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CH. X] DRAWING WITH HOUSE CURRENT 343
If direct current is used, the lamp will burn for about six minutes and the carbons should be fed together every three to five minutes. (See fig. 205).
CONDENSER, STAGE AND MICROSCOPE FOR DRAWING WITH THE HOUSE LIGHTING SYSTEM
§ 492. Drawing outfit. — If one has a drawing outfit consisting of the projection apparatus shown in figure 182, all that is necessary to do is to place the arc lamp with its small carbons in the lamphouse and arrange it exactly as for projection.
The procedure is precisely as described above for the ordinary arc lamp on the usual special lantern lighting system (Ch. IX).
§ 493. Small Current Outfit. — This consists of an arc lamp using small carbons (6 to 8 mm. in diameter) and a rheostat or an inductor (fig. 197) not allowing over 5 to 6 amperes of current to flow. Instead of the usual large condenser (fig. 121), a small, single, convex lens is used. This is of 70 to 100 mm. (3 to 4 in.) focus, and 37 to 50 mm. (i>£ to 2 in.) in diameter, and is placed in a tube extending straight out from the upper carbon. Usually, also, the lens is in a sliding tube, so that it may be varied in distance from the source of light. If it is at its focal distance from the light, the beam will be approximately parallel (fig. 189); if farther from the light, the beam will be converging.
§ 494. Method of using the lamp with a special condenser. —
There are three methods of using this arc lamp and special condenser :
(1) The lamp can be put in line with the drawing microscope and a converging beam thrown directly on the specimen as for the large apparatus (fig. 132), the mirror and sometimes the substage condenser having been removed or turned aside.
(2) The mirror is removed from the microscope, but the substage condenser is left in position, and a parallel beam of light thrown directly into the substage condenser along the optic axis
(fig. 20lA).