Part 8 (2/2)

[Illustration: _Fig 79_ ILlustRATING VIBRATIONS]

SOUND WAVES--Sound waves are long and short; the long waves giving sounds which are low in the hexperiht, 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 a your ear close to the string you will find that while A has a low pitch, C is very ed instruth, which represents the distance between the dotted lines A is reater than E

HEARING ELECTRICITY--In electricity, mechanism has been made to enable man to note the action of the current By net, we can see its manifestations It is now but a step to devise so else electrically, the80_ THE MAGNETIC FIELD]

In the chapter on netic field extended out beyond the netis like a80

The80 at only one end, but its manifestations are alike at both ends It will be seen that the netic field extends out to a considerable distance and has quite a radius of influence

THE DIAPHRAGM IN A MAGNETIC FIELD--If, noe put a diaphragnet, 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 net, and this disturbance of the netic 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 sam (A) disturbed it

A SIMPLE TELEPHONE CIRCUIT--Fronets are connected up in the sanets have the sam will affect the other sile net are disturbed in unison

HOW TO MAKE A TELEPHONE--For experi 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 dia, bored and threaded at one end to receive a screw (C) which passes 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 dia 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 sht upon the end of the core, and when in position rests against an annular shoulder in the recess A hollow space (F) is thus provided behind the spool (D), so the tires froe 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

[Illustration: _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 froht out, and threaded through holes (K) drilled longitudinally through the walls of the case, and affixed to the end by ether 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 si 83 This has le 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

[Illustration: _Fig 83_ SIMPLE TELEPHONE CONNECTION]

COMPLETE INSTALLATION--To install a more complete system requires, at each end, a switch, a battery and an electro-neto bell You may use, for this purpose, a bell,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-throitches (D, D'), one tero to electric bells (F, F'), and froround The ground wires also have wires (H, H'), which go to the other terminals of the switch (D, D') The double-throitch (D, D'), in the two stations, is thrown over so the current, if any should pass through, will go through the bell to the ground, through the wires (E, G or E', G')

[Illustration: _Fig 84_ TELEPHONE STATIONS IN CIRCUIT]