Electricity for Boys Part 3

Now, before the pole piece (C) is put on, we will slip on a disc (E), made of hard rubber, then a thin rubber tube (F), and finally a rubber disc (G), so as to provide a positive insulation for the wire coil which is wound on the bobbin thus made.

HOW TO WIND.--In practice, and as you go further along in this work, you will learn the value, first, of winding one layer of insulated wire on the spool, coating it with sh.e.l.lac, and then putting on the next layer, and so on; when completely wound, the two wire terminals may be brought out at one end; but for our present purpose, and to render the explanation clearer, the wire terminals are at the opposite ends of the spool (H, H').

THE DYNAMO FIELDS.--Two of these spools are so made and they are called the _fields_ of the dynamo.

We will next prepare an iron bar (I), 5 inches long and 1/2 inch thick and 1-1/2 inches wide, then bore two holes through it so the distance measures 3 inches from center to center. These holes are to be threaded for the 3/4-inch cores (A). This bar holds together the upper ends of the cores, as shown in Fig. 23.

[Ill.u.s.tration: _Fig. 23._ BASE AND FIELDS a.s.sEMBLED]

We then prepare a base (J) of any hard wood, 2 inches thick, 8 inches long and 8 inches wide, and bore two 3/4-inch holes 3 inches apart on a middle line, to receive a pair of 3/4-inch cap screws (K), which pa.s.s upwardly through the holes in the base and screw into the pole pieces (C). A wooden bar (L), 1-1/2" 1-1/2", 8 inches long, is placed under each pole piece, which is also provided with holes for the cap screws (K). The lower side of the base (J) should be countersunk, as at M, so the head of the nut will not project. The fields of the dynamo are now secured in position to the base.

[Ill.u.s.tration: _Fig. 24._ DETAILS OF THE ARMATURE, CORE

_Fig. 25._ DETAILS OF THE ARMATURE, BODY]

THE ARMATURE.--A bar of iron (Fig. 24), 1" 1" and 2-1/4 inches long, is next provided. Through this bar (1) are then bored two 5/16-inch holes 1-3/4 inches apart, and on the opposite sides of this bar are two half-rounded plates of iron (3) (Fig. 25).

ARMATURE WINDING.--Each plate is 1/2 inch thick, 1-3/4 inches wide and 4 inches long, each plate having holes (4) to coincide with the holes (2) of the bar (1), so that when the two plates are applied to opposite sides of the bar, and riveted together, a cylindrical member is formed, with two channels running longitudinally, and transversely at the ends; and in these channels the insulated wires are wound from end to end around the central block (1).

MOUNTING THE ARMATURE.--It is now necessary to provide a means for revolving this armature. To this end a bra.s.s disc (5, Fig. 26) is made, 2 inches in diameter, 1/8 inch thick. Centrally, at one side, is a projecting stem (6) of round bra.s.s, which projects out 2 inches, and the outer end is turned down, as at 7, to form a small bearing surface.

[Ill.u.s.tration: _Fig. 26._ JOURNALS _Fig. 27._ COMMUTATOR, ARMATURE MOUNTINGS]

The other end of the armature has a similar disc (8), with a central stem (9), 1-1/2 inches long, turned down to 1/4-inch diameter up to within 1/4 inch of the disc (7), so as to form a shoulder.

THE COMMUTATOR.--In Fig. 27 is shown, at 10, a wooden cylinder, 1 inch long and 1-1/4 inches in diameter, with a hole (11) bored through axially, so that it will fit tightly on the stem (6) of the disc (5). On this wooden cylinder is driven a bra.s.s or copper tube (12), which has holes (13) opposite each other. Screws are used to hold the tube to the wooden cylinder, and after they are properly secured together, the tube (12) is cut by a saw, as at 14, so as to form two independent tubular surfaces.

[Ill.u.s.tration: _Fig. 28._ END VIEW ARMATURE, MOUNTED]

These tubular sections are called the commutator plates.

[Ill.u.s.tration: _Fig. 29._ TOP VIEW OF ARMATURE ON BASE]

In order to mount this armature, two bearings are provided, each comprising a bar of bra.s.s (15, Fig. 28), each 1/4 inch thick, 1/2 inch wide and 4-1/2 inches long. Two holes, 3 inches apart, are formed through this bar, to receive round-headed wood screws (16), these screws being 3 inches long, so they will pa.s.s through the wooden pieces (I) and enter the base (J). Midway between the ends, each bar (15) has an iron bearing block (17), 3/4" 1/2" and 1-1/2 inches high, the 1/4-inch hole for the journal (7) being midway between its ends.

COMMUTATOR BRUSHES.--Fig. 28 shows the base, armature and commutator a.s.sembled in position, and to these parts have been added the commutator brushes. The brush holder (18) is a horizontal bar made of hard rubber loosely mounted upon the journal pin (7), which is 2-1/2 inches long. At each end is a right-angled metal arm (19) secured to the bar (18) by screws (20). To these arms the brushes (21) are attached, so that their spring ends engage with the commutator (12). An adjusting screw (22) in the bearing post (17), with the head thereof bearing against the brush-holder (18), serves as a means for revolubly adjusting the brushes with relation to the commutator.

DYNAMO WINDINGS.--There are several ways to wind the dynamos. These can be shown better by the following diagrams (Figs. 30, 31, 32, 33):

THE FIELD.--If the field (A, Fig. 30) is not a permanent magnet, it must be excited by a cell or battery, and the wires (B, B') are connected up with a battery, while the wires (C, C') may be connected up to run a motor. This would, therefore, be what is called a "separately excited"

dynamo. In this case the battery excites the field and the armature (D), cutting the lines of force at the pole pieces (E), so that the armature gathers the current for the wires (C, C').

[Ill.u.s.tration: _Fig. 30._ FIELD WINDING]

[Ill.u.s.tration: _Fig. 31._ SERIES-WOUND]

SERIES-WOUND FIELD.--Fig. 31 shows a "series-wound" dynamo. The wires of the fields (A) are connected up in series with the brushes of the armature (D), and the wires (G, G') are led out and connected up with a lamp, motor or other mechanism. In this case, as well as in Figs. 32 and 33, both the field and the armature are made of soft gray iron. With this winding and means of connecting the wires, the field is constantly excited by the current pa.s.sing through the wires.

SHUNT-WOUND FIELD.--Fig. 32 represents what is known as a "shunt-wound"

dynamo. Here the field wires (H, H) connect with the opposite brushes of the armature, and the wires (I, I') are also connected with the brushes, these two wires being provided to perform the work required.

This is a more useful form of winding for electroplating purposes.

[Ill.u.s.tration: _Fig. 32._ SHUNT-WOUND _Fig. 32._ COMPOUND-WOUND]

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 pa.s.sing through a wire will cause a current to pa.s.s 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.

[Ill.u.s.tration: _Fig. 34._ _Fig. 35._ _Fig. 36._ TO THE RIGHT, COMPa.s.s 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 pa.s.sed 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 pa.s.sing 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 a.s.sume 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 a.s.sume 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).

[Ill.u.s.tration: _Fig. 37._ INDICATING DIRECTION OF CURRENT]

SIMPLE CURRENT DETECTOR.--A simple current detector may be made as follows:

Prepare a base 3' 4' in size and 1 inch thick. At each corner of one end fix a binding post, as at A, A', Fig. 37. Then select 20 feet of No.

28 cotton-insulated wire, and make a coil (B) 2 inches in diameter, leaving the ends free, so they may be affixed to the binding posts (A, A'). Now glue or nail six blocks (C) to the base, each block being 1"

1" 2", and lay the coil on these blocks. Then drive an L-shaped nail (D) down into each block, on the inside of the coil, as shown, so as to hold the latter in place.

[Ill.u.s.tration: _Fig. 38._ THE BRIDGE]

Now make a bridge (E, Fig. 38) of a strip of bra.s.s 1/2 inch wide, 1/16 inch thick and long enough to span the coil, and bend the ends down, as at F, so as to form legs. A screw hole (G) is formed in each foot, so it may be screwed to the base.

Midway between the ends this bridge has a transverse slot (H) in one edge, to receive therein the pivot pin of the swinging magnet. In order to hold the pivot pin in place, cut out an H-shaped piece of sheet bra.s.s (I), which, when laid on the bridge, has its ends bent around the latter, as shown at J, and the crossbar of the H-shaped piece then will prevent the pivot pin from coming out of the slot (H).

[Ill.u.s.tration: _Fig. 39._ DETAILS OF DETECTOR]

The magnet is made of a bar of steel (K, Fig. 39) 1-1/2 inches long, 3/8 inch wide and 1/16 inch thick, a piece of a clock spring being very serviceable for this purpose. The pivot pin is made of an ordinary pin (L), and as it is difficult to solder the steel magnet (K) to the pin, solder only a small disc (M) to the pin (L). Then bore a hole (N) through the middle of the magnet (K), larger in diameter than the pin (L), and, after putting the pin in the hole, pour sealing wax into the hole, and thereby secure the two parts together. Near the upper end of the pin (L) solder the end of a pointer (O), this pointer being at right angles to the armature (K). It is better to have a metal socket for the lower end of the pin. When these parts are put together, as shown in Fig. 37, a removable gla.s.s top, or cover, should be provided.

This is shown in Fig. 40, in which a square, wooden frame (P) is used, and a gla.s.s (Q) fitted into the frame, the gla.s.s being so arranged that when the cover is in position it will be in close proximity to the upper projecting end of the pivot pin (L), and thus prevent the magnet from becoming misplaced.