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CH. X] DRAWING WITH HOUSE CURRENT 341
Sh Shield to stop stray light and to aid in centering.
C Carbons with alternating current. They are of the same size.
D Carbons with direct current. The upper one is 8 mm. and the lower one 6 mm. in diameter.
E Shield or disc at the end of the condenser tube showing the opening of the condenser (C) and the spot of light at the right.
§ 488. Arc lamp and small carbons. — The form of arc lamp to use on the house circuit is not of particular importance. It may be very conveniently one of the small lamps shown in fig. 41—44, 201, 205, or it can be an ordinary arc lamp for greater currents, but supplied with long clamping screws, bushings or adapters for the small carbons (§ 127). The small lamps are generally of the hand-feed type and move the upper and the lower carbons equally.
§ 489. Size of carbons for direct current. — A. — The carbons found useful for direct current are as follows, all being of the softcored variety:
(1) Upper or positive carbon 7 mm. in diameter, lower or negative carbon 5 mm.
(2) Upper carbon 8 mm., lower 6 mm.
(3) Upper carbon u mm., lower 8 mm.
B. — The carbons for alternating current with an equal feed for the upper and the lower carbon, should be of the same size, and this size should not exceed 8 mm. in diameter for 5 to 6 amperes. If only three or four amperes are used, then it is better to have carbons not greater than 6 mm. in diameter.
§ 490. Reason for using small carbons. — In order to have the light steady and thus have the field continuously bright, the entire end of the upper carbon should be white hot.
If the carbon is so large that the crater covers only a part of the tip, the crater will wander about on the end of the carbon. Every change in the position of the crater changes the direction of the light, beam. While the crater is in one position the entire field of a high power objective may be brilliantly illuminated; if the crater wanders to a new position, the field will be only partly or not at all illuminated. In such a case, one must constantly change the position of the mirror of the microscope to keep the field bright. If, however, the crater is nearlv as large as the end of the carbon, it