. The difference between direct and alternating current is, 

 in general terms, this : the direct current has a constant polarity 

 and one carbon is always positive; while the alternating current 

 has an alternation of polarity, as the current flows in one direction 

 for an instant and then in the opposite direction. The result is 

 that each carbon is positive half the time and negative half the 

 time, hence both carbons have brilliant craters from which light 

 for the screen image might be obtained. Sometimes an effort is 

 made to utilize the light from both craters by the arrangement of 

 the carbons in the form of a V, the apex of the V pointing toward 

 the condenser (fig. 230). 



INSTALLATION OF A MAGIC LANTERN WITH AN ALTERNATING 

 CURRENT ARC LIGHT 



104. Wiring from the supply to the lantern. This is pre- 

 cisely as for the direct current lamp. If the lantern is to be used 

 for experimental purposes it is advantageous to have an incandes- 

 cent lamp inserted in the circuit as shown in fig. 2. 



105. Rheostat or other regulating device. There must be 

 introduced along one of the supply wires to the lantern some form 

 of balancing device. This may be in the form of a rheostat like 

 that vised for the direct current ( 6); an inductor or choke-coil, 

 a transformer, or a mercury arc rectifier may be used. For the 

 special advantages and disadvantages of the different balancing 

 devices (see 736-738). 



106. Wiring the lamp. For the alternating current it makes 

 no difference which supply wire is connected with the upper carbon, 

 as each carbon has an approximately equally brilliant crater. 



