_Apparatus._ A compass (No. 18); a dry cell, D C (No. 51); wires with spring connectors attached (§ 226) for making connections. Fig. 66 shows a plan or top view of the arrangement. Any other form of cell will do in place of D C. =226. Electrical Connections.= One must constantly join wires, connect wires with apparatus, or connect one piece of apparatus to another, to make the proper electrical connections. A very simple method of connections has been used in all the apparatus described in these experiments. A little arrangement which we shall call a spring connector, S C, (Fig. 61), gives us a means of quickly making connections; that is, it does away with expensive binding-posts. It is made of brass, nickel plated, and may be used anywhere without affecting the magnetic needle. Six or eight wires, about No. 24 gauge, each about 1-1/2 ft. long, should be prepared with a connector at each end. You may use wire furnished (No. 53). Scrape the insulation from the ends of the wires for about 1-1/2 inches, then twist the bare ends around the connectors as shown in Fig. 61. The wire should pass around tightly at least 4 or 5 times and then be twisted a little, as shown, to help tighten it. Do not put it on so poorly or in such quantity that the part, B, will spread. =227.= Fig. 62 shows how the connector should be slipped upon a thin piece of metal, M, like that on the galvanoscope, for example. The wire, W, from the apparatus itself is permanently fastened under the head of the screw, S, while the wire from any other apparatus is one of those kept on hand as above mentioned and connected with S C. [Illustration: Fig. 62.] [Illustration: Fig. 63.] [Illustration: Fig. 64.] =228.= Fig. 63 shows how several wires may be quickly joined, electrically, by slipping the connectors at their ends upon a thin metal plate, M P, which may be a piece of tin, zinc, copper, etc. M P should not be too thick. In case the connectors become too much spread to pinch the plate, squeeze the part, A, a little more together. =229.= Fig. 64 shows the method of connecting with the special form of binding-post used, for example, upon the resistance coil, R C. The end, C, of S C, is pressed down into the tube, T, until you feel, by moving it, that it springs firmly against the sides of the tube. In case you wish S C to fit very tightly in T, one of the legs may be slightly bent outwards. [Illustration: Fig. 65.] =230.= The connector may be used in still another way; that is, by pushing the part, A, into the hole of an ordinary binding-post, (Fig. 65), and using it just the same as a thick wire.