Fig. 87 shows such an arrangement, in which a large number of elements are placed in a small space. The junctures are so arranged that the alternate ones come together at one side. Fig. 88 shows a thermopile connected with a galvanometer. The heat of a match, or the cold of a piece of ice, will produce a current, even if held at some distance from the thermopile. The galvanometer should be a short-coil astatic one. (See "Study," Chapter XXIV., for experiments and home-made thermopile.) [Illustration: Fig. 88.] CHAPTER XI. MAGNETIC EFFECTS OF THE ELECTRIC CURRENT. =91. Electromagnetism= is the name given to magnetism that is developed by electricity. We have seen that if a magnetic needle be placed in the field of a magnet, its N pole will point in the direction taken by the lines of force as they pass from the N to the S pole of the magnet. [Illustration: Fig. 89.] =92. Lines of Force about a Wire.= When a current passes through a wire, the magnetic needle placed over or under it tends to take a position at right angles to the wire. Fig. 89 shows such a wire and needle, and how the needle is deflected; it twists right around from its N and S position as soon as the current begins to flow. This shows that the lines of force pass _around_ the wire and not in the direction of its length. The needle does not swing entirely perpendicular to the wire, that is, to the E and W line, because the earth is at the same time pulling its N pole toward the N. Fig. 90 shows a bent wire through which a current passes from C to Z. If you look along the wire from C toward the points A and B, you will see that _under_ the wire the lines of force pass to the left. Looking along the wire from Z toward D you will see that the lines of force pass opposite to the above, as the current comes _toward_ you. This is learned by experiment. (See "Study," Exp. 152, § 385, etc.) [Illustration: Fig. 90.] [Illustration: Fig. 91.] _Rule._ Hold the right hand with the thumb extended (Fig. 89) and with the fingers pointing in the direction of the current, the palm being toward the needle and on the opposite side of the wire from the needle. The north-seeking pole will then be deflected in the direction in which the thumb points. =93. Current Detectors.= As there is a magnetic field about a wire when a current passes through it, and as the magnetic needle is affected, we have a means of detecting the presence of a current. When the current is strong it is simply necessary to let it pass once over or under a needle; when it is weak, the wire must pass several times above and below the needle, Fig. 91, to give the needle motion. (See "Apparatus Book," Chapter XIII., for home-made detectors.) [Illustration: Fig. 92.]