COMPOUND-WOUND FIELD.--Fig. 33 is a diagram of a "compound-wound" dynamo. The regular field winding (J) has its opposite ends connected directly with the armature brushes. There is also a winding, of a comparatively few turns, of a thicker wire, one terminal (K) of which is connected with one of the brushes and the other terminal (K') forms one side of the lighting circuit. A wire (L) connects with the other armature brush to form a complete lighting circuit. CHAPTER V HOW TO DETECT AND MEASURE ELECTRICITY MEASURING INSTRUMENTS.--The production of an electric current would not be of much value unless we had some way by which we might detect and measure it. The pound weight, the foot rule and the quart measure are very simple devices, but without them very little business could be done. There must be a standard of measurement in electricity as well as in dealing with iron or vegetables or fabrics. As electricity cannot be seen by the human eye, some mechanism must be made which will reveal its movements. THE DETECTOR.--It has been shown in the preceding chapter that a current of electricity passing through a wire will cause a current to pass through a parallel wire, if the two wires are placed close together, but not actually in contact with each other. An instrument which reveals this condition is called a _galvanometer_. It not only detects the presence of a current, but it shows the direction of its flow. We shall now see how this is done. For example, the wire (A, Fig. 35) is connected up in an electric circuit with a permanent magnet (B) suspended by a fine wire (C), so that the magnet (B) may freely revolve. [Illustration: _Fig. 34._ _Fig. 35._ _Fig. 36._ TO THE RIGHT, COMPASS MAGNET, TO THE LEFT] For convenience, the magnetic field is shown flowing in the direction of the darts, in which the dart (D) represents the current within the magnet (B) flowing toward the north pole, and the darts (E) showing the exterior current flowing toward the south pole. Now, if the wire (A) is brought up close to the magnet (B), and a current passed through A, the magnet (B) will be affected. Fig. 35 shows the normal condition of the magnetized bar (B) parallel with the wire (A) when a current is not passing through the latter. DIRECTION OF CURRENT.--If the current should go through the wire (A) from right to left, as shown in Fig. 34, the magnet (B) would swing in the direction taken by the hands of a clock and assume the position shown in Fig. 34. If, on the other hand, the current in the wire (A) should be reversed or flow from left to right, the magnet (B) would swing counter-clock-wise, and assume the position shown in Fig. 36. The little pointer (G) would, in either case, point in the direction of the flow of the current through the wire (A). [Illustration: _Fig. 37._ INDICATING DIRECTION OF CURRENT] SIMPLE CURRENT DETECTOR.--A simple current detector may be made as follows: