This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1916 Excerpt: ...example, the amperage will depend upon the resistance encountered. Having first carefully read and considered the text matter under "Resistance," Page 34, let us examine the resistance of the projection circuit, laying aside, however, the resistance of the line and carbons, which is, in itself, a small quantity, usually ignored when figuring projection circuit resistance. If we were to connect a projection lamp to the supply lines as indicated at A, Fig. 142, when the carbons were brought together a dead short circuit would be established, which would instantly blow a fuse. To avoid this we establish resistance in the form of a "rehostat," as at C, sketch B, Fig. 142. This resistance operates precisely the same as does the resistance in the filament of an incandescent lamp. It only allows a certain given amperage to pass, the amperage being dependent upon the voltage and the number of ohms resistance contained in resistance C. But right here another equation enters. The foregoing is true only so long as the carbons remain in contact with each other. The instant they are separated an arc is struck, and additional resistance is established in the arc itself, the amount of which will vary somewhat with the amperage, but more largely with the distance the carbons are separated from each other. However, in picture projection it is found that, with a given amperage, there is one certain distance at which the carbons must be separated from each other in order to secure the best possible projection light, and this distance cannot be allowed to vary appreciably without injuring the illumination of the screen, nor does the resistance vary to any large extent with ordinary differences in amperage. Therefore the resistance of the D. C. arc, when it ...