Electricity for Boys Part 6

WIRING UP A HOUSE.--The system of wiring up a house so that all doors and windows will be connected to form a burglar alarm outfit, is shown in Fig. 61. It will be understood that, in practice, the bell is mounted on or at the annunciator, and that, for convenience, the annunciator box has also a receptacle for the battery. The circuiting is shown diagramatically, as it is called, so as fully to explain how the lines are run. Two windows and a door are connected up with an annunciator having three drops, or numbers 1, 2, 3. The circuit runs from one pole of the battery to the bell and then to one post of the annunciator. From the other post a wire runs to one terminal of the switch at the door or window. The other switch terminal has a wire running to the other pole of the battery.

A, B, C represent the circuit wires from the terminals of the window and door switches, to the annunciators.

It is entirely immaterial which side of the battery is connected up with the bell.

From the foregoing it will readily be understood how to connect up any ordinary apparatus, remembering that in all cases the magnet must be brought into the electric circuit.

CHAPTER VIII

ACc.u.mULATORS. STORAGE OR SECONDARY BATTERIES

STORING UP ELECTRICITY.--In the foregoing chapters we have seen that, originally, electricity was confined in a bottle, called the Leyden jar, from which it was wholly discharged at a single impulse, as soon as it was connected up by external means. Later the primary battery and the dynamo were invented to generate a constant current, and after these came the second form of storing electricity, called the storage or secondary battery, and later still recognized as acc.u.mulators.

THE ACc.u.mULATOR.--The term _acc.u.mulator_ is, strictly speaking, the more nearly correct, as electricity is, in reality, "_stored_" in an acc.u.mulator. But when an acc.u.mulator is charged by a current of electricity, a chemical change is gradually produced in the active element of which the acc.u.mulator is made. This change or decomposition continues so long as the charging current is on. When the acc.u.mulator is disconnected from the charging battery or dynamo, and its terminals are connected up with a lighting system, or with a motor, for instance, a reverse process is set up, or the particles re-form themselves into their original compositions, which causes a current to flow in a direction opposite to that of the charging current.

It is immaterial to the purposes of this chapter, as to the charging source, whether it be by batteries or dynamos; the same principles will apply in either case.

[Ill.u.s.tration: _Fig. 62._ ACc.u.mULATOR GRIDS]

ACc.u.mULATOR PLATES.--The elements used for acc.u.mulator plates are red lead for the positive plates, and precipitated lead, or the well-known litharge, for the negative plates. Experience has shown that the best way to hold this material is by means of lead grids.

Fig. 62 shows the typical form of one of these grids. It is made of lead, cast or molded in one piece, usually square, as at A, with a wing or projection (B), at one margin, extending upwardly and provided with a hole (C). The grid is about a quarter of an inch thick.

THE GRID.--The open s.p.a.ce, called the grid, proper, comprises cross bars, integral with the plate, made in a variety of shapes. Fig. 62 shows three forms of constructing these bars or ribs, the object being to provide a form which will hold in the lead paste, which is pressed in so as to make a solid-looking plate when completed.

THE POSITIVE PLATE.--The positive plate is made in the following manner: Make a stiff paste of red lead and sulphuric acid; using a solution, say, of one part of acid to two parts of water. The grid is laid on a flat surface and the paste forced into the perforations with a stiff knife or spatula. Turn over the grid so as to get the paste in evenly on both sides.

The grid is then stood on its edge, from 18 to 20 hours, to dry, and afterwards immersed in a concentrated solution of chloride of lime, so as to convert it into lead peroxide. When the action is complete it is thoroughly rinsed in cold water, and is ready to use.

THE NEGATIVE PLATE.--The negative plate is filled, in like manner, with precipitated lead. This lead is made by putting a strip of zinc into a standard solution of acetate of lead, and crystals will then form on the zinc. These will be very thin, and will adhere together, firmly, forming a porous ma.s.s. This, when saturated and kept under water for a short time, may be put into the openings of the negative plate.

[Ill.u.s.tration: _Fig. 63._ a.s.sEMBLAGE OF ACc.u.mULATOR PLATES]

CONNECTING UP THE PLATES.--The next step is to put these plates in position to form a battery. In Fig. 63 is shown a collection of plates connected together.

For simplicity in ill.u.s.trating, the cell is made up of gla.s.s, porcelain, or hard rubber, with five plates (A), A, A representing the negative and B, B the positive plates. A base of grooved strips (C, C) is placed in the batteries of the cell to receive the lower ends of the plates. The positive plates are held apart by means of a short section of tubing (D), which is clamped and held within the plates by a bolt (E), this bolt also being designed to hold the terminal strip (F).

In like manner, the negative plates are held apart by the two tubular sections (G), each of which is of the same length as the section D of the positives. The bolt (H) holds the negatives together as well as the terminal (I). The terminals should be lead strips, and it would be well, owing to the acid fumes which are formed, to coat all bra.s.s work, screws, etc., with paraffine wax.

The electrolyte or acid used in the cell, for working purposes, is a pure sulphuric acid, which should be diluted with about four times its weight in water. Remember, you should always add the strong acid to the water, and never pour the water into the acid, as the latter method causes a dangerous ebullition, and does not produce a good mixture.

Put enough of this solution into the cell to cover the tops of the plates, and the cell is ready.

[Ill.u.s.tration: _Fig. 64._ CONNECTING UP STORAGE BATTERY IN SERIES]

CHARGING THE CELLS.--The charge of the current must never be less than 2.5 volts. Each cell has an output, in voltage, of about 2 volts, hence if we have, say, 10 cells, we must have at least 25 volts charging capacity. We may arrange these in one line, or in series, as it is called, so far as the connections are concerned, and charge them with a dynamo, or other electrical source, which shows a pressure of 25 volts, as ill.u.s.trated in Fig. 64, or, instead of this, we may put them into two parallel sets of 5 cells each, as shown in Fig. 65, and use 12.5 volts to charge with. In this case it will take double the time because we are charging with only one-half the voltage used in the first case.

The positive pole of the dynamo should be connected with the positive pole of the acc.u.mulator cell, and negative with negative. When this has been done run up the machine until it slightly exceeds the voltage of the cells. Thus, if we have 50 cells in parallel, like in Fig. 64, at least 125 volts will be required, and the excess necessary should bring up the voltage in the dynamo to 135 or 140 volts.

[Ill.u.s.tration: _Fig. 65._ PARALLEL SERIES]

[Ill.u.s.tration: _Fig. 66._ CHARGING CIRCUIT]

THE INITIAL CHARGE.--It is usual initially to charge the battery from periods ranging from 36 to 40 hours, and to let it stand for 12 or 15 hours, after which to re-charge, until the positive plates have turned to a chocolate color, and the negative plates to a slate or gray color, and both plates give off large bubbles of gas.

In charging, the temperature of the electrolyte should not exceed 100 Fahrenheit.

When using the acc.u.mulators they should never be fully discharged.

THE CHARGING CIRCUIT.--The diagram (Fig. 66) shows how a charging circuit is formed. The lamps are connected up in parallel, as ill.u.s.trated. Each 16-candle-power 105-volt lamp will carry 1/2 ampere, so that, supposing we have a dynamo which gives 110 volts, and we want to charge a 4-volt acc.u.mulator, there will be 5-volt surplus to go to the acc.u.mulator. If, for instance, you want the cell to have a charge of 2 amperes, four of these lamps should be connected up in parallel. If 3 amperes are required, use 6 lamps, and so on.

CHAPTER IX

THE TELEGRAPH

The telegraph is a very simple instrument. The key is nothing more or less than a switch which turns the current on and off alternately.

The signals sent over the wires are simply the audible sounds made by the armature, as it moves to and from the magnets.

MECHANISM IN TELEGRAPH CIRCUITS.--A telegraph circuit requires three pieces of mechanism at each station, namely, a key used by the sender, a sounder for the receiver, and a battery.

THE SENDING KEY.--The base of the sending instrument is six inches long, four inches wide, and three-quarters of an inch thick, made of wood, or any suitable non-conducting material. The key (A) is a piece of bra.s.s three-eighths by one-half inch in thickness and six inches long. Midway between its ends is a cross hole, to receive the pivot pin (B), which also pa.s.ses through a pair of metal brackets (C, D), the bracket C having a screw to hold one of the line wires, and the other bracket having a metal switch (E) hinged thereto. This switch bar, like the brackets, is made of bra.s.s, one-half inch wide by one-sixteenth of an inch thick.

Below the forward end of the key (A) is a cross bar of bra.s.s (F), screwed to the base by a screw at one end, to receive the other line wire. Directly below the key (A) is a screw (G), so that the key will strike it when moved downwardly. The other end of the bar (F) contacts with the forward end of the switch bar (E) when the latter is moved inwardly.

[Ill.u.s.tration: _Fig. 67._ TELEGRAPH SENDING KEY]

The forward end of the key (A) has a k.n.o.b (H) for the fingers, and the rear end has an elastic (I) attached thereto which is secured to the end of the base, so that, normally, the rear end is held against the base and away from the screw head (G). The head (J) of a screw projects from the base at its rear end. Key A contacts with it.

When the key A contacts with the screw heads G, J, a click is produced, one when the key is pressed down and the other when the key is released.

You will notice that the two plates C, F are connected up in circuit with the battery, so that, as the switch E is thrown, so as to be out of contact, the circuit is open, and may be closed either by the key A or the switch E. The use of the switch will be ill.u.s.trated in connection with the sounder.

[Ill.u.s.tration: _Fig. 68._ TELEGRAPH SOUNDER]

When the key A is depressed, the circuit of course goes through plate C, key A and plate F to the station signalled.

THE SOUNDER.--The sounder is the instrument which carries the electro-magnet.

In Fig. 68 this is shown in perspective. The base is six inches long and four inches wide, being made, preferably, of wood. Near the forward end is mounted a pair of electro-magnets (A, A), with their terminal wires connected up with plates B, B', to which the line wires are attached.