[Illustration: Fig. 577.--Connections and circuits of Queen acme portable testing set. There are three rows of blocks, LL', MM', NN'. LL is connected to NN' by means of a heavy copper bar, joining L' and N'. LL' and NN' constitute the rheostat, from which any resistance from 1 ohm to 11,110 ohms may be obtained by removing the proper plugs. The block N of the rheostat is connected to the lower line post D. The upper line post C is connected to the block X of the commutator. The block C has no other permanent connection, except key G. The block R of the commutator is connected to the block L of the rheostat, and has no other connection, excepting by plugs. Each half of MM' constitutes a bridge arm, designated A and B respectively. Beginning at the lower line post D, the connections form a continuous circuit through the rheostat, thence through the bridge arm B, thence through the bridge arm A, thence to the upper line post C. The commutator serves merely to reverse the bridge arms A and B. The battery terminals are connected as shown: the positive terminal directly to the common junction of the two bridge arms, and the negative terminal through the battery key to the rheostat. The positive terminal of the galvanometer is connected through the galvanometer key with the block X, and the negative terminal with the block R of the commutator or what is equivalent, with the block L of the rheostat. The commutator blocks A, B, R and X, are connected by plugs as shown. When the commutator plugs are in the position PQ, the bridge arm B is connected to the rheostat and the bridge arm A is connected to the line, the ratio between the bridge arms ratio being A ÷ B = X ÷ R but when the plugs are in the position ST, the bridge arms are reversed in position A, being connected with the rheostat and B, with the line, and the bridge arm ratio becomes A ÷ B = R ÷ X. The connections of the testing set may be more readily understood from the simplified diagram fig. 578.] [Illustration: Fig. 578.--Simplified diagram showing connections of Queen Acme portable testing set.] Ques. Describe the operation of the Queen Acme testing set figs. 576 and 577, in measuring resistance.