. American telephone practice . epresented by the top and bottomrow of blocks, and if all plugs are removed the resistance in this arm 860 AMERICAN TELEPHONE PRACTICE. will amount to 11,110 ohms. The ratio arms A and B are repre-sented by the left and right-hand halves respectively of the centerrow. A galvanometer and suitable battery, together with batteryand galvanometer keys, are all mounted in a carrying case as connections of this instrument are indicated in Fig. 628, andare as follows: The top row of blocks is connected to the bottomrow by a heavy copper bar joining the right-h
. American telephone practice . epresented by the top and bottomrow of blocks, and if all plugs are removed the resistance in this arm 860 AMERICAN TELEPHONE PRACTICE. will amount to 11,110 ohms. The ratio arms A and B are repre-sented by the left and right-hand halves respectively of the centerrow. A galvanometer and suitable battery, together with batteryand galvanometer keys, are all mounted in a carrying case as connections of this instrument are indicated in Fig. 628, andare as follows: The top row of blocks is connected to the bottomrow by a heavy copper bar joining the right-hand blocks. Thisconnection is made very heavy so as to interpose no extra resistancein the rheostat. On the rheostat formed by the upper and lowerrows of blocks any resistance from 1 to 11,110 ohms may be obtained,the resistance being added by leaving out plugs. The lower left-hand block of the rheostat is connected to the lower binding post, D,forming one terminal of the unknown resistance. The upper post, 7B QDODDOO££T ■]. FIG. OF PORTABLE TESTING SET. C, forming the other terminal of the unknown resistance, is con-nected to block, X, which block is also joined to the galvanometerkey. The block, R, is connected to the upper left-hand block ofthe rheostat. The end blocks of the middle row are connected to-gether and to the + terminal of the battery. The — terminal ofthe battery is connected through the battery key to the lower left-hand end of the rheostat. One galvanometer terminal is connecteddirectly to the block, R, the left-hand block of the rheostat, and tothe back contact of the galvanometer key. The other galvanometerterminal is connected through the key to the block, X. By carefully following out these connections it will be apparent 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-respond
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