=Note.= The student will be surprised, perhaps, to find that different results are obtained for the resistance of a given wire in case he uses different standard resistances in the various tests; that is, he will probably get a different result in Exp. 127 (A) from the result of Exp. 127 (B). The difference here, however, may not be large. The best results are obtained by making the standard resistance as nearly equal as possible to the resistance to be measured, so that a balance can be found when the end of wire 3 (Fig. 105) is near the center of the bridge-wire. If R, Fig. 105, is much larger or smaller than X, the point desired on G-s W will be near one of its ends, and large errors thereby produced. The approximate resistance of X can be found by trial, then more or less resistance can be used for R to suit. The student should make several coils as explained in Apparatus Book, Chapter XVII. The resistance of the different coils furnished should be measured and marked. These can be used to vary the value of R. =EXPERIMENT 130.= =334. Directions.= (A) Measure the resistance of the coil of No. 25 copper wire (No. 90). (See Exp. 126 for details and the Note, Exp. 129.) =EXPERIMENT 131.= =335. Directions.= (A) Measure the combined resistance of the two coils used in Exps. 129 and 130, when they are joined in "series"; that is, when one end of one coil is joined to one end of the other by means of a metal plate, the free ends being connected to the bridge (Exp. 126). The current has to travel through the entire length of both coils. (B) Compare this result with the sum of their separate resistances found in Exps. 129 and 130. (See Exp. 129, Note.) =EXPERIMENT 132.= =336. Directions.= (A) Measure the resistance of the two coils (Exp. 131) when they are joined "in parallel." (See § 293.) They may be joined in parallel by connecting them both to the bridge at the same time, one end of each being slipped onto 2 (Fig. 103), the other end of each being joined to 3. In this way the current has two paths, side-by-side, to get from 2 to 3. (See Exp. 129, Note.) (B) Compare this resistance with that of Exp. 131. =EXPERIMENT 133.= =337. Directions.= (A) Measure the resistance of 1 meter of No. 28 German-silver wire. Use the wire as arranged on a board, Exp. 122 (Figs. 96 and 98), making the connections with the bridge from binding-posts, X and Z. (See Exp. 129, Note.) The wires connecting the bridge with the ends of the G-s wire will each have to be about 2 ft. long. In making deductions (§ 330) figure according to the length used. (B) Divide the total resistance by 100 to get the resistance of 1 cm. of the wire, and carefully mark off the board into cm. This will give 100 parts between X and Z. =EXPERIMENT 134.=