Electricity for Boys Part 7

Midway between the magnets and the rear end of the base is a pair of upwardly projecting brackets (C). Between these are pivoted a bar (D), the forward end of which rests between the magnets and carries, thereon, a cross bar (E) which is directly above the magnets, and serves as the armature.

The rear end of the base has a screw (F) directly beneath the bar D of such height that when the rear end of the bar D is in contact therewith the armature E will be out of contact with the magnet cores (A, A). A spiral spring (G) secured to the rear ends of the arm and to the base, respectively, serves to keep the rear end of the key normally in contact with the screw F.

CONNECTING UP THE KEY AND SOUNDER.--Having made these two instruments, we must next connect them up in the circuit, or circuits, formed for them, as there must be a battery, a key, and a sounder at each end of the line.

In Fig. 69 you will note two groups of those instruments. Now observe how the wires connect them together. There are two line wires, one (A) which connects up the two batteries, the wire being attached so that one end connects with the positive terminal of the battery, and the other end with the negative terminal.

[Ill.u.s.tration: _Fig. 69._ A TELEGRAPH CIRCUIT]

The other line wire (B), between the two stations, has its opposite ends connected with the terminals of the electro-magnet C of the sounders.

The other terminals of each electro-magnet are connected up with one terminal of each key by a wire (D), and to complete the circuit at each station, the other terminal of the key has a wire (E) to its own battery.

TWO STATIONS IN CIRCUIT.--The ill.u.s.tration shows station 2 telegraphing to station 1. This is indicated by the fact that the switch F' of that instrument is open, and the switch F of station 1 closed. When, therefore, the key of station 2 is depressed, a complete circuit is formed which transmits the current through wire E' and battery, through line A, then through the battery of station 1, through wire E to the key, and from the key, through wire D, to the sounder, and finally from the sounder over line wire B back to the sounder of station 2, completing the circuit at the key through wire D'.

When the operator at station 2 closes the switch F', and the operator at station 1 opens the switch F, the reverse operation takes place. In both cases, however, the sounder is in at both ends of the line, and only the circuit through the key is cut out by the switch F, or F'.

THE DOUBLE CLICK.--The importance of the double click of the sounder will be understood when it is realized that the receiving operator must have some means of determining if the sounder has transmitted a dot or a dash. Whether he depresses the key for a dot or a dash, there must be one click when the key is pressed down on the screw head G (Fig. 62), and also another click, of a different kind, when the key is raised up so that its rear end strikes the screw head J. This action of the key is instantly duplicated by the bar D (Fig. 68) of the sounder, so that the sounder as well as the receiver knows the time between the first and the second click, and by that means he learns that a dot or a dash is made.

ILl.u.s.tRATING THE DOT AND THE DASH.--To ill.u.s.trate: Let us suppose, for convenience, that the downward movement of the lever in the key, and the bar in the sounder, make a sharp click, and the return of the lever and bar make a dull click. In this case the ear, after a little practice, can learn readily how to distinguish the number of downward impulses that have been given to the key.

_The Morse Telegraph Code_

A . - N - . & . ...

B - ... O .. 1 . - - .

C .. . P ..... 2 .. - ..

D - . . Q .. - . 3 ... - .

E . R . .. 4 .... - F . - . S ... 5 - - - G - - . T - 6 ......

H .... U .. - 7 - - ..

I .. V ... - 8 - ....

J - . - . W . - - 9 - .. - K - . - X . - .. 0 ---- ------ L -- Y .. ..

M - - Z ... .

EXAMPLE IN USE.--Let us take an example in the word "electrical."

E L E C T R I C A L . -- . .. . - . .. .. .. . . - --

The operator first makes a dot, which means a sharp and a dull click close together; there is then a brief interval, then a lapse, after which there is a sharp click, followed, after a comparatively longer interval, with the dull click. Now a dash by itself may be an L, a T, or the figure 0, dependent upon its length. The short dash is T, and the longest dash the figure 0. The operator will soon learn whether it is either of these or the letter L, which is intermediate in length.

In time the sender as well as receiver will give a uniform length to the dash impulse, so that it may be readily distinguished. In the same way, we find that R, which is indicated by a dot, is followed, after a short interval, by two dots. This might readily be mistaken for the single dot for E and the two dots for I, were it not that the time element in R is not as long between the first and second dots, as it ordinarily is between the single dot of E when followed by the two dots of I.

CHAPTER X

HIGH TENSION APPARATUS, CONDENSERS, ETC.

INDUCTION.--One of the most remarkable things in electricity is the action of induction--that property of an electric current which enables it to pa.s.s from one conductor to another conductor through the air.

Another singular and interesting thing is that the current so transmitted across s.p.a.ces changes its direction of flow, and, furthermore, the tension of such a current may be changed by transmitting it from one conductor to another.

LOW AND HIGH TENSION.--In order to effect this latter change--that is, to convert it from a low tension to a high tension--coils are used, one coil being wound upon the other; one of these coils is called the primary and the other the secondary. The primary coil receives the current from the battery, or source of electrical power, and the secondary coil receives charges, and transmits the current.

For an ill.u.s.tration of this examine Fig. 70, in which you will note a coil of heavy wire (A), around which is wound a coil of fine wire (B).

If, for instance, the primary coil has a low voltage, the secondary coil will have a high voltage, or tension. Advantage is taken of this phase to use a few cells, as a primary battery, and then, by a set of _Induction Coils_, as they are called, to build up a high-tension electro-motive force, so that the spark will jump across a gap, as shown at C, for the purpose of igniting the charges of gas in a gasoline motor; or the current may be used for medical batteries, and for other purposes.

[Ill.u.s.tration: _Fig. 70._ INDUCTION COIL AND CIRCUIT]

The current pa.s.ses, by induction, from the primary to the secondary coil. It pa.s.ses from a large conductor to a small conductor, the small conductor having a much greater resistance than the large one.

ELASTIC PROPERTY OF ELECTRICITY.--While electricity has no resiliency, like a spring, for instance, still it acts in the manner of a cus.h.i.+on under certain conditions. It may be likened to an oscillating spring acted upon by a bar.

Referring to Fig. 71, we will a.s.sume that the bar A in falling down upon the spring B compresses the latter, so that at the time of greatest compression the bar goes down as far as the dotted line C. It is obvious that the spring B will throw the bar upwardly. Now, electricity appears to have a kind of elasticity, which characteristic is taken advantage of in order to increase the efficiency of the induction in the coil.

[Ill.u.s.tration: _Fig. 71._ ILl.u.s.tRATING ELASTICITY]

THE CONDENSER.--To make a condenser, prepare two pine boards like A, say, eight by ten inches and a half inch thick, and sh.e.l.lac thoroughly on all sides. Then prepare sheets of tinfoil (B), six by eight inches in size, and also sheets of paraffined paper (C), seven by nine inches in dimensions. Also cut out from the waste pieces of tinfoil strips (D), one inch by two inches. To build up the condenser, lay down a sheet of paraffined paper (C), then a sheet of tinfoil (B), and before putting on the next sheet of paraffined paper lay down one of the small strips (D) of tinfoil, as shown in the ill.u.s.tration, so that its end projects over one end of the board A; then on the second sheet of paraffine paper lay another sheet of tinfoil, and on this, at the opposite end, place one of the small strips (D), and so on, using from 50 to 100 of the tinfoil sheets. When the last paraffine sheet is laid on, the other board is placed on top, and the whole bound together, either by wrapping cords around the same or by clamping them together with bolts.

[Ill.u.s.tration: _Fig. 72._ CONDENSER]

You may now make a hole through the projecting ends of the strips, and you will have two sets of tinfoil sheets, alternately connected together at opposite ends of the condenser.

Care should be exercised to leave the paraffine sheets perfect or without holes. You can make these sheets yourself by soaking them in melted paraffine wax.

CONNECTING UP A CONDENSER.--When completed, one end of the condenser is connected up with one terminal of the secondary coil, and the other end of the condenser with the other secondary terminal.

[Ill.u.s.tration: _Fig. 73._ HIGH-TENSION CIRCUIT]

In Fig. 73 a high-tension circuit is shown. Two coils, side by side, are always used to show an induction coil, and a condenser is generally shown, as ill.u.s.trated, by means of a pair of forks, one resting within the other.

THE INTERRUPTER.--One other piece of mechanism is necessary, and that is an _Interrupter_, for the purpose of getting the effect of the pulsations given out by the secondary coil.

A simple current interrupter is made as follows: Prepare a wooden base (A), one inch thick, six inches wide, and twelve inches long. Upon this mount a toothed wheel (B), six inches in diameter, of thin sheet metal, or a bra.s.s gear wheel will answer the purpose. The standard (C), which supports the wheel, may be of metal bent up to form two posts, between which the crankshaft (D) is journaled. The base of the posts has an extension plate (E), with a binding post for a wire. At the front end of the base is an L-shaped strip (F), with a binding post for a wire connection, and the upwardly projecting part of the strip contacts with the toothed wheel. When the wheel B is rotated the spring finger (F) snaps from one tooth to the next, so that, momentarily, the current is broken, and the frequency is dependent upon the speed imparted to the wheel.

[Ill.u.s.tration: _Fig. 74._ CURRENT INTERRUPTER]

USES OF HIGH-TENSION COILS.--This high-tension coil is made use of, and is the essential apparatus in wireless telegraphy, as we shall see in the chapter treating upon that subject.

CHAPTER XI

WIRELESS TELEGRAPHY

TELEGRAPHING WITHOUT WIRES.--Wireless telegraphy is an outgrowth of the ordinary telegraph system. When Maxwell, and, later on, Hertz, discovered that electricity, magnetism, and light were transmitted through the ether, and that they differed only in their wave lengths, they laid the foundations for wireless telegraphy. Ether is a substance which is millions and millions of times lighter than air, and it pervades all s.p.a.ce. It is so unstable that it is constantly in motion, and this phase led some one to suggest that if a proper electrical apparatus could be made, the ether would thereby be disturbed sufficiently so that its impulses would extend out a distance proportioned to the intensity of the electrical agitation thereby created.

SURGING CHARACTER OF HIGH-TENSION CURRENTS.--When a current of electricity is sent through a wire, hundreds of miles in length, the current surges back and forth on the wire many thousands of times a second. Light comes to us from the sun, over 90,000,000 of miles, through the ether. It is as reasonable to suppose, or infer, that the ether can, therefore, convey an electrical impulse as readily as does a wire.