Electricity for Boys Part 5

SIMPLE SWITCHES.--We have now gone over the simpler or elementary outlines of electrical phenomena, and we may commence to do some of the practical work in the art. We need certain apparatus to make connections, which will be constructed first.

A TWO-POLE SWITCH.--A simple two-pole switch for a single line is made as follows:

A base block (A, Fig. 43) 3 inches long, 2 inches wide and 3/4 inch thick, has on it, at one end, a binding screw (B), which holds a pair of fingers (C) of bra.s.s or copper, these fingers being bent upwardly and so arranged as to serve as fingers to hold a switch bar (D) between them.

This bar is also of copper or bra.s.s and is pivoted to the fingers. Near the other end of the base is a similar binding screw (E) and fingers (F) to receive the blade of the switch bar. The bar has a handle (G) of wood. The wires are attached to the respective binding screws (B, E).

DOUBLE-POLE SWITCH.--A double-pole switch or a switch for a double line is shown in Fig. 44. This is made similar in all respects to the one shown in Fig. 43, excepting that there are two switch blades (A, A) connected by a cross bar (B) of insulating material, and this bar carries the handle (C).

[Ill.u.s.tration: _Fig. 43._ TWO-POLE SWITCH]

[Ill.u.s.tration: _Fig. 44._ DOUBLE-POLE SWITCH]

Other types of switch will be found very useful. In Fig. 45 is a simple sliding switch in which the base block has, at one end, a pair of copper plates (A, B), each held at one end to the base by a binding screw (C), and having a bearing or contact surface (D) at its other end. At the other end of the base is a copper plate (E) held by a binding screw (F), to the inner end of which plate is hinged a swinging switch blade (G), the free end of which is adapted to engage with the plates (A, B).

[Ill.u.s.tration: _Fig. 45._ SLIDING SWITCH]

SLIDING SWITCH.--This sliding switch form may have the contact plates (A, B and C, Fig. 46) circularly arranged and any number may be located on the base, so they may be engaged by a single switching lever (H). It is the form usually adopted for rheostats.

REVERSING SWITCH.--A reversing switch is shown in Fig. 47. The base has two plates (A, B) at one end, to which the parallel switch bars (C, D) are hinged. The other end of the base has three contact plates (E, F, G) to engage the swinging switch bars, these latter being at such distance apart that they will engage with the middle and one of the outer plates.

The inlet wires, positive and negative, are attached to the plates (A, B, respectively), and one of the outlet wires (H) is attached to the middle contact plate (F), while the other wire is connected up with both of the outside plates. When the switch bars (C, D) are thrown to the left so as to be in contact with E, F, the outside plate (E) and the middle plate (F) will be positive and negative, respectively; but when the switch is thrown to the right, as shown in the figure, plate F becomes positive and plate E negative, as shown.

[Ill.u.s.tration: _Fig. 46._ RHEOSTAT FORM OF SWITCH]

PUSH b.u.t.tONS.--A push b.u.t.ton is but a modified structure of a switch, and they are serviceable because they are operating, or the circuit is formed only while the finger is on the b.u.t.ton.

[Ill.u.s.tration: _Fig. 47._ REVERSING SWITCH]

In its simplest form (Fig. 48) the push b.u.t.ton has merely a circular base (A) of insulating material, and near one margin, on the flat side, is a rectangular plate (B), intended to serve as a contact plate as well as a means for attaching one of the wires thereto. In line with this plate is a spring finger (C), bent upwardly so that it is normally out of contact with the plate (B), its end being held by a binding screw (D). To effect contact, the spring end of the finger (C) is pressed against the bar (B), as at E. This is enclosed in a suitable casing, such as will readily suggest itself to the novice.

ELECTRIC BELL.--One of the first things the boy wants to make, and one which is also an interesting piece of work, is an electric bell.

To make this he will be brought, experimentally, in touch with several important features in electrical work. He must make a battery for the production of current, a pair of electro-magnets to be acted upon by the current, a switch to control it, and, finally, he must learn how to connect it up so that it may be operated not only from one, but from two or more push b.u.t.tons.

[Ill.u.s.tration: _Fig. 48._ PUSH b.u.t.tON]

HOW MADE.--In Fig. 49 is shown an electric bell, as usually constructed, so modified as to show the structure at a glance, with its connections.

A is the base, B, B' the binding posts for the wires, C, C the electro-magnets, C' the bracket for holding the magnets, D the armature, E the thin spring which connects the armature with the post F, G the clapper arm, H the bell, I the adjusting screw on the post J, K the wire lead from the binding post B to the first magnet, L the wire which connects the two magnets, M the wire which runs from the second magnet to the post J, and N a wire leading from the armature post to the binding post B'.

[Ill.u.s.tration: _Fig. 49._ ELECTRIC BELL]

The principle of the electric bell is this: In looking at Fig. 49, you will note that the armature bar D is held against the end of the adjusting screw by the small spring E. When a current is turned on, it pa.s.ses through the connections and conduits as follows: Wire K to the magnets, wire M to the binding post J, and set screw I, then through the armature to the post F, and from post F to the binding post B'.

[Ill.u.s.tration: _Fig. 50._ ARMATURE OF ELECTRIC BELL]

ELECTRIC BELL--HOW OPERATED.--The moment a current pa.s.ses through the magnets (C, C), the core is magnetized, and the result is that the armature (D) is attracted to the magnets, as shown by the dotted lines (O), when the clapper strikes the bell. But when the armature moves over to the magnet, the connection is broken between the screw (I) and armature (D), so that the cores of the magnets are demagnetized and lose their pull, and the spring (E) succeeds in drawing back the armature.

This operation of vibrating the armature is repeated with great rapidity, alternately breaking and re-establishing the circuit, by the action of the current.

In making the bell, you must observe one thing, the binding posts (B, B') must be insulated from each other, and the post J, or the post F, should also be insulated from the base. For convenience we show the post F insulated, so as to necessitate the use of wire (N) from post (F) to binding post (B').

The foregoing a.s.sumes that you have used a cast metal base, as most bells are now made; but if you use a wooden base, the binding posts (B, B') and the posts (F, J) are insulated from each other, and the construction is much simplified.

It is better, in practice, to have a small spring (P, Fig. 50) between the armature (D) and the end of the adjusting screw (I), so as to give a return impetus to the clapper. The object of the adjusting screw is to push and hold the armature close up to the ends of the magnets, if it seems necessary.

If two bells are placed on the base with the clapper mounted between them, both bells will be struck by the swinging motion of the armature.

An easily removable cap or cover is usually placed over the coils and armature, to keep out dust.

A very simple annunciator may be attached to the bell, as shown in the following figures:

[Ill.u.s.tration: _Figs. 51-54._ ANNUNCIATOR]

ANNUNCIATORS.--Make a box of wood, with a base (A) 4" 5" and 1/2 inch thick. On this you can permanently mount the two side pieces (B) and two top and bottom pieces (C), respectively, so they project outwardly 4-1/2 inches from the base. On the open front place a wood or metal plate (D), provided with a square opening (D), as in Fig. 54, near its lower end. This plate is held to the box by screws (E).

Within is a magnet (F), screwed into the base (A), as shown in Fig. 51; and pivoted to the bottom of the box is a vertical armature (G), which extends upwardly and contacts with the core of the magnet. The upper end of the armature has a shoulder (H), which is in such position that it serves as a rest for a V-shaped stirrup (I), which is hinged at J to the base (C). This stirrup carries the number plate (K), and when it is raised to its highest point it is held on the shoulder (H), unless the electro-magnet draws the armature out of range of the stirrup. A spring (L) bearing against the inner side of the armature keeps its upper end normally away from the magnet core. When the magnet draws the armature inwardly, the number plate drops and exposes the numeral through the opening in the front of the box. In order to return the number plate to its original position, as shown in Fig. 51, a vertical trigger (M) pa.s.ses up through the bottom, its upper end being within range of one of the limbs of the stirrup.

This is easily made by the ingenious boy, and will be quite an acquisition to his stock of instruments. In practice, the annunciator may be located in any convenient place and wires run to that point.

[Ill.u.s.tration: _Fig. 55._ ALARM SWITCH ON WINDOW]

[Ill.u.s.tration: _Fig. 56._ BURGLAR ALARM ATTACHMENT TO WINDOW]

BURGLAR ALARM.--In order to make a burglar alarm connection with a bell, push b.u.t.tons or switches may be put in circuit to connect with the windows and doors, and by means of the annunciators you may locate the door or window which has been opened. The simplest form of switch for a window is shown in the following figures:

The base piece (A), which may be of hard rubber or fiber, is 1/4 inch thick and 1" 1-1/2" in size.

[Ill.u.s.tration: _Fig. 57._ BURGLAR ALARM CONTACT]

At one end is a bra.s.s plate (B), with a hole for a wood screw (C), this screw being designed to pa.s.s through the plate and also into the window-frame, so as to serve as a means of attaching one of the wires thereto. The inner end of the plate has a hole for a round-headed screw (C') that also goes through the base and into the window-frame. It also pa.s.ses through the lower end of the heart-shaped metal switch-piece (D).

The upper end of the base has a bra.s.s plate (E), also secured to the base and window by a screw (F) at its upper end. The heart-shaped switch is of such length and width at its upper end that when it is swung to the right with one of the lobes projecting past the edge of the window-frame, the other lobe will be out of contact with the plate (E).

[Ill.u.s.tration: _Fig. 58._ NEUTRAL POSITION OF CONTACT]

The window sash (G) has a removable pin (H), which, when the sash moves upwardly, is in the path of the lobe of the heart-shaped switch, as shown in Fig. 56, and in this manner the pin (H) moves the upper end of the switch (D) inwardly, so that the other lobe contacts with the plate (E), and establishes an electric circuit, as shown in Fig. 57. During the daytime the pin (H) may be removed, and in order to protect the switch the heart-shaped piece (D) is swung inwardly, as shown in Fig.

58, so that neither of the lobes is in contact with the plate (E).

WIRE CIRCUITING.--For the purpose of understanding fully the circuiting, diagrams will be shown of the simple electric bell with two push b.u.t.tons; next in order, the circuiting with an annunciator and then the circuiting necessary for a series of windows and doors, with annunciator attachments.

[Ill.u.s.tration: _Fig. 59._ CIRCUITING FOR ELECTRIC BELL]

CIRCUITING SYSTEM WITH A BELL AND TWO PUSH b.u.t.tONS.--Fig. 59 shows a simple circuiting system which has two push b.u.t.tons, although any number may be used, so that the bell will ring when the circuit is closed by either b.u.t.ton.

THE PUSH b.u.t.tONS AND THE ANNUNCIATOR BELLS.--Fig. 60 shows three push b.u.t.tons and an annunciator for each b.u.t.ton. These three circuits are indicated by A, B and C, so that when either b.u.t.ton makes contact, a complete circuit is formed through the corresponding annunciator.

[Ill.u.s.tration: _Fig. 60._ _Annunciators_]

[Ill.u.s.tration: _Fig. 61._ WIRING SYSTEM FOR A HOUSE]