. American telephone practice . parent thatthe parts as connected form three arms of a Wheatstone bridge, thefourth, of course, being the unknown resistance joined to the line TESTING. 861 posts. This is shown diagrammatically in Fig. 629, where the cor-responding parts are similarly lettered. It will be noticed that this latter figure is practically the sameas Fig. 626, with the addition of the center blocks, A, B, X, R,forming a sort of commutator. The object of this arrangementis to make it possible to reverse the connections of arms, A andB, with R and X. Thus with the plugs in the positio
. American telephone practice . parent thatthe parts as connected form three arms of a Wheatstone bridge, thefourth, of course, being the unknown resistance joined to the line TESTING. 861 posts. This is shown diagrammatically in Fig. 629, where the cor-responding parts are similarly lettered. It will be noticed that this latter figure is practically the sameas Fig. 626, with the addition of the center blocks, A, B, X, R,forming a sort of commutator. The object of this arrangementis to make it possible to reverse the connections of arms, A andB, with R and X. Thus with the plugs in the position shown bythe black dots the connection is precisely as shown in Fig. 626, and the equation — = — holds true. If, however, the plugs are inserted in the holes on the other diagonal, arm, A, will be con-nected to arm, X, and arm, B, to arm, R, and the equation of /? /? the bridge will be —- == -^.fe AX The bridge arms, A and B, have not the same range of resist-ances in this bridge, A, having only 1, 10, and 100 ohm coils,. FIG. 629.—CIRCUITS OF PORTABLE TESTING SET. while the resistances of B are 10, 100, and 1000 ohms. Therefore,if a ratio of 1000 to 1 for measuring large resistances is desired,the plugs are inserted in the commutator along the arrow H () ; while an opposite arrangement of the plugs along the arrowL will give a ratio of 1 to 1000 for very small resistances. In thisbridge the galvanometer key is so arranged as to short-circuit thegalvanometer while the key is up. The galvanometers usually furnished with the complete bridgesconsist of a needle so mounted as to swing freely in a hori-zontal plane. This needle is given a tendency to point in onedirection, sometimes by the action of the earths magnetic field andsometimes by the field of a powerful permanent magnet. By caus-ing the current through the bridge wire to flow through a coil. 862 AMERICAN TELEPHONE PRACTICE. either stationary and surrounding the needle, or movable and car-ried on the needle
Size: 2069px × 1208px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., boo, bookcentury1900, bookdecade1900, bookidamericantelepho00mill
