[Electric engineering.] . ry purposes, the carbons are arrangedvertically, one above the other, as shown in Fig. 2. Whenarranged in this way, the top carbon should always be thepositive one when direct current is used, otherwise thecrater will be formed in the bottom carbon and most ofthe light will be thrown up instead of down. When lampsare first connected up, they should be allowed to burn fora short time, and if the crater makes its appearance in thebottom carbon, the connections to the lamp terminals shouldbe reversed. Of course, with alternating current it makesno difference how the lamp
[Electric engineering.] . ry purposes, the carbons are arrangedvertically, one above the other, as shown in Fig. 2. Whenarranged in this way, the top carbon should always be thepositive one when direct current is used, otherwise thecrater will be formed in the bottom carbon and most ofthe light will be thrown up instead of down. When lampsare first connected up, they should be allowed to burn fora short time, and if the crater makes its appearance in thebottom carbon, the connections to the lamp terminals shouldbe reversed. Of course, with alternating current it makesno difference how the lamp is connected in circuit, as thecurrent is continually reversing anyway and both carbonsburn alike. It is an easy matter to tell when a direct-cur-rent lamp is correctly connected. Allow the lamp to burnfor a short time, then switch it off and see which carbonremains bright the longer. The positive carbon is muchhotter than the negative, hence the negative carbon is theone that becomes dull first, 12 ELECTRIC LIGHTING. 18. Fig. 7. 15. For use in stereopticons and other projection appa-ratus, the carbons are often inclined at an angle, as shown in Fig. 7. The objectin doing this is to allowmore of the light fromthe crater to reach thelenses. searchlightsa similar arrarigementis used, only the car-bons are often slantedthe other way and thelight is reflected froma parabolic reflector or Mangin mirror, as shown in Fig. 8,which shows the arc placed at the focus of a parabolic re-flector M. The raysof light upon strikingthe mirror are reflectedout parallel to eachother, and as they are mthus kept bunched to-gether the light maybe made to penetratelong distances. Asmall concave reflect-or is usually placed, asshown at r, to throw the rays of the arc that would ordi-narily pass outwards back towards the main parabolic ground-glass silvered mirror is used in the United States Navy, but forordinary commercial workthe Mangin mirror is used, asit is cheaper and easier toma
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