Part 7 (1/2)

Reference was nition of internal-coines (p 101) to the _induction coil_ This is a device for increasing the _voltage_, or pressure, of a current The two-cell accue (otherwise called electro-motive force = EMF) of 44 volts If you attach a wire to one terminal of the accumulator and brush the loose end rapidly across the other terht spark passes between the wire and the terminal In reality there are two sparks, one when they touch, and another when they separate, but they occur so closely together that the eye cannot separate the two impressions A spark of this kind would not be sufficiently hot to ignite a charge in a motor cylinder, and a spark from the induction coil is therefore used

[Illustration: FIG 53--Sketch of an induction coil]

We give a sketch of the induction coil in Fig 53 It consists of a core of soft iron wires round which is wound a layer of coarse insulated wire, denoted by the thick line One end of the winding of this _primary_ coil is attached to the battery, the other to the base of a ha between the end of the core and a screw, S, passing through an upright, T, connected with the other terminal of the battery

The action of the hammer is precisely the same as that of the armature of an electric bell Outside the primary coil are wound many turns of a much finer wire completely insulated from the primary coil The ends of this _secondary_ coil are attached to the objects (in the case of aand a wire projecting fro) bethich a spark has to pass As soon as H touches S the circuit is conet with external lines of force passing fro the turns of the secondary coil H is almost instantaneously attracted by the core, and the break occurs The lines of force now (at least so it is supposed) sink into the core, cutting through the turns of the ”secondary,” and causing a powerful current to flow through thereater the nuer is the current If sufficiently intense, it ju the intermediate air to a state of incandescence

THE CONDENSER

The sudden parting of H and S would produce strong sparking across the gap between them if it were not for the condenser, which consists of a number of tinfoil sheets separated by layers of paraffined paper All the ”odd” sheets are connected with T, all the ”even” with T1 Now, the netism in the core after ”break” of the primary circuit, the more rapidly will the lines of force collapse, and the more intense will be the induced current in the secondary coil

The condenser dithening the period of netization after the ”th of the reverse current

TRANSFORMATION OF CURRENT

The difference in the voltage of the prith of the windings If there are 100 turns of wire in the prie will be increased 1,000 times; so that a 4-volt current is ”stepped up” to 4,000 volts In the largest induction coils the secondary winding absorbs 200-300up to four feet in length Such a spark would pierce a glass plate two inches thick

It must not be supposed that an induction coil increases the _aiven off by a battery It merely increases its pressure at the expense of its voluh to do what a low-tension flow could not effect A fair coy of the low-tension current as the momentum of a number of small pebbles thrown in succession at a door, say 100 a ht make no impression on it, but if you could knead every 100 pebbles into a single stone, and throw these stones one per minute, you would soon break the door in

Any interards its intensity

Youits rate of flow, or _ae_; or decrease its pressure and increase its flow In the case that we have considered, a continuous battery current is rendered intermittent by a mechanical contrivance But if the current co” dynamo--that is, is already intermittent--the contact-breaker is not needed There will be raphs

USES OF THE INDUCTION COIL

The induction coil is used--(1) For passing currents through glass tubes alases The luminous effects of these ”Geissler” tubes are very beautiful (2) For producing the now faiven off at the negative terminal (cathode) of a vacuuht on a fluorescent screen, which reduces their rate of vibration sufficiently for the eye to be sensitive to the ht, such as metals, stone, wood, etc, and as a consequence have proved of great use to the surgeon in localizing or deter the nature of an internal injury

They also have a deterrent effect upon cancerous growths (3) In wireless telegraphy, to cause powerful electric oscillations in the ether (4) On es (5) For electrical e of the body

[11] ”What is Electricity?” p 46

[12] If a netized bar be heated to white heat and tapped with a hanetism, because the distance between the molecules has increased, and the inal positions

Chapter VI

THE ELECTRIC TELEGRAPH

Needle instrunetic needle--Method of reversing the current--Sounding instruh-speed telegraphy

Take a small pocket compass and wind several turns of fine insulated wire round the case, over the top and under the bottom Now lay the compass on a table, and turn it about until the coil is on a line with the needle--in fact, covers it Next touch the terminals of a battery with the ends of the wire The needle at once shi+fts either to right or left, and re as the current flows If you change the wires over, so reversing the direction of the current, the needle at once points in the other direction It is to this conduct on the part of a netic field” that e the existence of the needle telegraph instrument

NEEDLE INSTRUMENTS