_Bolt Cores._ Fig. 131. After winding on the coils, as directed in Apparatus Book, remove the nut and put on an extra washer, E W, so that the ends of the coils will not be pressed against the tin, but come out between the two washers. Push the screw-end of the bolt through holes (about 2 in. apart) punched in the tin, then put on the nut, as shown. Do not force the nut on too far,--just far enough to hold the cores in place. The ends of the wires are not shown in Figs. 130, 131. Connectors are fastened to them (§ 408). [Illustration: Fig. 132.] =408. Method of Joining Coils.= To produce the best results the poles of the horseshoe electromagnet should be unlike. As the coils are wound alike, their ends must be joined in such a manner that the current will pass around them in opposite directions; that is, if the current enters one coil at the outside end, O E, it must enter the other coil at the inside end, I E. Fig. 132 shows a plan of the connections, spring connectors being fastened to the coil-ends, to allow rapid and easy changes in the arrangement. L, M, and R are pieces of metal fastened to a strip of wood (No. 95), used to make connections from cells or other apparatus. They are turned up at each end as in Fig. 104, 3. Care should be taken not to get short circuits by allowing two wires to touch the tin base. By changing the ends of the coils upon L, M, and R (left, middle, and right), and by changing the direction in which the current enters the "combination connecting plates" (No. 95), it is evident that the nature of the poles can be regulated to suit. =EXPERIMENTS 164-173. To study horseshoe electromagnets.= _Apparatus._ Coils of wire with cores and yoke like those explained in this chapter. Coils fastened to tin base or yoke with wires leading from them to the combination connecting plates (No. 95, Fig. 132), are very handy. Cells; iron filings; compass; iron strip (No. 76). =EXPERIMENT 164. To test the poles.= =409. Directions.= (A) Arrange as in Fig. 126, but use the experimental magnets and combination connections (Fig. 132) in place of the single coil shown in Fig. 126. Join O of the key with L, and Zn of the cell with R of Fig. 132. When the key is pressed the current will enter the magnets from L and leave at R. (B) With the compass test the polarity of the cores as in Exp. 163, B, C. Make a sketch of the arrangement, and note which pole is N and which S. (C) See which way the current must pass around each coil, by the way it is wound, and compare the results of (B) with Exp. 154, Fig. 123. =EXPERIMENT 165. To test the poles.=