The Elements of Electrical Engineering; A First Year's Course for Students

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1902 edition. Excerpt: ...bright spots. These parts being at a much higher temperature, decompose the benzene vapour faster than the remaining portions, and so carbon is deposited at these weak parts most rapidly, and the bright spots soon disappear. The current is then increased, and so carbon is deposited till the filament is perfectly uniform and its resistance brought down to the required value, which is known by putting down a double contact key connecting the filament to a Wheat stone bridge after switching off the current, and the flashing is repeated till a balance is obtained on the bridge, as seen by a sensitive galvanometer close by. The filaments being all alike in length, it follows that they are also alike in sectional area when they have the same resistance, and will all absorb the same amount of power when connected to the same difference of potential. The filaments have now to be sealed in glass globes, which are blown from a piece of glass tubing about in. in diameter and in. thick in the wall, after which a tube is blown on the end, about T3ff in. in diameter and 3 in. long, to attach the bulb to the vacuum pump. The junction is constricted so that it may be easily sealed after exhausting, as shown in Fig. 128. The large end of the tube is now sealed up near the bulb, then blown out and expanded with a tool to admit the filament, which partly springs in. The platinum ends are stuck on to the side of the opening and adjusted in position so as to put no strain on the filament, and to hold it symmetrically in the bulb (Fig. 129). The opening is then closed by squeezing the sides together, and while the glass is still soft the comers arc snipped off. Two small dents are made in the top of the bulb with a blunt tool, and the projecting platinum wires...