Electricity for Boys Part 8

It is on this principle that impulses are sent for thousands of miles, and no doubt they extend even farther, if the proper mechanism could be devised to detect movement of the waves so propagated.

THE COHERER.--The instrument for detecting these impulses, or disturbances, in the ether is generally called a _coherer_, although detector is the term which is most satisfactory. The name coherer comes from the first practical instrument made for this purpose.

[Ill.u.s.tration: _Fig. 75._ WIRELESS TELEGRAPHY COHERER]

HOW MADE.--The coherer is simply a tube, say, of gla.s.s, within which is placed iron filings. When the oscillations surge through the secondary coil the pressure or potentiality of the current finally causes it to leap across the small s.p.a.ce separating the filings and, as it were, it welds together their edges so that a current freely pa.s.ses. The bringing together of the particles, under these conditions, is called cohering.

Fig. 75 shows the simplest form of coherer. The posts (A) are firmly affixed to the base (B), each post having an adjusting screw (C) in its upper end, and these screw downwardly against and serve to bind a pair of horizontal rods (D), the inner ends of which closely approach each other. These may be adjusted so as to be as near together or as far apart as desired. E is a gla.s.s tube in which the ends of the rods (D) rest, and between the separated ends of the rods (D) the iron filings (F) are placed.

THE DECOHERERS.--For the purpose of causing the metal filings to fall apart, or decohere, the tube is tapped lightly, and this is done by a little object like the clapper of an electric bell.

In practice, the coils and the parts directly connected with it are put together on one base.

THE SENDING APPARATUS.--Fig. 76 shows a section of a coil with its connection in the sending station. The spark gap rods (A) may be swung so as to bring them closer together or farther apart, but they must not at any time contact with each other.

The induction coil has one terminal of the primary coil connected up by a wire (B) with one post of a telegraph key, and the other post of the key has a wire connection (C), with one side of a storage battery. The other side of the battery has a wire (D) running to the other terminal of the primary.

[Ill.u.s.tration: _Fig. 76._ WIRELESS SENDING APPARATUS]

The secondary coil has one of its terminals connected with a binding post (E). This binding post has an adjustable rod with a k.n.o.b (F) on its end, and the other binding post (G), which is connected up with the other terminal of the secondary coil, carries a similar adjusting rod with a k.n.o.b (H).

From the post (E) is a wire (I), which extends upwardly, and is called the aerial wire, or wire for the antennae, and this wire also connects with one side of the condenser by a conductor (J). The ground wire (K) connects with the other binding post (G), and a branch wire (L) also connects the ground wire (K) with one end of the condenser.

[Ill.u.s.tration: _Fig. 77._ WIRELESS RECEIVING APPARATUS]

THE RECEIVING APPARATUS.--The receiving station, on the other hand, has neither condenser, induction coil, nor key. When the apparatus is in operation, the coherer switch is closed, and the instant a current pa.s.ses through the coherer and operates the telegraph sounder, the galvanometer indicates the current.

Of course, when the coherer switch is closed, the battery operates the decoherer.

HOW THE CIRCUITS ARE FORMED.--By referring again to Fig. 76, it will be seen that when the key is depressed, a circuit is formed from the battery through wire B to the primary coil, and back again to the battery through wire D. The secondary coil is thereby energized, and, when the full potential is reached, the current leaps across the gap formed between the two k.n.o.bs (F, H), thereby setting up a disturbance in the ether which is transmitted through s.p.a.ce in all directions.

It is this impulse, or disturbance, which is received by the coherer at the receiving station, and which is indicated by the telegraph sounder.

CHAPTER XII

THE TELEPHONE

VIBRATIONS.--Every manifestation in nature is by way of vibration. The beating of the heart, the action of the legs in walking, the winking of the eyelid; the impulses from the sun, which we call light; sound, taste and color appeal to our senses by vibratory means, and, as we have hereinbefore stated, the manifestations of electricity and magnetism are merely vibrations of different wave lengths.

THE ACOUSTIC TELEPHONE.--That sound is merely a product of vibrations may be proven in many ways. One of the earliest forms of telephones was simply a "sound" telephone, called the _Acoustic Telephone_. The principle of this may be ill.u.s.trated as follows:

Take two cups (A, B), as in Fig. 78, punch a small hole through the bottom of each, and run a string or wire (C) from the hole of one cup to that of the other, and secure it at both ends so it may be drawn taut.

Now, by talking into the cup (A) the bottom of it will vibrate to and fro, as shown by the dotted lines and thereby cause the bottom of the other cup (B) to vibrate in like manner, and in so vibrating it will receive not only the same amplitude, but also the same character of vibrations as the cup (A) gave forth.

[Ill.u.s.tration: _Fig. 78._ ACOUSTIC TELEPHONE]

[Ill.u.s.tration: _Fig. 79._ ILl.u.s.tRATING VIBRATIONS]

SOUND WAVES.--Sound waves are long and short; the long waves giving sounds which are low in the musical scale, and the short waves high musical tones. You may easily determine this by the following experiment:

Stretch a wire, as at B (Fig. 79), fairly tight, and then vibrate it.

The amplitude of the vibration will be as indicated by dotted line A.

Now, stretch it very tight, as at C, so that the amplitude of vibration will be as shown at E. By putting your ear close to the string you will find that while A has a low pitch, C is very much higher. This is the principle on which stringed instruments are built. You will note that the wave length, which represents the distance between the dotted lines A is much greater than E.

HEARING ELECTRICITY.--In electricity, mechanism has been made to enable man to note the action of the current. By means of the armature, vibrating in front of a magnet, we can see its manifestations. It is now but a step to devise some means whereby we may hear it. In this, as in everything else electrically, the magnet comes into play.

[Ill.u.s.tration: _Fig. 80._ THE MAGNETIC FIELD]

In the chapter on magnetism, it was stated that the magnetic field extended out beyond the magnet, so that if we were able to see the magnetism, the end of a magnet would appear to us something like a moving field, represented by the dotted lines in Fig. 80.

The magnetic field is shown in Fig. 80 at only one end, but its manifestations are alike at both ends. It will be seen that the magnetic field extends out to a considerable distance and has quite a radius of influence.

THE DIAPHRAGM IN A MAGNETIC FIELD.--If, now, we put a diaphragm (A) in this magnetic field, close up to the end of the magnet, but not so close as to touch it, and then push it in and out, or talk into it so that the sound waves strike it, the movement or the vibration of the diaphragm (A) will disturb the magnetic field emanating from the magnet, and this disturbance of the magnetic field at one end of the magnet also affects the magnetic field at the other end in the same way, so that the disturbance there will be of the same amplitude. It will also display the same characteristics as did the magnetic field when the diaphragm (A) disturbed it.

A SIMPLE TELEPHONE CIRCUIT.--From this simple fact grew the telephone.

If two magnets are connected up in the same circuit, so that the magnetic fields of the two magnets have the same source of electric power, the disturbance of one diaphragm will affect the other similarly, just the same as the two magnetic fields of the single magnet are disturbed in unison.

HOW TO MAKE A TELEPHONE.--For experimental and testing purposes two of these telephones should be made at the same time. The case or holder (A) may be made either of hard wood or hard rubber, so that it is of insulating material. The core (B) is of soft iron, 3/8 inch in diameter and 5 inches long, bored and threaded at one end to receive a screw (C) which pa.s.ses through the end of the case (A).

The enlarged end of the case should be, exteriorly, 2-1/4 inches in diameter, and the body of the case 1 inch in diameter.

[Ill.u.s.tration: _Fig. 81._ SECTION OF TELEPHONE RECEIVER]

Interiorly, the large end of the case is provided with a circular recess 1-3/4 inches in diameter and adapted to receive therein a spool which is, diametrically, a little smaller than the recess. The spool fits fairly tight upon the end of the core, and when in position rests against an annular shoulder in the recess. A hollow s.p.a.ce (F) is thus provided behind the spool (D), so the two wires from the magnet may have room where they emerge from the spool.

The spool is a little shorter than the distance between the shoulder (E) and the end of the casing, at G, and the core projects only a short distance beyond the end of the spool, so that when the diaphragm (H) is put upon the end of the case, and held there by screws (I) it will not touch the end of the core. A wooden or rubber mouthpiece (J) is then turned up to fit over the end of the case.

[Ill.u.s.tration: _Fig. 82._ THE MAGNET AND RECEIVER HEAD]

The spool (D) is made of hard rubber, and is wound with No. 24 silk-covered wire, the windings to be well insulated from each other.

The two ends of the wire are brought out, and threaded through holes (K) drilled longitudinally through the walls of the case, and affixed to the end by means of screws (L), so that the two wires may be brought together and connected with a duplex wire (M).

As the screw (C), which holds the core in place, has its head hidden within a recess, which can be closed up by wax, the two terminals of the wires are well separated so that short-circuiting cannot take place.

TELEPHONE CONNECTIONS.--The simplest form of telephone connection is shown in Fig. 83. This has merely the two telephones (A and B), with a single battery (C) to supply electricity for both. One line wire (D) connects the two telephones directly, while the other line (E) has the battery in its circuit.

[Ill.u.s.tration: _Fig. 83._ SIMPLE TELEPHONE CONNECTION]

COMPLETE INSTALLATION.--To install a more complete system requires, at each end, a switch, a battery and an electro-magneto bell. You may use, for this purpose, a bell, made as shown in the chapter on bells.

Fig. 84 shows such a circuit. We now dispense with one of the line wires, because it has been found that the ground between the two stations serves as a conductor, so that only one line wire (A) is necessary to connect directly with the telephones of the two stations.

The telephones (B, B', respectively) have wires (C, C') running to the pivots of double-throw switches (D, D'), one terminal of the switches having wires (E, E'), which go to electric bells (F, F'), and from the bells are other wires (G, G'), which go to the ground. The ground wires also have wires (H, H'), which go to the other terminals of the switch (D, D'). The double-throw switch (D, D'), in the two stations, is thrown over so the current, if any should pa.s.s through, will go through the bell to the ground, through the wires (E, G or E', G').

[Ill.u.s.tration: _Fig. 84._ TELEPHONE STATIONS IN CIRCUIT]