94. Astatic Needles and Detectors. By arranging two magnetized needles with their poles opposite each other, Fig. 92, an astatic needle is formed. The pointing-power is almost nothing, although their magnetic fields are retained. This combination is used to detect feeble currents. In the ordinary detector, the tendency of the needle to point to the N and S has to be overcome by the magnetic field about the coil before the needle can be moved; but in the astatic detector and galvanoscope this pointing-power is done away with. Fig. 93 shows a simple astatic galvanoscope. Fig. 67 shows an astatic galvanometer for measuring weak currents.
Fig. 93.
95. Polarity of Coils. When a current of electricity passes through a coil of wire, the coil acts very much like a magnet, although no iron enters into its construction. The coil becomes magnetized by the electric current, lines of force pass from it into the air, etc. Fig. 94 shows a coil connected to copper and zinc plates, so arranged with cork that the whole can float in a dish of dilute sulphuric acid. The current passes as shown by the arrows, and when the N pole of a magnet is brought near the right-hand end, there is a repulsion, showing that that end of the coil has a N pole.
Rule. When you face the right-hand end of the coil, the current is seen to pass around it in an anti-clockwise direction; this produces a N pole. When the current passes in a clockwise direction a S pole is produced.
Fig. 94.
96. Electromagnets. A coil of wire has a stronger field than a straight wire carrying the same current, because each turn adds its field to the fields of the other turns. By having the central part of the coil made of iron, or by having the coil of insulated wire wound upon an iron core, the strength of the magnetic field of the coil is greatly increased.
Lines of force do not pass as readily through air as through iron; in fact, lines of force will go out of their way to go through iron. With a coil of wire the lines of force pass from its N pole through the air on all sides of the coil to its S pole; they then pass through the inside of the coil and through the air back to the N pole. When the resistance to their passage through the coil is decreased by the core, the magnetic field is greatly strengthened, and we have an electromagnet.