Now that the condenser has become so important a factor in telephone

work, many schemes for cheapening and facilitating their manufacture

have been devised. One in particular merits description, the

"rolled-up" condenser having come largely into use. The tin-foil is

supplied in rolls containing many yards of foil of the requisite width

for the condenser to be made. Likewise rolls of paper are provided,

exceeding in width, however, those of tin-foil. These rolls are

arranged upon horizontal spindles in front of an empty spindle, or

mandrel, upon which the condenser is to be formed. A few turns of the

paper ribbon are made around the mandrel, then the foil is brought

forward and a few turns made, then follows a turn of paper ribbon

and another of foil, and finally a paper layer; and the mandrel being

rotated, the alternate layers of foil and paper are laid on and rolled

around each other on the mandrel until the requisite quantity is

obtained. It then becomes an easy matter to cut the paper ends so no

contact is possible between the layers of foil. The whole thing is

slipped off the mandrel, secured by a rubber band or two, placed in a

hot paraffin bath, and left to become saturated while still warm and

before the paraffin has time to harden; the cylinder is put under a

press and squeezed flat, driving out excess paraffin, and leaving the

condenser in a convenient shape to handle. Connections are then made to

the foil leaves, and a case of wood or metal completes the work.

There is no reason why aluminum foil or lead foil, or, in fact, any

thin sheet metal should not be used in condensers. In telephone work,

paper covered with gilt paint was tried, and worked fairly well, but

was ultimately rejected in favor of tin-foil. In some cases, when

it is desired to construct a condenser for high potential work, the

oil-tank apparatus can be used. This is readily made of any desired

dimensions, as follows: Procure a square glass jar, such as is made

for storage batteries, a few pieces of sheet metal cut to fit loosely

in the jar, some glass rods and sufficient clean "transformer oil"

or heavy paraffin oil to nearly fill the jar. The sheets of metal

can then be hung from the glass rods into the jar, being separated

one-half inch, and the oil poured in. Two plates, about 8 inches by 6

inches, will hang nicely into a type D^3 Chloride Battery jar, which

is 7⅞ inches long by 9½ inches high by 3¼ inches wide. Altering the

relative distances between the plates will give considerable adjustment

to this simple condenser, or, if desired, more plates may be inserted

and connected up, as in the tin-foil condensers. This type can be made

portable, but it is not to be recommended unless no objection is had

to emptying and refilling the jar with oil.

ADJUSTABLE CONDENSERS.

In operating large coils, it is convenient to be able to vary the

capacity of the condenser on the primary circuit. To make an adjustable

condenser presents no more difficulty than a non-adjustable one, simply

more labor. For example, the large condenser used with the 6-inch

spark coil might be divided into four sections, containing 2000 square

inches, 500 square inches, 300 square inches, and 200 square inches of

surface (see Fig. 34). Wires leading from the ends of the foil sheets

_C C_ are to be brought to the brass plates _G G_. The brass rods

_B B_ are connected by binding posts to the coil, each strip being

well insulated from its neighbor. Any combination is possible by the

insertion of brass plugs in holes drilled between the strips. The plugs

must be fully large enough to make good contact on each of the two

strips between which they are inserted, and should be turned taper.

With the largest coils the condenser and contact breaker are generally

mounted separately, and are fully adjustable.

[Illustration: FIG. 34.]

SPECIFIC INDUCTIVE CAPACITY.

Dry air 1.000

Sulphur 2.590

Hard rubber 2.290

Paraffin 1.996

Shellac 2.750

Kerosene 2.225

Paraffin oil 2.710

Castor oil 4.962

Olive oil 3.575

Condensers made with dielectric of high inductive capacity (insulation

being equal) will retain greater charge than those made with

dielectrics of low inductive capacity. Thus, one made with shellac

would be nearly half as great again as with paraffin.

Capacity of a condenser increases with area of foil surface, with

diminished distance between foil plates and with increase of

insulation.

CHAPTER V.

EXPERIMENTS.

The luminous effects that can be obtained by means of a Ruhmkorff coil

are exceedingly beautiful and instructive. The simplest experiment

of this nature is the production of the spark consequent on the

approximation of the electrodes attached to the secondary coil. This

spark can be varied in both length, intensity, or shape by the form

and nature of the substances between which it is permitted to pass.

Attach to each end of the discharger a fine steel needle, and bring

them together until the spark jumps from one to the other. A long thin

snapping spark will pass, which, however, appears to be trying to take

any but a straight path across the air gap. The peculiar crookedness

of this, as in a lightning flash, is credited to the fact of particles

of matter floating in the air conducting the current better than the

pure air. The curious odor noticed in these discharges, as, in fact, in

the working of all high-tension apparatus, is ozone—O_{3}, triatomic

oxygen. This gas, so noticeable after a thunderstorm, has a powerful

effect on the mucous membranes of the throat and nasal passages,

and must be inhaled with caution. It is being used by the medical

profession for the destruction of germs and for general therapeutic

service.

Substitute pieces of fine iron wire for the needles, and bring the ends

together about one quarter the distance through which the normal spark

will pass. The spark will be found to have changed its appearance, now

being thick and redder, or, rather, of a deep yellow, and to possess

vast heating qualities.

The iron wire will melt at one electrode, and if the other be examined

it will be perceived that it has not even become warm. The cold wire

will be the one connected to the positive pole of the coil.

Connecting the poles together with a piece of very fine iron wire will

result in the deflagration of the wire in a vivid light.

The short thick spark is termed the calorific spark, and believed to

possess its yellow color from the combustion of the sodium in the air.

This spark will easily ignite a piece of paper held in its path.

Take a sheet of hard rubber and breathe on its surface; lay a wire from

each pole of the secondary to points on the sheet, about twice as far

apart as the spark would pass over in the air. The electric current

will strive to complete its circuit; streams of violet light forming a

perfect network will issue from each pole, until, provided the rubber

is sufficiently damp, they will unite in a spark far exceeding its

normal length in the air. It is curious to watch how the streams branch

out from these two points, and how persistently they strive to meet

each other. Scatter some finely powdered carbon on this sheet (crushed

lead-pencil or electric light carbon is good material). The points may

now be removed to still further distant places, and yet the current

will work across. Each particle of carbon seems to be provided with

innumerable scintillating diamonds, so sparkling is this effect.

Hard rubber is not absolutely necessary for these experiments; glass

will do, but the black background of the rubber intensifies the

luminosity of the discharges. Take a teaspoonful of powdered carbon and

scatter it between the points on the rubber, so that the spark can find

a ready path, evidenced by but little visible light. It will be seen

that this powder is blown away from one electrode after a few minutes,

leaving the latter in the centre of a clear space, but at the other

electrode not much disturbed.

Bring the points so close to one another that the spark becomes short

and fat; soon the carbon will commence to burn, forming a veritable arc

light. Take two pointed lead-pencils and wrap a few turns of wire from

the electrodes round the blunt ends of them; bring the pointed ends

together, and an arc will soon be established; but at various points

where the wire is wrapped the current will burn through the wood, and a

number of incandescent points will ensue.

In these experiments on the rubber sheet it will be noticed that the

spark acts as it does in the air, inasmuch as it does not take a direct

path, but jumps in an irregular track from point to point.

If two small metal balls be substituted (Fig. 35) for the points

between which the sparks be passing, it will be noted that the sparks

do not pass through so great an air gap as before, or even as rapidly.

The spark between two balls is much noisier than that passing between

points, and if the balls be of about 1 inch in diameter, a curious

effect ensues on the passage of the current (Fig. 36). This effect has

been likened to a stream of water issuing from a horizontal nozzle into

a cavity when the nozzle is moved up and down slowly in the space of a

few inches.

[Illustration: FIG. 35.]

[Illustration: FIG. 36.]

THE LUMINOUS PANE.

[Illustration: FIG. 37.]

This easily made exhibit (Fig. 37) is one that is susceptible of quite

a number of applications. In its simple form it is but an enlarged

version of the rubber sheet scattered with carbon dust. The old way

to make it was to take a plate of glass and cement on one face of it

a sheet of tinfoil, using shellac varnish preferably. When dry, the

tinfoil was scored across and across in such manner as to divide

it up into little squares or diamonds. When the current was applied

to each end of the plate, the spark divided into innumerable little

ones; between each bit of tinfoil and its neighbors there would be

many little sparks, and the effect was very pretty, somewhat as

was described before when the carbon dust was strewn between the

electrodes. It is more easily and quickly prepared by giving a sheet

of glass a coating of shellac varnish, and then sparingly dusting any

powdered conductor over its surface, using perhaps carbon dust or

filings of metal. By cutting out a stencil from a piece of thin card

and laying it over the sparkling plate, the design shows out very

strikingly, and various designs in stencils can be prepared, different

powdered conductors giving different colored sparks.

A long glass tube moistened inside with mucilage or shellac varnish and

then having some conducting dust shaken through will also give quite a

pleasing effect.

LUMINOUS DESIGNS.

Coat one side of a glass plate with tinfoil, leaving an attached strip

for connection. Shellac a piece of paper of a size corresponding to the

design to be rendered luminous. When the shellac has dried so far as to

become "tacky," lay a sheet of foil on it and press it down evenly all

over.

Then draw on the paper a design that can be readily cut out. Use a pair

of scissors or a very sharp knife. If the latter, lay the sheet on a

piece of glass; but there is a greater tendency to tear the design when

a knife is used if an unpractised hand wields it.

This design may either be stuck on to the plain side of the glass plate

with varnish or simply laid on (Fig. 38). Connect one secondary wire to

the foil coating of the plate and the other to the design. This must be

shown in the dark, and the luminosity will not be strikingly apparent

until the eyes become accustomed to the darkness—that is, when the

room has been previously lighted.

One of the most beautiful and easily obtained phenomena of the

high-tension discharge is the "electric brush" (Fig. 39). This occurs

when the secondary electrodes of the coil are too far apart to allow

of the free passage of the spark, and can only be seen at its best in

a perfectly dark place. The ball tips before mentioned show this brush

very plainly, or two sheets of tinfoil in circuit hung far enough apart

to prevent vivid sparking will cause this so-called "silent" discharge.

This latter arrangement should not be used for over fifteen minutes,

as the ozone which is liberated in large quantities will affect those

persons in the vicinity.

[Illustration: FIG. 38.]

[Illustration: FIG. 39.]

In fact, when a rapid vibrator is being used with the coil, the leading

wires from the secondary terminals present this brush appearance, the

curious threads of light resembling luminous hairs waving in the air.

The more rapid the vibrations the more prominent the brush effect, as

will be seen in the Tesla coils. The positive ball of the discharger

shows the brush as a spreading mass of luminous threads reaching out

toward the negative ball, which latter resembles a star, as in the

figure.

The intensely disruptive power of the long spark is readily shown by

its power to perforate substances, but great care must be taken that

the secondary wires of a coil are led away from the body of the coil. A

good plan is to hang two silk cords or stout threads from the ceiling,

to which the secondary wires may be attached and kept in sight when

experimenting at any distance from the coil.

To pierce a piece of thin glass, take two lumps of paraffin about the

size of a walnut, and, warming them and the glass sheet, stick them on

opposite sides of the glass facing each other. Then warm the ends of

the two pointed wires and thrust them into the lumps of paraffin, that

they terminate on the glass surface directly opposite each other. On

connecting these to the secondary coil a few impulses to the contact

breaker will start an electric discharge sufficient to pierce the glass

if the thickness be proportioned to the power of the apparatus. The

great Spottiswood coil pierced a block of glass 6 inches in thickness.

There is, however, a certain element of danger to the secondary

insulation in performing this experiment.

CHAPTER VI.

SPECTRUM ANALYSIS.

If a metal or the salt of a metal be burned in a flame it imparts to

the flame a distinctive color; table salt thrown into the fire burns

with a yellowish flame, denoting the presence of sodium, and a greenish

tint, indicating the combustion of chlorine. Violet flames accompany

the burning of the salts of potassium, and barium burns green. Lithium

and strontium give a red hue. But to be ordinarily perceptible,

the salts require for the most part to be present in considerable

quantities. By the use of the spectroscope, however, extremely small

proportions of these metals and salts can be readily detected and

classified.

[Illustration: FIG. 40.]

If a beam of light be transmitted through a prism of glass the rays

are decomposed, and what is known as a spectrum is formed (Fig.

40). The most generally observed spectrum is the rainbow. When the

light from a flame in which is burning some suitable substance be

transmitted through the prism, the color which predominates in the

flame will predominate in its spectrum. The combination of a prism and

tubes for observing these effects is a spectroscope (Fig. 41). The

short fat spark from the Ruhmkorff coil is most useful in this work.

The electrodes are provided with a portion of the substance to be

examined, and the spark is passed and viewed through the spectroscope.

[Illustration: FIG. 41.]

The spectroscope is shown in connection with the coil in Fig. 41. _A_

is the aperture in the screen through which the rays from the metal

burning at the discharger balls _D D_ passes. The lens at _L_ is used

to view these rays after they have been decomposed by the prism _P_,

which, as well as the lens, can be rotated. _I_ is the coil, _P P_ the

primary and _S S_ the secondary wires, _C_ being a condenser bridged

across the circuit.

The screen should be pierced by a very narrow aperture, _A_, and be

placed at a considerable distance from the prism _P_, that the rays

issuing through the aperture may not strike the prism until they have

widely diverged and become separated from each other. The aperture is

practically formed of perfectly parallel knife edges, forming a slit

not exceeding one hundredth of an inch in width.

The colored spaces in the solar spectrum do not occupy an equal extent

of area; the violet is the most extended, the orange the least. The

proportion is in three hundred parts: Violet, 80; green, 60; yellow,

48; red, 45; indigo, 40; orange, 27.

The solar rays exhibit on careful examination dark lines crossing

the spectrum at right angles to the order of the colors, and always

occupying the same relative positions. These are called Fraunhofer's

lines.

If, however, the spectra of metals, gases, and other elements be

examined they will be found to present certain characteristic _bright_

lines, the body of the spectrum being often feeble or entirely dark.

The spectrum of hydrogen gives two very bright lines of red and orange.

An extremely minute quantity of an element is necessary to give

distinct lines. Sodium gives a single or double line of yellow light in

a position agreeing with that of the orange rays in the solar spectrum.

Potassium gives a red line in the red end and a violet line in the

violet end of the solar spectrum. Strontium presents eight bright

lines; calcium gives mainly one broad green band and one bright orange

band.

In practical work with the spectroscope a solar spectrum is often

arranged that it can be used as a comparison with the spectrum being

investigated, one spectrum being formed above the other, and the

observation made as to which lines coincide. Iron gives nearly sixty

bright lines coinciding with the same number of dark lines of the solar

spectrum.

The violet rays of the solar spectrum are the rays which possess the

maximum chemical action, the yellow the maximum light effect, the red

the maximum heating effect. Beyond the violet band of the spectrum

exist certain rays termed the invisible rays or ultra-violet rays,

which in themselves are not luminous. Their vibratory rate is higher

and their wave length shorter than the violet rays, according to the

most generally accepted theory of light. These rays, when passed

through certain substances, suffer a change and become visible

in a luminous state of the substance, which luminosity is termed

fluorescence.

The bright yellow line of sodium in the orange rays is found in nearly

all spectra, owing to its extensive diffusion in the atmosphere.

Tesla has succeeded in producing electric waves of length approximating

to those of white light, which appear to have very little heat. The

ideal light is that which shows no heat and does not liberate noxious

gases in the air, and were it not for its feeble luminosity, the light

of the electric spark passing through a carbonic acid vacuum would

approximate this most nearly.

The present mode of obtaining light—that of raising to a high

temperature some substance or collection of particles—seems certainly

somewhat antiquated. The following notes may be of interest and

assistance in researches bearing on the lighting question.

Solid bodies, when heated, show a red glow in daylight at an elevation

of temperature corresponding to 1000° Fahr.

Temperature, Color of

degrees F. Substance.

1000 Red.

1200 Orange.

1300 Yellow.

1500 Blue.

1700 Indigo.

2000 Violet.

2130 All colors—_i.e._, white.

The number of vibrations per second necessary for the production of

light, and the velocity of light being determined, the calculation of

the wave lengths of the colored rays becomes possible.

The following table (Sprague) shows this in ten-millionths of a

millimetre (a millimetre = .039 inch) measured in the dark lines of the

solar spectrum, from red to violet:

Orange = 6.88

Orange, Higher = 6.56

Yellow = 5.89

Green = 5.26

Blue = 4.84

Blue, Higher = 4.29

Violet = 3.93

CHAPTER VII.

CURRENTS IN VACUO.

Notwithstanding it requires an intensely high potential to enable the

current to jump an air gap of 1 inch, the same potential will produce a

luminous discharge through exhausted glass tubes aggregating 8 feet or

even more.

But the exhaustion can be carried so far that there is no apparent

discharge; and, on the contrary, air at as high a pressure as 600

pounds per square inch will resist the passage of the spark over an

extremely short space. If the tubes be filled with various gases

and then partially exhausted, the length of tube through which the

luminous discharge will pass varies with the gas, becoming shorter in

the following order: Hydrogen, nitrogen, air, oxygen, and carbonic

acid—the shortest.

[Illustration: FIG. 42.]

Before detailing some of the more striking phenomena connected with

high-tension discharges in vacuo, a description of a few forms of

simple mercurial air pumps will be serviceable.

Fig. 42: If a glass tube, _F_, stopped at one end, 3 feet long or

over, be filled with mercury and the open end immersed in a vessel

of mercury, _T_, the column of metal in the tube will sink until it

attains a height, _M_, of about 30 inches, varying according to the

condition of the atmosphere.

The space between the mercury column and the top of the tube will be

a fairly good vacuum. This fact was noted many years ago, and the

gradual evolution of the mercurial air-pump based on this result can

be followed in the articles on the mercurial air-pump by Silvanus P.

Thompson, read before the Society of Arts, England, some years ago.

Geissler, the first manufacturer of the "Geissler" or vacuum tube for

electrical research, seeing the inconvenience of the above-described

operation and the meagre results obtained, invented the pump called by

his name (Fig. 43).

_F E_ is a stout glass tube some 3 feet long, having a bulb, _B_, at

its upper extremity, and a rubber tube, _S_, attached to the curved

end. A reservoir of mercury, _R_, connects with this rubber tube, and a

special glass tap is fixed in the upper end of the glass tube at _E_,

beyond which tap being the point of attachment for the object to be

exhausted. The operation is as follows: On turning the tap a part of

the way it allows a passage between the tube _F E_ and the atmosphere.

The reservoir _R_ is then raised until the mercury flows into the bulb

and up the tube to the tap. The tap is then turned a fraction, and the

communication with the air is shut off and opened between the object to

be exhausted and the tube _F E_. The reservoir is then lowered and the

mercury falls, drawing down the air from the object into the tube. The

tap is then turned as in the first place, and the reservoir _R_ raised,

when the air drawn into the tube is forced out by the rising column of

metal. This operation being repeated many times, withdraws nearly all

the air from the object—in fact, makes a fairly good vacuum. This pump

has been much modified from the simple form described.

The form of pump most used in the United States lamp factories is based

on the application of the piston-like action of a quantity of mercury

dropping down a tube. This is known as the Sprengel pump, after the

inventor.

[Illustration: FIG. 43.]

[Illustration: FIG. 44.]

Fig. 44: _F_ is a stout glass tube about 40 inches long by

one-twelfth of an inch internal diameter, carrying the reservoir

funnel _R_ at the top, a piece of soft rubber tubing, _S_, nipped by a

pinch-cock being interposed to admit of the regulation of the mercurial

drops. The lower end of this "fall tube," as it is called, is immersed

in mercury contained in a suitable vessel, _V_, a branch tube being

blown or cemented into the fall tube to admit of the connection of the

object to be exhausted at _E_. _S_ is another piece of rubber tubing

with a pinch-cock regulation. The point _H_ is the normal barometric

height of the mercury—about 30 inches. On attaching a bulb, for

example, at _E_, and regulating the pinch-cock at the top of the fall

tube _F_, a succession of drops of mercury falls down the tube, each

drop acting as a piston to drive the air before it, sucking the same

from the bulb, and forcing it down through the tube and vessel out into

the atmosphere.

On its first being set into operation, the cushions of air between the

drops silence their fall; but as a higher degree of rarefaction occurs,

the air cushions become insufficient, and the drops fall with a sharp

click on the top of the barometric column.

One great disadvantage in this form of pump is the tendency to fracture

of the glass tube that is manifested by the concussion of the drops of

mercury at the barometric height. However, this has to a certain extent

been obviated in later forms of this useful and efficient pump.

For many electrical experiments, the simple exhaust tube (Fig.

42) mentioned at the beginning of the article will be found very

satisfactory. The top end need not necessarily be sealed off with

glass, a cork having a wire, _W_, run through for connection being

driven in, and a coat of paraffin or one of the cements mentioned in a

later chapter be laid on.

The second electrical connection is made by a wire dipping in the

tumbler of mercury.

DISCHARGES IN VACUO.

In a simple glass tube having two wires carrying balls inserted

through its ends, from which the air has been partially exhausted, the

study of the changes shown by the passage of the spark is extremely

interesting. Before the commencement of exhaustion no luminous effect

can be discerned; at a low degree of exhaustion a luminosity appears

between the ends of the wires, the negative pole being surrounded by a

violet glow and a larger pear-shaped red discharge from the positive.

An interval near the negative electrode is in darkness, widening as the

exhaustion progresses. When the degree of exhaustion is very high, a

series of arches concentric with the positive ball appear and become

broader and more distinct as the rarefaction progresses. The arches or

bands are called striæ, and are most distinct when the tube is made in

the form of a narrow cylinder, with a bulb at each end. Carbonic acid

gas vacua give the best results. If the finger be placed on the bulb at

either end a luminous spot appears, and by using a very rapid contact

breaker in the primary circuit, the luminous discharges become highly

sensitive, being diverted from their regular path on the approach of

the hand, a magnet, or a grounded wire. An extended treatment of these

phenomena would be out of place here, but can be found in nearly all

comprehensive works on electricity.

If an incandescent-lamp bulb be held in the hand and one end be brought

near to a terminal of the coil, a beautiful bluish light appears.[2]

The carbon filament, if long, and not held by its loop, becomes

electrified and oscillates, often giving out a clear, high, bell-like

sound as it strikes the glass. Particles of carbon deposited on the

glass during the burning of the lamp, shown in daylight as a blackening

deposit, generally show little sparks, like stars scattered over the

inside of the globe.

[2] This depends on the degree of exhaustion.

A vacuum tube will phosphoresce if held in the hand near a secondary

terminal, or even if laid on the table near the coil, and will light

quite brilliantly if one end be held against a terminal. This latter

method, however, is generally inconvenient, as a certain amount of

physical pain ensues from the discharge into the skin.

Different gases in the tubes give characteristic colors. In carbonic

acid gas the whitish green hue prevails; in hydrogen, white and red; in

nitrogen, orange yellow. The characteristic spectra are given by the

gases in the tubes, and can be readily examined in the spectroscope.

But there is sometimes a slight variation in these colors, dependent

upon changes in the current.

In many Geissler tubes, a portion of the bulbs is made of uranium

glass. On the passage of the spark in the tube this glass glows with

a magnificent emerald green hue. Other tubes are constructed with an

outside enveloping glass tube fitted with a corked orifice into which

can be poured different solutions.

[Illustration: FIG. 45.]

[Illustration: FIG. 46.]

[Illustration: FIG. 47.]

Fig. 45 shows a solution tube to be filled with solution of sulphate of

quinine, etc.

Fig. 46 shows three exhausted tubes arranged in series.

_A_ is of uranium glass, and glows dark green; _B_ of English glass,

showing a blue hue, and _C_ of soft German glass, glowing with a bright

apple-green tint.

Crystals of nitrate of calcium, nitrate of silver, benzoic acid,

tungstate of calcium, lithia benzoate, sodium salicylate, zinc

sulphide, and acetate of zinc fluoresce.

Fig. 47 is a highly exhausted tube, having at its lowest part a few

pieces of ruby. When the secondary current is turned on at _P_ and _N_

the rubies shine with a brilliant rich red, as if they were glowing hot.

[Illustration: FIG. 48.]

Fig. 48 shows the tube to exhibit the effect resulting from focussing

the electric rays on a piece of iridio-platinum at _B_.

The cup _A_ forms the negative pole; the metal disk _C_, the positive.

On increasing the intensity of the spark, the metal at _B_ glows with

extreme brilliancy, and melts if the intensity be carried too far.

CHAPTER VIII.

ROTATING EFFECTS.

Although the luminous discharges in the exhausted tubes are extremely

beautiful, yet the effect is indescribably enhanced when the tubes are

rotated. Gassiot's star was the name given to the earliest exhibit of

a rotating tube carrying a luminous discharge, owing to the curious

phenomenon ensuing from the interruptions of the spark. As the human

retina is only capable of retaining an impression for a fraction of a

second, and as the tube is only momentarily luminous during the passage

of the spark, the effect of the revolving tube is that of a series of

such arranged as the radii of a circle, the number apparent, being

governed by the rapidity of rotation and the rate of interruption of

the current.

[Illustration: FIG. 49.]

Fig. 49 represents a form of rotating wheel which is easily made, and

yet susceptible of many novel and attractive effects. Such a wheel,

placed in a store window, would undoubtedly attract many persons by

the beautiful variations of colored figures which it presents while

in motion. And once a crowd is collected and its attention attracted

to one spot, the capabilities of advertising the goods on sale are

apparent.

A pasteboard or light wooden disk _D_, 3 feet in diameter or over, is

mounted on a shaft, _S_, operated by an electric motor or such power

as may be attainable. Upon its surface are mounted the tube-holders _T

T T T_, connected, as shown, by wires leading from the secondary of

the Ruhmkorff coil. Starting at the shaft _S_, the circuit runs to the

first tube-holder, where the continuity of the wire is broken to allow

of the attachment of the vacuum tube. From the first tube-holder the

wire runs in turn to each of the other three tube-holders, terminating

at _R_, where it passes through a hole to a metal ring on the back of

the disk shown by the dotted circle. This ring and the shaft are in

connection with the secondary coil, by reason of its electrodes being

attached to two brushes or strips of metal pressing, one on the ring,

the other on the shaft; or the bearing in which the shaft turns may

displace one of the brushes. _W W_ are two counterbalance weights,

that the wheel may run smoothly and be not affected by the irregular

distribution of the tubes or its surface. _E E_ are elastic bands,

looped over the wire and through rings in the disk, that the wires may

not be liable to touch or short circuit.

At Fig. 50 is an enlarged view of a tube-holder, although, as it

is meant only as a diagram, considerable variation of design is

permissible. The springs at _H H_, to which the wires run, being bent

back, the metal pins _P P_ may be thrust through the rings on the ends

of the tube, and the elasticity and pressure of the spring will hold it

in place and make the necessary contact. A wooden block, _B_, secured

to the face of the disk, is provided with a thumb-screw, _S_, securing

the tube-holder to it, by means of which the tube-holders may be

turned a trifle upon their axes and so vary the effect of the wheel.

[Illustration: FIG. 50.]

[Illustration: FIG. 51.]

Fig. 51 is a side view of the wheel, showing one manner of mounting

the disk and its connections. The same figures apply to the parts as

in the preceding figure. _M N_ are the wires leading to the coil, _P_

is a pulley on the shaft whereby the rotary power may be applied. The

wires on the face of the disk are not shown, as they would impair the

clearness of the diagram unnecessarily.

The greatest danger in the operation of such a piece of apparatus

will be the tendency of the high tension spark to wander where it is

not wanted, and to take short but forbidden paths back to the coil.

However, care and perhaps experiment will prove the remedy. It will be

noticed by reference to Fig. 49 that a circle has been drawn almost

bisecting two of the tube-holders. This circle represents a circle of

danger, and where a thin material has been used for the disk, the disk

may very well be reinforced by a piece of stouter card cemented on its

face.

The disk, whether of wood or of pasteboard, must have a liberal coating

of insulation, either shellac varnish, paraffin, or beeswax, and be

absolutely free from unnecessary holes. Moreover, the ring _R_ must be

of such a distance from the support _F_, if the latter be metal, as

will preclude any jumping of the spark. A Ruhmkorff coil giving upward

of three quarters of an inch of spark will be large enough to operate a

wheel carrying four 8-inch tubes.

The wheel may be set back in a window and surrounded by dark fabrics,

or built in, as it were, in a cave of such. The judicious use of

pieces of looking-glass scattered on the sides of the cave, in such

manner as to reflect the light of the tubes, will enhance the effect.

There is no danger of fire where ordinary care is used, as the _long_

spark necessary to the production of the luminosity will hardly ignite

anything but gas, unless specially arranged to do so.

Fig. 52 is a triangle formed of three Geissler tubes, and intended for

rotation as a whole. _M M_ are two pieces of mica or glass, to prevent

any possibility of the spark jumping and short circuiting, in which

event the tubes would fail to light.

[Illustration: FIG. 52.]

This triangle is shown diagrammatically at _A B C_, Fig. 53, mounted

on an insulated rotating disk. Before commencement of rotation, and

upon the current being turned on to the tubes, a simple triangle will

result, but at a certain stage of rotation the Maltese cross shown

is formed. A still higher rate of rotation will produce the double

star, Fig. 54, and as the rotation and rate of vibration of the coil

contact-breaker is varied, an apparently endless succession of stars or

triangles appears to grow out into view.

[Illustration: FIG. 53.]

[Illustration: FIG. 54.]

Although Figs. 53 and 54 serve to illustrate a triangle of tubes and

its variations, a very pretty and simple effect can be obtained with it

as follows: Three strips of looking-glass are cut and scratched across

their silvered surface, as described for the luminous pane, Fig. 37.

The current then being allowed to pass, and the wheel being rotated,

the triangle acts as in the preceding paragraphs, multiplying and

forming figures, which are extremely interesting to watch.

While treating on the subject of store-window attractions, a few

suggestions on a display of stationary Geissler tubes may be made.

Starting with the assumption that the platform on which the goods

would be displayed is of wood, a very small amount of preparation is

necessary. The platform is covered with a dark material free from

gloss, such as canton flannel, on which the tubes are laid in any fancy

pattern, or may be scattered haphazard. Fine bare wire (No. 36 B. & S.

is not any too small) is run from tube to tube, using care that it does

not touch itself in such manner as to short circuit the current. There

is not much necessity to cover the wires, unless the rate of vibration

of the contact be so rapid as to show the brush discharge from the wire

strands. In a jewelry store the cylindrical portions of the tubes may

be covered with strips of dark cloth, concealing all but the bulbs.

The Uranium bulbs will resemble emeralds; the yellow bulbs, topaz; and

the blue, turquoise—certainly a very striking collection of gems. A few

diamond-shaped pieces of the foil-coated glass scratched across, by the

whiteness of the tiny sparks will aid to set off the whole. The outfit

is not expensive: a coil giving a one half inch spark will light from

four to six tubes to great brilliancy. Cloths with metallic threads

woven in them must not be used, nor any of the metallic powders known

in the trade as "glitters."

CHAPTER IX.

GAS LIGHTING.

When it is desired to light clusters of gas jets situated in

inaccessible places, or a number of them simultaneously, this method

finds ready application. It operates in the division of a long spark

among a number of burners, the gas being turned on at the main and

the primary circuit of a Ruhmkorff coil closed and opened until the

succession of sparks ignites the gas, Fig. 55. There are various

commercial forms of these burners, prominent among which is the "Smith

jump spark" burner.

[Illustration: FIG. 55.]

A lava tip is provided with a mica or isinglass flange midway between

the tip and the lower end of the burner. This flange isolates the

electrodes from any possibility of the spark straying away to the

metallic pillar in which the burner is inserted. The multiple lava tip

burner is intended for use where a very short burner is needed, also

for flash rings multiple lights. Here the tips are placed close enough

together to ignite by contagion. In this case one of the common tips

is removed from the ring and a multiple lava tip substituted. It is

customary to allow sixteen burners to one inch of spark. Any number of

series can be operated alternately by means of a suitable switch.

The wire used to connect the burners is generally bare copper, and as

small in diameter as will sustain its own weight without injury, the

amount of the current being infinitesimal. It is supported on porcelain

or glass knobs screwed to the wall or ceiling, being carefully planned

to avoid any metallic substances to which the spark might be tempted

to escape. In wiring chandeliers, the wire is run through glass tubes

wherever there is any liability of its coming near the metal pipes.

There is a very great danger of this jumping of the spark where it is

not wanted, and the utmost care must be taken in planning the course

the wires shall take. Even a damp wall will cause trouble or a gilt

cornice, although the latter may be entirely insulated from the ground.

The switch bases for the groups of circuits must be of hard rubber, and

the switch points and levers be placed so far apart that there is no

liability of the spark jumping, which it certainly will do if it gets

a chance. Ordinary insulated wires are ineffectually protected by the

rubber compounds used. Glass, mica, and better still, a large air gap

are the only insulations that will serve, for the tremendous potential

or voltage of the current must be carefully considered whenever

insulation is necessary. The coil is better provided with a spring key

in the primary circuit than a vibrator, it gives better control of the

circuit and probably a larger and better spark.

GAS LIGHTING IN MULTIPLE.

The spark which occurs at the contact breaker of a Ruhmkorff coil

is held in check by the condenser; were no condenser used, it would

possess considerable powers of combustion. Using a large primary coil

and a few cells of open circuit battery, this spark is made to pass

across the path of a gas jet, which it instantly ignites. The contact

breaker consists of a platinum point, fixed on the gas tip, and a

German silver spring, carried on a lever, which latter is pulled across

the tip so as to make and break the circuit at the burner orifice. Some

burners are provided with a ratchet arrangement, by which pulling the

lever once turns on and lights the gas, pulling again turns it off;

others require the gas to be turned on first.

[Illustration: FIG. 56.]

Reference to Fig. 56 will show the connections to two burners _P P_ and

an automatic burner _A_, to be described later. The coil _C_ is a core

of soft iron, about ¾ inch diameter and eight to ten inches long, wound

with from two to four pounds of magnet wire, Nos. 12 or 14 B & S. One

side of the battery goes to ground or to the gas pipe, thus forming the

return circuit. The wiring on the fixtures is done with No. 20 to 24 B

& S gas fixture wire, insulated with four windings of silk or cotton.

This is fastened to the lacquered brass work by means of thick shellac

varnish, it being tied on first with thread, which can readily be

removed when shellac is dry and hard. The wire is held on the insulated

collar of the burner by a small nut and screw, and great care must be

taken to ensure no grounding. The setting up of a gas-lighting outfit

is extremely simple, but it often fails for want of care. There must be

the best possible insulation between wire and metal work.

AUTOMATIC BURNERS.

There are several forms of these burners, but the principle of all is

the same. A gas burner protrudes from the top of a brass case which

encloses the actuating mechanism. This mechanism consists of two

electromagnets, the armature of one opening a valve and allowing the

gas to flow, at the same time vibrating a platinum-tipped rod, which

produces a series of sparks at the burner tip. These sparks ignite the

gas, and a second magnet is provided to shut off the flow of gas, thus

extinguishing the light. Some devices use one electromagnet for both

lighting or extinguishing, but the majority are with double magnets.

The circuit is worked from a push button situated at any desired

location, and having a white and black button, one for lighting and

the other for extinguishing. The principal automatic burners are the

Holtzer, the Boston, and the Bartholdi, between which there lies little

choice, so admirably are they constructed.

BARTHOLDI AUTOMATIC BURNER.

Instead of a rotating stop-cock, as in other automatics, a gravity

valve is employed in the Bartholdi, which is held to its seat by the

weight of the armature and connecting stem, as shown in Fig. 57. When

the gas is turned off the valve rests upon its seat, as indicated in

the cut. By a closure of the electric circuit at the turn-on button,

two of the helices _M P_ are energized, causing the armature _J_ to

be lifted, thus, by means of the stem _H_, raising the valve _G_ from

its seat into the dotted position, and opening the gas way so that

the gas may issue to the tip, as shown by the arrows. At the same

time, the top of the valve strikes against the end of the lever _W_,

causing the circuit to be broken at the spark points _T U_, resulting

in a continuous sparking as long as the finger presses the button.

The magnet when raising the armature has also twisted or partially

revolved it, so as to bring the notch _d_ in the armature over the end

of the hook _e_, as shown in dotted lines. When the circuit is broken

by lifting the finger from the button the notch falls into the hook and

the valve is locked open.

[Illustration: FIG. 57.]

To extinguish the flame the turn-off button is pressed, when a second

magnet (not shown in cut) lifts the armature and twists it in the

opposite direction, so that when the circuit is broken the armature

falls free to its normal position, closing the valve.

[Illustration: FIG. 58.]

In wiring up an automatic burner it is necessary to run two wires to

it, one from the white button and another from black button on push

plate _S_. Reference to Fig. 58 will make this clear. Most burners

are provided with two binding posts inside the brass case, and the

wires are run through a rubber-bushed hole in the base. If the push

has already been set in position and wired up, as per Fig. 58, have

the buttons pressed alternately, when on touching the binding posts

on automatic with the wires, the lighting or extinguishing connection

is easily selected. The lighting armature in most automatic burners

buzzes violently, while the extinguishing one only strikes once on

contact being made. Fig. 58 shows how to connect up two pushes to one

automatic, one push, perhaps, being located downstairs and the other

upstairs in the case of a hall lamp. In setting up these burners care

must be taken not to bend contacts or alter adjustment, and absolute

precaution is necessary that no crosses or weakly insulated places

are in circuit. After burning for some time it often happens that the

burner refuses to light, only buzzing feebly or not at all. If feebly,

the trouble is in battery, which should consist of, at least, four or

six cells of open circuit battery with low internal resistance, such as

Samson-Law carbon cylinder, or for occasional use large, dry cells.

If no click is heard on pressing white button, examine all connections;

if still no trouble is found, examine the platinum break. The platinum

tip may be bent by the continual hammering against the platinum tip on

vibrating rod, preventing contact on collar, or that soot has formed

there. These are the commonest maladies of automatic burners, and can

be easily remedied by readjusting platinum tip and cleaning. Contacts

here must be clean. In general wiring use waterproof office wire or,

better still, rubber-covered wire; for fixtures use the fixture wire

before described. When shellacking the wire to the fixture don't

attempt to connect up batteries until the shellac is dry and hard,

say for half a day. Electric gas-lighting is fruitful of trouble if

the work is not well done. Another cause of trouble may arise from a

dirty burner not allowing the gas to strike near the contact (clean

the burner), or the collar carrying contact may have shifted, perhaps

short-circuited; it should be insulated with a thin strip of asbestos.

Although white lead at the joints makes a fairly good contact, some

persons prefer to use tin-foil, a piece of foil being worked around

screw thread and the burner screwed on; it prevents leaks as well as

lead if well done, and makes better contact. As a short circuit on

the wires will cause all the burners to fail, many devices have been

invented to open the circuit upon such an occurrence. These will be

found described in the catalogues of electrical stores; they do not

come within the province of this book for description.

CHAPTER X.

BATTERIES FOR COILS.

In selecting a battery to operate the coil, one is needed which will

supply a large steady current for a considerable period. Although the

primary circuit is opened and closed rapidly, yet the class known

as open circuit cells is not suitable, even though they have a low

internal resistance, and thereby render a large current. Such cells

are only suitable for the uses for which they are mostly designed,

bell-ringing or annunciator work. There is one case, however, where an

open circuit cell may be used with an induction coil, and that is in

gas lighting as previously described; but here a dozen or so impulses

of current are generally sufficient, followed by long periods of rest.

For the latter work the cells in common use are the Samson, Champion,

and Monarch, all of which are of low internal resistance and great

recuperative power.

The reason that such cells will not work for long periods, is that they

polarize. This latter action takes place in these open circuit cells,

which are of the Leclanché type as follows: A positive plate of zinc

is immersed in a solution of ammonium chloride (or salammoniac), and a

negative plate of carbon and peroxide of manganese, contained either in

a porous cup or compressed into a block also stands in the solution.

Care is taken that these two plates do not touch each other. When the

outside circuit is closed the zinc combines with the chlorine of the

solution liberating free hydrogen and ammonia. The hydrogen appears

at the negative plate, where it is acted upon by the oxygen of the

peroxide of manganese to form water.

But when the circuit is of too low resistance, the oxidizing action of

the peroxide of manganese is not rapid enough, and a film of hydrogen,

which is a poor conductor, forms over the negative plate, increasing

the internal resistance of the cell and setting up local action. In

the best class of these open circuit cells, this hydrogen is absorbed

after a rest, and the battery recuperates and is ready for work again.

The circuit of the Ruhmkorff coil is low, and this polarization always

occurs a few minutes after the contact-breaker is started.

[Illustration: FIG. 59.]

In the class of closed circuit cells, chosen for the present purpose,

the Grenet or bottle bichromate is one of the handiest for occasional

use. A glass bottle-shaped jar, _J_, Fig. 59, is provided with a hard

rubber cap, _G_, on which are mounted the binding posts _A B_. To

the underside of this cap are attached two carbon plates _C C_, which

reach nearly to the bottom of the jar, being connected together on the

cap by a varnished copper strip, the latter being in turn connected to

one binding post. Through the centre of the cap passes a brass rod,

_R_, having attached to its lower end a piece of sheet zinc, _Z_, well

amalgamated with mercury. This process of amalgamation consists in

cleaning the zinc, then rubbing its surface with a rag dipped in dilute

sulphuric acid, and pouring a few drops of mercury on the wet zinc.

The mercury will spread readily over the zinc, provided it has been

well cleaned, and if properly done should give the zinc plate a bright,

shining appearance.

When the cell is not in use, the zinc is drawn up into the neck of

the bottle and clamped by a set screw against the brass rod. A copper

spring pressing on the rod serves to carry the current to the second

binding post.

This cell originated in France, whence its name, but a cheaper form is

now made in the United States known as the Novelty Grenet. The shape of

the jar is somewhat different, and the carbon is moulded, whereas the

French carbon is sawed from the carbon deposited in the gas retort; but

the American form is practically of as great utility as the French, and

the cost recommends it.

The bichromate solutions are affected by light, and deteriorate less it

kept in stoneware jugs. The Grenet battery can very well be fitted into

a neat wood case, which will serve the further purpose of preventing

chance knocks from fracturing the glass jar.

Carbons which are used in batteries containing the foregoing solution

should be well washed in warm water whenever the solution is changed,

and especially when it is intended to put the battery out of active

service. When the solution acquires a decidedly green hue it should

be replaced with fresh. The electromotive force of this cell varies

from 1.90 to 2 volts, and the amperage is dependent on the size of the

plates, running from 5 amperes upward.

The glass jar is filled up to the commencement of the neck with a

solution of bichromate of potash or sodium, called electropoion fluid,

and prepared as follows: To 1 gallon of water add 1 pound of bichromate

of sodium, mixing in a stoneware vessel. When dissolved add 3 pounds

of sulphuric acid in a thin stream, stirring slowly. As the mixture

heats on the introduction of the acid, care must be used to pour in the

latter slowly. This solution should not be used until quite cold.

The sodium salt is preferable to the potassium, owing to its not

forming the crystals of chrome alum, and also on account of its lower

cost and greater solubility, the latter being four times greater than

that of the potassium salt. The commercial acid used should contain at

least 90 per cent pure acid and should be free from impurities. On

filling the battery use utmost care not to splash the solution on any

of the metal work, or it will cause corrosion. Although the salts in

the solution will most likely make a stain, the corrosive action of the

acid can be arrested if the solution be splashed on the clothes by the

prompt application of ammonia solution.

[Illustration: FIG. 60.]

The "Fuller" cell, Fig. 60, which is another type of the bichromate

cell, is one from which a steady current can be obtained for a

longer interval than from the Grenet, but the current is less. The

electromotive force is the same, but the current is only 3 amperes,

except in certain modifications.

In the porous cup is a cone-shaped zinc having a stout copper wire cast

in. This wire is occasionally covered with rubber insulation, but,

as a rule, is bare. The porous cup is of unglazed porcelain, thick,

but very porous. This sets in the glass jar, a wooden cover fitting

_loosely_ over the whole to exclude dust. Through this cover passes

the wire leading from the zinc, and also the carbon plate carrying a

machine screw and check nuts for connection. The cover is dipped in

melted paraffin, as is also the upper end of the carbon and the rim

of the glass jar. This is to prevent the creeping of the salts in the

solutions and the corrosion of the brass work.

Into the porous cup is poured a solution composed of 18 parts by weight

of common salt and 72 parts by weight of water. Electropoion fluid is

held by the glass jar, the two solutions reaching a level of two thirds

the height of the jar. One ounce of mercury is added to the porous cup

solution to ensure the complete and continuous amalgamation of the

zinc. The salt can be more readily dissolved in warm water, but _all_

solutions must be used _cold_. It is not always necessary to renew the

solutions when the battery fails to give out its accustomed strength,

but several ounces of water can be substituted for a similar amount of

fluid in the porous cup. Stir the solution by moving the zinc up and

down, and a temporary improvement will be noticed.

To obtain a greater current from this cell, use a larger zinc, such as

a well-amalgamated zinc plate, and add a teaspoonful of sulphuric acid

to clean water for the porous cup solution. Additional carbon plates

connected together and placed round the porous cup will lower the

resistance of the cell and increase the current, and also tend to keep

down the polarization.

A new form of this battery was described by M. Morisot a short time ago.

The positive pole is of retort carbon in the outer cell in a

depolarizing mixture made of 1 part sulphuric acid, 3 parts saturated

solution bichromate of potash, crystals of the latter salt being

suspended in the cell to keep up the saturation. A porous cup contains

a solution of caustic soda. The zinc is in a second porous cup placed

within the first, which holds a caustic soda solution of greater

density. The electromotive force is 2½ volts when the cell is first

placed in circuit, and will remain at 2.4 for some hours. The internal

resistance is low, but varies with the thickness of the porous cups.

This cell is not suitable for any but use for a few hours at one time.

The Dun cell has a negative electrode of a carbon porous cup filled

with broken carbon. The zinc is in the form of a heavy ring, and hangs

at the top of the solution in the outer jar. Permanganate of potash

crystals are placed in the porous cup, and the entire cell filled with

a solution of caustic potash 1 part to water 5 parts. The voltage is

1.8, and the internal resistance being low the resultant current is

large.

A cell with an electrode of aluminum in a solution of caustic potash

and carbon in strong nitric acid in porous cup is claimed to have an

electromotive force of 2.8, but the nitric acid is not a desirable acid

to handle.

Metallic magnesium in a salammoniac solution with a copper plate in a

hydrochloric acid and sulphate of copper mixture is of high voltage,

nearly 3 volts being obtained, and the current is large, but it is a

new combination and has not as yet stood the test of time.

There are other formulæ for solutions to be used in Fuller or Grenet

cells which may be useful to the experimenter. Trouvé's is as follows:

Water, 36 parts; bichromate of potash, 3 parts; sulphuric acid, 15

parts, all by weight. Bottone's: Chromic acid, 6 parts; water, 20

parts; chlorate of potassium (increases electromotive force), ⅓ part;

sulphuric acid, 3½ parts, all by weight. A convenient "red salt" or

"electric sand": Sulphate of soda, 14 parts; sulphuric acid, 68 parts;

bichromate of potash, 29 parts; soda dissolved in heated acid, and

potash stirred in slowly. When cold can be broken up and prepared when

required by dissolving in five times its weight of water.

The chromic acid used in Bottone's solution is very soluble in water,

it being possible to dissolve five or six times the amount in the same

quantity of water as of bichromate of potash. The simple solution of

chromic acid is 1 pound to 1 pint of water, to which is added 6 ounces

of sulphuric acid.

When it becomes necessary to cut zinc plates, it may be readily done by

making a deep scratch on the surface, filling the scratch first with

dilute sulphuric acid, and then with mercury. The mercury will quickly

eat into the metal, and the plate may be easily broken across or cut

with a saw. Zinc plates can be bent into shape by the application of

heat. Hold the plate in front of a hot fire until it cannot be touched

by the bare hand: it will be found that it has softened so that it

can be bent around a suitable wooden form. As zinc plates are most

attacked at the surface of the acid solution, it is advisable to coat

the extreme upper portion of them with varnish or paraffin. Rolled

zinc is always preferable to cast, especially so when immersed in acid

solutions.

To avoid confusion, it may be stated here that it is the rule to speak

of the zinc element as the positive plate and the negative electrode or

pole, and the carbon _vice versa_. The portion of the element immersed

in the solution is the plate, the part outside, the pole or electrode.

In diagrams and also in formulæ positive is shown by a + (plus) sign

and negative by a-(minus) sign.

The relation of cost of the materials most used is shown in the

subjoined table, which cost, however, varies with the market:

Sulphuric acid, chemically pure 18

" " commercial 1.5

Muriatic " 1.12

Nitric " 3.5

Electropoion fluid 2

Bichromate of potash 10.5

" " soda 8.5

Caustic soda 9

Salammoniac 7

Chromic acid 19

Blue vitriol 4

Litharge 5.75

Mercury bisulphate 94

Paraffin 9

Beeswax 35 to 45

Shellac varnish 87

Tinfoil 35

GRAVITY BATTERY.

A cheap modification of the Daniell cell. A glass jar has at the bottom

a copper plate consisting of 4 to 6 leaves of thin sheet copper, set

on their edges in a starlike shape, a copper wire being attached to

the copper rivet which holds the leaves together. A mass of crystals

of sulphate of copper is filled in and laid on the top of the copper

electrode an inch or so above its top. The negative plate is a

variously shaped plate of cast zinc hung from the edge of the jar and

reaching about 2 inches from the top into the fluid. Water is poured in

until it covers the zinc, and the battery is complete. The sulphate of

copper deposits its metallic copper on the copper leaves and liberates

sulphuric acid, which rises and attacks the zinc, setting free sulphate

of zinc. The sulphate of zinc solution being of greater density remains

near the bottom, and the sulphate of zinc solution stays near the zinc.

When the cell is left too long on an open circuit the two solutions

tend to mix, and copper is deposited on the zinc. The sulphate of zinc

finally saturates the top solution, which has to be partly drawn off

and replaced by fresh water and crystals of sulphate of copper dropped

into the jar to take the place of that which has been decomposed.

Electromotive force 1 volt, current from 3∕10 to 5∕10 of an ampere.

The practical working of this cell will be treated of later on in

these pages.

[Illustration: FIG. 61.]

The Gethins (Fig. 61) and the Hussey bluestone cells both have the

zincs standing in porous cups (shown by dotted lines), which in turn

are supported half-way down the jar, generally resting on the copper

strip acting as a porous partition between the fluids. The zinc stands

in a solution of zinc sulphate, or a weak sulphuric acid solution.

The internal resistance is low, and the current large, being from 1

to 5 amperes. These cells are the ideal bluestone cells for charging

storage batteries requiring very little attention. The special Gethins

cell shown in the figure has the copper made with a collar, which

encircles the porous cup, and thereby lowers the internal resistance

of the battery. The voltage not being over 1 volt, however, renders

these cells hardly suitable for direct connection. Five cells connected

in multiple would give all of 10 amperes of current, and 1 volt, and

a number of these multiple groups could be connected in series for a

higher voltage.

GORDON BATTERY

is similar in operation to the Edison-Lalande, but differs in details

of construction. The zinc is a heavy ring suspended outside, but not

touching a perforated tin cylinder closed at the bottom, containing the

oxide of copper in flakes. Its internal resistance is slightly higher

than the Edison-Lalande cell, otherwise there is little choice. The 6 ×

8 size is excellent for coil work, giving 250 actual ampere hours and

remaining on open circuit for long periods without deterioration.

EDISON-LALANDE CELL.

This is a practical form of the old Lalande-Chaperon cell, and gives

a steady, large current, being of low internal resistance, but is of

low electromotive force, being less than .70 volt on closed circuit

of medium resistance. Being of low internal resistance, however, its

output is large—three cells of the type _S_; internal resistance, 0.025

ohm. Capacity, 300 ampere hours, will about equal one cell type E 5

of the Chloride Storage Battery. The elements of this cell consist of

positive plates of amalgamated zinc, suspended on each side of negative

plates of the black oxide of copper in an electrolyte solution of

caustic potash. In action the decomposition of water forms an oxide of

zinc from the positive element, which with the potash in combination

leaves a soluble salt of zinc and potash. The hydrogen of the water

acts on the oxide plates to form metallic copper, thus really reducing,

instead of increasing, the internal resistance of the cell. A layer

of heavy paraffin oil is poured on top of the solution to prevent the

action of air.

NEW STANDARD,

or Roche dry cell. This cell possesses remarkable recuperative powers

and low internal resistance. Made in many sizes, the best suited for

medical coils is No. 2; dimensions, 5-7∕8 × 2-7∕16 inches. For heavier

work the No. 5, 6 × 2-9∕16 inches, and known as the Navy Standard, is

recommended. A convenient size for portable medical coils is No. 3, 3¾

× 1⅞ inches, taking up very little room, yet giving a large output.

Two of these latter cells enclosed in the coil case will give with a

suitably wound primary (No. 18 to 20 B & S) as strong a current as can

be used in electrotherapy. For Ruhmkorff coils cells Nos. 6 and 7 (6

× 3 inches and 7 × 3 inches) furnish a most desirable battery for all

work not needing the constant operation of the contact breaker, such

as wireless telegraphy, gas-lighting, etc. They will do service on

X-ray work, but the writer prefers a storage cell or the copper oxide

types. The E. M. F. of the above cells is one and six-tenths volts, and

current from 9 amperes to the No. 7 size, which gives 24 amperes on

short circuit.

DRY-CELL CONSTRUCTION.

As a matter of practice, there is no really dry cell; all so-called

cells contain liquid held in suspension, and their output is limited

to the amount of fluid. One of this type can easily be made in the

following manner: A containing jar is made up of first-quality sheet

zinc, the edges being joined by a turned seam and then soldered, the

bottom of zinc being also soldered in. In soldering here, as actually

in all such operations, be _absolutely sure_ the edges of the metal

are clean. The jar is partially filled with the following composition:

Oxide of zinc, 1 part; sal ammoniac, 1 part; plaster of paris, 3

parts; chloride of zinc, 1 part; water, 2 parts, all by weight; or sal

ammoniac, 1 part; chloride of calcium, 5 parts; calcined magnesia, 5

parts; water, 2 parts, or enough water to make a thin paste. A brass

binding post is soldered to the zinc case and a carbon plate having a

binding post is inserted in the centre of the cell, care being taken

that it does not touch the zinc. A small disc of wood laid in the

bottom of the cell will prevent contact at the bottom. Molten pitch

or a composition of pitch and rosin in the proportion of 6 to 1 is

poured on top, so as to seal the cell. As gas is generated in the cell,

a safety valve should be provided, either a piece of porous cane or

a short length of hard rubber tube, inside of which have been placed

a few strands of woollen thread. This class of cell is so cheap and

so many forms are available for choice that it is rarely desirable

to make one's own. They will not do for steady current, but only for

intermittent work. The large sizes being of low internal resistance,

can be used for signalling in wireless telegraphy, where it is not

possible to use wet (or free fluid) cells. The principal dry cells on

the market are the Mesco, the O. K., the Nungesser, and the Samson

semi-dry cell.

CHAPTER XI.

STORAGE OR SECONDARY CELL.

The development of the storage or secondary cell has been one of the

most important electrical advances of the century. For purposes of

experiment or work, where a large or steady current is required from

compact and readily tended apparatus, the storage cell proves its

utility. The simplest form was that used by the early experimenters,

and as it is easy to make, a form of it may very well be described.

[Illustration: FIG. 62.]

[Illustration: FIG. 63.]

From a sheet of lead ⅛ inch thick two or more pieces are cut of

the requisite size, say, 5 inches square. In making these plates,

they should be cut so as to leave a strip 1 inch wide and 3 inches

long, projecting from one corner, _A_ (Fig. 62), for the purpose of

connection. This is for the reason that the fumes of the sulphuric

acid solution would quickly corrode any wires or screws in the plates,

and also to give a better connection. The number of plates cut must

be an odd one, as it is general to make the two outside plates of the

same polarity—viz., negative. These plates are then scored with a steel

point across and across on both sides to perhaps a depth of 1∕64 of an

inch. This scoring is not absolutely necessary; it somewhat hastens the

formation of the plates. The plates are then laid face to face, being

separated by pieces of wood, rubber, or, still better, by a piece of

grooved wood, Fig. 63 having a thin piece of asbestos on each side.

These grooves are to carry off the gas, and should run up and down the

board, as in the figure. The wood is ⅛ of an inch thick or thereabouts,

and preferably perforated with holes ¼ of an inch or larger. When laid

together, a few strong rubber bands hold the plates from coming apart.

To prevent lateral motion, a few rubber pins may be thrust through

the plates. The alternate strips are to be connected together in two

series, as in a condenser, and the complete series placed in a jar

containing a mixture of seven parts of water to one of sulphuric acid.

The terminal of the strips connected to the smallest number of plates

is to be marked _P_ or +, for positive.

This terminal is now to be connected to a charging current (not over 1

ampere), as described in the directions for charging batteries, for

eight hours, and then discharged at a rate not over 1 ampere for six

hours. Then the connections are to be reversed and the cell charged

backward, as it were, and discharged. This has to be repeated for a

long period, perhaps a month, before the cell is in good condition; on

the final charge it is to be connected positive to positive of charging

source. This operation is called "forming," and the result is to change

the metallic lead of the positive plate into red-brown peroxide of

lead, and the lead negative plates into spongy lead.

In modern commercial cells this operation is no longer pursued, the

plates are variously constructed of lead frameworks holding plugs of

litharge or lead oxide, which is "formed" with great facility. For many

purposes other than operating Ruhmkorff coils, a few simple cells made,

as described, are handy to have and easy to make. In sealing the cells

up for portability, care must always be taken to leave a small hole in

the cover for the escape of the sulphurous acid gas.

CHARGING STORAGE BATTERIES.

Although the charging of a storage or secondary battery is by no means

a difficult operation, yet it requires care, and one unaccustomed

to the work will meet many slight difficulties which may seriously

affect the results. Pre-eminently the best charging source is a direct

current, constant potential electric-light circuit. The amount of

current required varies according to the type and make of the cell, but

we will select one of a capacity of 50 ampere hours for illustration.

By 50 ampere hours is meant a delivery of 1 ampere per hour for fifty

hours, or a rate of discharge equal to the above, as 2 amperes per hour

for twenty-five hours. In practice a secondary cell will not be found

to act exactly as above, the total amount of current decreasing as

the discharge is greater. Each cell is constructed to discharge at a

certain rate, above which it is not safe to go. Five amperes per hour

is a suitable rate for a fifty-hour cell, and should not be greatly

exceeded. The Chloride type, however, is one which can be discharged

at a higher rate than normal without any serious results, the latter

being generally a bulging or "buckling," as it is called, of the plates

whereby they short circuit or fall apart. The voltage of the charging

source should be at least 10 per cent over that of the battery when

fully charged. The voltage of a cell of storage battery varies from

about 2.3 at commencement of discharge to 1.7, at which latter voltage

discharge must be stopped and charging recommenced.

Fig. 64 shows the connections to charge a storage battery from an

electric-light circuit. The latter must be direct current and of low

tension. The circuit from the negative lead runs to the rheostat handle

_R_, thence through as many coils as are in circuit (varied by moving

the handle over the contact pieces in connection with the resistance

coils). The positive of the cell is connected to the positive main.

In connecting storage cells to the mains the utmost care must be taken

that the terminals are correctly attached. It happens in isolated

plants that some change is made in the wiring or the switchboard,

which reverses the current without warning being given to the battery

charger. It is the safest way to test the polarity of the terminals of

_both_ battery and mains each time charging is commenced. For polarity

tests see Chapter I. It is immaterial on which side of the battery the

rheostat or similar device is placed.

[Illustration: FIG. 64.]

[Illustration: FIG. 65.]

Fig. 65 shows the employment of lamps instead of the rheostat. The

lamps _L L_ regulate the current flow by the manner in which the

circuit is arranged. If only one lamp be turned on, the current

necessary for only one lamp circulates through the battery. Each

additional lighted lamp adds to the current by decreasing the

resistance of the circuit. _S_ is a switch which must always be left

open when the dynamos are to be stopped.

CHARGING FROM PRIMARY BATTERY.

In many instances an electric-light circuit is not available for

charging purposes, in which event recourse must be made to a primary

battery. The one most suited for the work is the modified Daniell,

or copper and zinc combination in solutions of sulphate of copper

(bluestone) and sulphate of zinc respectively.

There are many good forms of this cell on the market, chief of which

are the simple gravity, the Gethins, and the Hussey, which have

been previously described. An example will now be described of the

operations necessary with the gravity cell, charging one 50-ampere

hour storage cell. At least six cells of gravity will be required, as

the voltage of each cell is never over 1 volt, and is dependent on

the resistance in the external circuit falling as the resistance is

lowered. Place the six clean glass jars on a firm foundation, where

there is no liability of shaking and no dust likely to settle. Unfold

the copper strips into the form of a star, bending the corners for half

an inch so as to give an anchorage in the bluestone. Place them into

the bottom of the jars and pour in water enough to cover them at least

3 inches below the surface. Now carefully drop in 4 pounds of clean

bluestone, which will fill in the angles between the copper wings, at

the same time holding the element down to the bottom of the jar. Hang

the zincs from the top edge of the jar, and fill up with water to 1

inch from the top. The addition of 5 ounces of sulphate of zinc per

cell will render the cells immediately available, and for the further

hastening of the chemical action, the copper wire from each copper may

be inserted in the binding post-hole of the zinc belonging to its own

cell and screwed tight for a few hours; or the cells may be connected

together in series, and the wire from the last copper be screwed to the

zinc of the first, thus putting the whole series on short circuit. The

only advantage of the first method being a saving of time when a number

of cells is being set up. This saving of time is often of consequence,

as the longer the newly set-up cell is on open circuit, the more

copper will be deposited on the zinc, which is highly undesirable.

This is shown by the blackening of the zinc as soon as it is put in

the solution, which blackening it is hard to prevent entirely. When

the cell is working satisfactorily it will show a clearly defined line

between the colorless solution above and the deep blue solution beneath.

Gravity cells should never be moved. If no sulphate of zinc is

available, half a teaspoonful of sulphuric acid may be poured in over

the zinc, which will tend to form the sulphate of zinc. Without any of

these helps the cell will take at least twenty-four hours on a short

circuit before it will give its normal current. This current should

be from 4∕10 to 5∕10 of an ampere. Five cells set up by the writer

varied after the addition of the zinc sulphate from 200 milli-amperes

(thousandths of an ampere) to 300 milli-amperes, although they were

apparently all set up alike; but after twelve hours' short circuiting

they all gave a fairly uniform current of from 470 to 500 milli-amperes.

[Illustration: FIG. 66.]

From time to time on storage battery work, say, every week, the

specific gravity of the top solution must be tested with a hydrometer

(see Fig. 66), which should be put into the solution and allowed to

come to rest. The indicated number at the level of the liquid should

be 25°. If the number is higher some solution should be drawn off

and clear water added, until the hydrometer settles down to 25° or

thereabouts. The inside of the glass jar for 1 inch from the top may be

greased to prevent the salts of zinc creeping over the edge, or half an

inch of heavy paraffin oil be poured on the top to prevent evaporation

and creeping. When the zinc gets very much coated with the dark deposit

it must be taken out and scraped and washed. When the bluestone needs

replenishing, drop in carefully and be sure none lodges on the zinc

element.

SETTING UP THE STORAGE CELL.

Each manufacturer of storage cells issues specific directions for the

charging of his own make, but generally the method is as follows:

The acid solution is prepared by mixing one volume of sulphuric acid

to from four to seven volumes of water, according to the make of the

cell. The sulphuric acid should have a specific gravity of 1.82 and be

chemically pure. _The acid must always be poured into the water, and

slowly, stirring all the time, then set aside for the mixture to cool._

It is best to mix the solution in a separate earthenware vessel, and

when two or more cells are to be set up, to mix all the solution at

one time, to ensure the same strength, unless a hydrometer is used to

determine this.

A good method to ascertain the exact quantity of solution required

is to place the elements in the jar and cover 1 inch deep at least

with water, then remove the elements and pour off the volume of water

corresponding to the proportion of acid to be added, and lastly pouring

the remaining water into the mixing vessel, prepare the solution, or

electrolyte, as it is called. New elements should be wetted with pure

water before being immersed in the solution. An ordinary charge of

the electrolyte requires from six to ten hours to cool thoroughly, as

considerable heat is evolved in the mixing.

Having now prepared the storage battery solution and set up the primary

cells, the charging can be proceeded with. The current must be turned

on the storage cell immediately the elements are placed in the acid.

Connect the wire from the zinc of the primary battery to the negative

of the storage cell and the copper wire to the positive. As the current

from a gravity cell is but small, it will take quite a time to charge a

storage cell of 50 ampere hours' capacity fully; it is a good scheme to

get the cell charged up from a dynamo source, and use the gravity cells

to keep it charged; but this cannot always be done, and the gravity

battery will do the work in time. As the best storage cells render but

90 per cent of the current put into them, they must be charged over the

number of hours for which they are required to deliver current.

When the cell is fully charged the solution will become milky and

give off gas freely. This gas in large quantities is detrimental to

health, and on no account should a storage cell be _charged_ in a

sleeping apartment. It affects the throat and lungs, and renders them

susceptible to take cold under suitable circumstances. The average

voltage of storage cells, when tested with the charging current on, is

2.4 volts, and the lowest they should be allowed to reach is 1.9 volts,

unless otherwise specified by the manufacturers.

Cells in poor condition are liable to form a _white_ deposit of

sulphate of lead, this fault being known as "sulphating." This trouble

requires much careful nursing, and the cells must be charged for a long

time at a very low rate until the plates of the positive element regain

their normal gray color. Chips of straw or excelsior, etc., falling in

between the plates will carbonize and cause trouble.

Most portable cells are sealed, but all cells can be easily sealed with

paraffin wax for amateur use. Cover the elements fully ½ inch above

the normal height of the electrolyte with water before pouring in the

electrolyte. Melt some paraffin in an earthenware jar and pour it on

top of the water, about the middle of the surface, when it will spread,

and care having been taken to have the jar sides dry, will cake solid

and form a good seal. Then bore a hole with a brace and bit or some

such tool through the wax and pour out the water. The cell can then

be set up as usual, the hole being only partly closed to allow of the

escape of the generated gas. A glass or rubber tube can be sealed into

the hole in the wax, and makes a more finished job.

While on the subject of primary batteries for charging storage cells, a

few remarks on their electromotive force may not be amiss. Although the

specifications issued by the manufacturers specify an excess charging

voltage of 10 per cent over the total voltage of the storage cells,

this does not apply to primary cells in its entirety. The voltage of

five gravity cells in series would aggregate 5 volts, and the voltage

of one storage cell but 2 volts, but there would not be 5 volts

available to force the charging current through the latter. In the

first place there is the counter electromotive force of the storage

cell working against the gravity battery. Simple subtraction would show

only 3 volts excess in favor of the primary electromotive force; but

the working voltage of a galvanic cell varies according to external

resistance of the cell and the external resistance of the circuit.

When the internal resistance is high, as in the gravity cell, and the

circuit resistance is low, in this case being the storage cell, the

available electromotive force of the primary is low also.

In many cases it is desired to operate a Ruhmkorff coil from an

electric-light main direct. This can readily be done if the circuit be

of the constant potential class—that is, one constructed to furnish

current for incandescent lamps in multiple. With the direct current,

such as the Edison, all that is necessary is either to interpose a

rheostat, as in Fig. 64, or to use the lamps, as in Fig. 65. The

manner of connecting up is the same as if the storage cell B be

replaced by the coil. Using the formula _C_ = _E_∕_R_, for example,

if the circuit be at 110 volts and the coil require 10 amperes, a

resistance of 11 ohms will be required. Or using the lamps in the

diagram, Fig. 65, about 20 lamps are to be put in circuit. If the

current be an alternating one, the contact-breaker will have to be

screwed down or short circuited.

THE "U. S." STORAGE CELL.

This cell is of the lead-zinc type, being the practical form of the

Reynier cell. It is to be recommended for working Ruhmkorff coils, its

output weight for weight being far in excess of the lead-lead types.

This cell is readily portable and easy of operation, the zinc electrode

being the only one needing renewal, and that at very infrequent

intervals.

The lead electrode consists of plates of peroxide clamped together,

and presents quite a large surface. The zinc in most types is of

the circular sheet form, and encloses the lead block, being kept

amalgamated by mercury lying in the bottom of the cell. The E. M. F.

on open circuit is about 2.5 volts, which is higher than any lead-lead

combination. On closed-circuit work this drops to from 2.35 volts

downwards. During action, when a large amount of current is being drawn

from the cell, a white sulphate appears, but this disappears upon

the cell being recharged or even left to rest. Bubbles of gas, which

sometimes form under the peroxide block, should be removed by gently

tilting the cell or hitting the table or shelf upon which it stands

a smart blow. The large type No. 3 is suitable for X-ray work, and a

still larger cell is made, which is preferable for heavy or continuous

discharges of current.

HARRISON CELL.

The No. 1 cell recently put upon the market has given excellent results

for open circuit work. It consists of a negative element with lead

peroxide as a depolarizer. The positive element is self-amalgamating

zinc, which is free from local action. The electrolyte is dilute

pure sulphuric acid. The potential is high, being 2.5 volts, and

the internal resistance is 0.14 ohm. This cell belongs to a group

which is midway between primary and storage, or secondary cells. Its

construction is similar to the lead-zinc secondary cell, in place of

which it may be used, it being easy to recharge an exhausted cell by

passing a weak current through it in reverse direction, thus recharging

the peroxide of lead grid and renewing the zinc and electrolyte.

The large size, or type No. 3, which the manufacturers are producing,

differs from the No. 1 cell in that it has a larger negative element,

or grid, and has two zincs, instead of one; consequently, it has a

lower internal resistance—0.07 ohm—and a higher discharge rate with

a capacity of 150 ampere hours. The potential is 2.5 volts. It is

suitable for coil work or for sparking gas engines, and for ease of

manipulation and convenience is to be highly recommended.

[Illustration: FIG. 67.]

The elements are shown in Fig. 67, lead grid _L_, which is filled

in with paste of peroxide of lead, and which neither buckles nor

disintegrates. The zinc _Z_, however, possesses a novel feature.

A cavity is cast in the zinc element and filled with an amalgam of

mercury, the copper electrode passing through this amalgam into the

solid zinc, as shown in the cut. As the action of the battery proceeds,

this amalgam forces its way into the pores of the element and keeps

up so good an amalgamation of both copper rod and zinc that zincs can

be used up to a point when the rising internal resistance makes it

economy to throw them away, and absolutely no perceptible local action

takes place in the cell upon continued open circuit. A preparation

is furnished if desired, which forms a jelly of the electrolyte,

making the cell readily portable. Like all of these combinations, its

electromotive force exceeds two volts, and its internal resistance is

low enough to advise its employment in coil work.

When a storage battery is to remain unused for a long time it must

first be fully charged, and then every week or so the charging current

passed through it until it bubbles. Where it is to be laid away

for a long period of time, and weekly charging is not feasible, the

following operations are necessary: First, fully charge battery, remove

electrolyte, and replace by water immediately. Discharge at normal rate

until voltage runs down to 1.7 per cell. Gradually decrease resistance

until battery is almost on short circuit. Let it stand for a day, then

pour off the water, and keep elements in a dry, clean place.

CHAPTER XII.

TESLA AND HERTZ EFFECTS.

The currents of high frequency used by Tesla in his researches are

produced by electrical rather than mechanical means. The alternating

current dynamo used by him renders a current of 10,000 alternations

per second, but the actual current necessary to the performance of

the luminous effects has a frequency of millions of oscillations per

second, produced by the discharge of Leyden jars or condensers.

Dr. Oliver J. Lodge, in his "Modern Views of Electricity," shows

that the discharge of the Leyden jar is in general oscillatory, the

apparently single and momentary spark, when analyzed in a very rapidly

rotating mirror, is shown to consist of a series of alternating

flashes, rapidly succeeding one another and lasting individually

less than one hundred thousandth of a second. The capacity of the

condenser and inertia of the circuit regulate the rapidity of these

oscillations. A 1 microfarad condenser discharging through a coil of

large self-induction, such as one having an iron core, may oscillate

only a few hundred times per second. On the other hand, a Leyden jar

of the 1 pint size discharging through a short circuit will set up

oscillations, perhaps ten million per second; and a still smaller jar

would give oscillations away up in the billions. But these small jars

are quickly discharged, and require a constant replenishing.

The discharge actually consists of a principal discharge in one

direction, and then several reflex actions back and forth, becoming

feebler until their cessation. In their vibration they generate waves

in the surrounding medium, similar in many respects to sound waves, but

of infinitely higher velocity. Their length depends on the rate of

vibration of the source and their velocity. The microfarad discharge

before mentioned will have a wave length of perhaps 1200 miles, the

small jar not over 70 feet; and yet the true light wave has only an

average length of one fifty thousandth of 1 inch. These waves travel

into space until they either die out from exhaustion or are absorbed

by some suitable body; but they possess the quality of resonance in a

degree like those of sound. Two tuning forks of the same pitch will

influence one another—that is, one on being vibrated will start the

other in vibration, even at a considerable distance, but the electric

waves far surpass them in this respect.

[Illustration: FIG. 68.]

Dr. Hertz made the first practical experiments in this direction with

his electric resonator (Fig. 68). This apparatus consisted of a 3-inch

spark induction coil, _I_, the secondary wires _S S_ being connected

to the copper rods _R R_, provided with metal balls _B B_, nearly 11

inches in diameter. The discharging balls _D D_ were approximated

until a satisfactory discharge passed between them. A large wire ring

having a spark gap in its circuit was so influenced by the resonance

as to show minute sparks passing across this gap even when the ring

was situated in a distant room. In many experiments with a rapidly

vibrating induction coil current, a sparking has been noticed in

metallic objects in the same room, in one instance it being discovered

in the metallic designs on a wall-paper.

THE "TESLA" EFFECTS.

In exploring the comparatively new field opened up by Professor

Crookes, Nikola Tesla has stimulated research into the mysteries of

high tension and frequency currents and their effects. In the majority

of his experiments Tesla uses alternating currents generated by

machinery of his own design, but in a large number of cases his effects

can be duplicated with an induction coil suitably energized. In the

latter case the apparatus consists of a battery, a Ruhmkorff coil, two

condensers, and a second specially constructed induction or disruptive

coil, with some few subsidiary implements. The contact-breaker or

rheotome must be one giving interruptions of very rapid sequence.

[Illustration: FIG. 69.]

Fig. 69 shows a diagram of the Tesla arrangement with a Ruhmkorff

coil. The terminals of the secondary coil of the Ruhmkorff coil _I_

terminate at the condensers _C C_. Bridged across the wires before

they reach the condensers is the discharger _D_. The second terminals

of the condensers are led through the split primary of the disruptive

coil, terminating at the points _B B_ of the second discharger. The

secondary of the disruptive coil is either outside or inside the

primary coil. The condensers are of special design, being small, but of

high insulation. They each consist of two plates of metal a few inches

square immersed in oil and arranged so they can be brought nearer

together or further apart, as necessary. Within limits, the smaller

these plates are the more frequent will be the oscillations of their

discharge. They also fulfil another purpose, they help nullify the high

self-induction of the secondary coil by adding capacity to it.

[Illustration: FIG. 70.]

The discharger tips are preferably metal balls under 1 inch

in diameter. Tesla uses various forms of dischargers, but for

experimental purposes the two metal balls will answer. They are

adjusted when the whole apparatus is working according to the results

desired. The mica plates serve to establish an air current up through

the gap, making the discharge more abrupt, an air blast being also

used at times for the furtherance of this object. The device (Fig. 70)

consists of an electro-magnet, _C_, set with its poles _P_ across the

air gap, helping to wipe out the spark, as in a well-known form of

lightning arrester. This form, described by Tesla, has the pole pieces

_P_ shielded by mica plates _M_, to prevent the sparks jumping into the

magnets. Fig. 70 is an elevation and Fig. 71 a plan of this device.

[Illustration: FIG. 71.]

The resonance effects obtained during the operation of a Tesla coil

are very marked, and their study may lead to the solution of the

problems of communication between distant points without the use of

other conducting media than the atmosphere. But the main use to which

the Tesla currents have been put is that of artificial illumination.

These currents have enabled experimenters to obtain a high luminosity

in vacua by the aid of only one conducting wire—in fact, in some cases

without the utilization of any conductor than the air. An ordinary

incandescent lamp connected to one terminal of the coil will show in

a fair degree some of the luminescent phenomena. The brush effects

from the terminals of the secondary coil are extremely marked and

interesting; but to detail the experiments that can be performed with

the Tesla disruptive coil would be an impossibility here. Reference is

recommended to the published works of Nikola Tesla, which happily are

readily procurable.

These currents of high frequency have of late been turned to account

in electrotherapeutics, principally for the stimulation they exert on

the nutritive process. They also exert a very great influence on the

vasomotor centres, as is evidenced by the reddening of the skin and

exudation of perspiration. This result is readily obtainable by placing

the patient in connection with one electrode on an insulating stool,

and terminating the other electrode at a large metal plate situated a

few feet distant; or the patient may be surrounded by a coil of wire in

connection with the coil of sufficient diameter, however, to prevent

contact.

CHAPTER XIII.

THE "ROENTGEN" RAYS AND RADIOGRAPHY.

Although the remarkable discovery that it was possible by electrical

means to depict an image of an object on a photographic sensitized

plate, despite the intervention of solid bodies, was first given to the

world at large by Professor Roentgen, yet he was undoubtedly led to

the results by consideration of the works of previous experimenters in

electrical discharges through vacua.

It is not intended here to trace the previous work of Professor

Crookes, the inventor of the radiometer, which is actuated by the

heat rays of light, nor of Hertz, who found that gold leaf was

transparent to rays emanating from certain vacuum tubes carrying

a luminous electrical discharge. It is mainly the purpose of these

pages to give directions for practical work, and not deal in theories,

interesting though they be. At the beginning of X-ray investigation

many claims were made which have since been disproven, but the

fundamental operations remain the same. A Crookes tube of special

design is energized from a coil or similar electrical distributor,

and by means of the resultant rays otherwise opaque objects appear

partially transparent, their shadows being cast upon the screen of a

fluoroscope, or these shadows are allowed to act upon a sensitized

photographic plate, and subsequent development reveals outlines or

shadowgraphs. The general arrangement of apparatus is shown in Fig. 72.

_C_ is a Ruhmkorff coil, giving not less than 2 inches of spark; _B_

the battery operating same; _T_ the modified form of Crookes tube used

most generally; _X_ the object under observation; _F_ the fluoroscope

or the sensitized photographic plate. The usual precautions are

taken to avoid the leakage of current from the secondary wires, the

tube _T_ being best mounted in a wooden stand (Fig. 72), and the wire

connections brought to it as direct as possible. No condenser, stand,

etc., are shown in drawing, to avoid unnecessary complication.

[Illustration: FIG. 72.]

THE FLUOROSCOPE.

This is a funnel-shaped cardboard box with an opening at the smaller

end for the eyes and a piece of card across the larger end. The inside

surface of this card is covered with crystals of barium platino

cyanide, the most satisfactory fluorescent substance obtainable. The

earlier fluoroscopes were made with tungstate of calcium, but the above

salt has proven far more satisfactory. The operation of the fluoroscope

is simple. It is held in the hand by a convenient handle, the open

end pressed close to the eyes, so as to exclude outside light, and

with the hand or other object held against the outside of the big end,

or screen, it is directed towards the Crookes tube. The screen then

appears to glow with a bluish light, and the shadow of the object is

distinctly seen on the screen. Different adjustments of the coil give

results which will be treated upon later.

PHOSPHORUS TUBE.

Messrs. Siemens and Halske manufactured a tube which allowed of a

slight variation of vacuum by using the vapor of phosphorus. An

auxiliary tube containing phosphorus was added to the main tube, and

upon heat being applied to it by means of a lamp, vapor is given off,

which materially reduces the vacuum of the main tube. When the opposite

result is desired part of the current is diverted through the auxiliary

tube, and the vapor is caused to solidify itself upon the walls of the

tube.

[Illustration: FIG. 73.]

THE CROOKES TUBE.

The most satisfactory tube for X-ray work is one where the vacuum is

readily adjustable. Reference to Fig. 73 shows the Queen form. A small

bulb, containing a chemical which gives off vapor when heated and

reabsorbs it when cooled, is directly connected to the main tube and

surrounded by an auxiliary tube, which is exhausted to a low vacuum. In

the auxiliary tube the cathode is opposite to the above-mentioned bulb,

so that any discharge through it will heat the bulb by the bombardment

of the cathode rays. The cathode is connected to a spark point, which

can be adjusted to any distance from the cathode of the main tube. The

anode of the small tube is directly connected to that of the main tube.

When the tube is put into operation the vacuum and, consequently, the

resistance of the main tube being high, the current preferably passes

by the spark point and auxiliary tube, heating the chemical for a

few seconds until sufficient vapor has been driven into the main tube

to permit the current to pass through the latter. After this only an

occasional spark will jump across the gap to counteract the tendency of

the reabsorption of the vapor and consequent raising in resistance of

the main tube.

This device presents easy means of adjusting the vacuum in the main

tube. With the spark point at a considerable distance from cathode the

vacuum will be high. When the spark gap is short the vacuum will become

low. The main bulb is about 4½ inches in diameter, and at the place

where the X-rays pass only 1∕64 of an inch in thickness. The cathode is

of aluminum, the anode of platinum. In starting this tube, it is best

to make the spark gap about one inch in width. When connected up and

working properly the main bulb will be filled with a green striated

luminosity between anode and cathode, and the tip of the chemical bulb

will have the shadow of the little platinum tip thrown upon it. The

green light is not always brilliant; at times it is quite weak, but yet

does its work well. A brilliant green light is often one of the signs

of wrong connection, and particularly so when the little shadow on the

chemical bulb is absent. Never run these or any other tubes backwards,

but be sure the current is flowing in correct direction at first

operation.

Other forms of Crookes tubes differ only in form, or are devoid of

adjustment, and the connections of coil, tube, etc., are the same.

GENERAL REMARKS.

A high vacuum gives greater penetrative power than a low vacuum. Where

the operator has not an adjustable tube it is imperative that he have

at least two tubes, one high and one low. It is the contrasts which

render the X-ray practical, and these contrasts are largely governed

by the vacuum. In locating a metallic substance in the human body a

high vacuum tube would be needed, that the bones and dense tissue be

rendered more transparent. On the other hand, to make a radiograph

of the bones, a lower vacuum is necessary in order to get a contrast

between the bones and the tissues. In general, a high vacuum is

best for fluoroscope work and a low vacuum for making pictures on a

photographic plate. Short exposures in radiography are obtained by

powerful rays and consequently by coils operating at considerable

energy. In extended examinations or where a subject is under the X-rays

for more than a minute or so, a screen should be interposed between

the subject and the tube to avoid the burning effect which is often

noticeable. This screen consists of a piece of cardboard well covered

with gold leaf, and should be grounded—that is, a connection be run

from the gold surface to a water-pipe or other ground connection.

Sheet lead is an efficient screen to the rays, and, if desired, a lead

screen can be made, partially enclosing the apparatus, to protect the

operator. But it must be large enough and far enough distant from the

coil and tube to avoid any possibility of leakage of current or even

inductive influence. In operating X-ray machines never attempt to alter

connections or make adjustments other than at coil platinum screw or

Crookes tube spark gap without first shutting off current. Remember

that a very unpleasant shock can be easily obtained from touching the

apparatus with only one hand. It is often advisable to remove one's

watch, particularly when using Ruhmkorff coils of large size.

The tube may be worked until it shows a slight redness in the centre of

the platinum, but care must then be taken not to increase current, or

the platinum will melt. Never allow the tube to come in contact with

any object other than its stand and connections while working, and be

sure the wires from secondary do not come near tube until they reach

places of attachment, or they may spark through glass and ruin the tube.

In making radiographs on sensitized plates the unused plates should

be kept at a considerable distance from the coil while working. Better

still if they are in another room. Plates for X-ray work are made by

most photographic supply dealers; in fact, almost any good brand of

sensitized plates or even films will answer. When making a radiograph,

the plate can either be left in the holder or well wrapped in black

paper, but current should never be turned on coil before the plate

and subject are in position. In photographing the chest, neck, etc.,

the plate can be strapped on to the part; but the subject must remain

absolutely still. The time of exposure varies considerably with the

size of coil, thickness of object, etc. Radiographs of the hand have

been taken by simply laying the hand on top of the plateholder and

operating tube for 100 seconds. But, as a rule, longer exposures are

necessary. Most radiographs will generally require that the plate be

"intensified" and a developer used that gives great detail, such as

metol quinol, etc. At any rate, great care should be exercised in

developing the plate, as many a good radiograph has been spoiled by

undue haste.

CHAPTER XIV.

WIRELESS TELEGRAPHY.

In Chapter XII. we showed how Dr. Hertz caused electric waves to pass

through space and become visible by sparks across an air gap in a wire

ring situated at a distance from the source of energy. The apparatus

used, and termed an electric resonator, is in principle similar to that

of the wireless telegraph. The minute sparks instead of idly passing

across the air gap are made to traverse a "coherer" (to be afterwards

more fully described). This "coherer" substantially consists of a

resistance, preferably metal filings placed in series, with a battery

and relay. Normally, the resistance is so adjusted that the battery

current is not strong enough to operate the relay. A wire is led from

one side of this coherer up into the air to intercept the Hertzian

waves, the other side of the coherer is put to earth, or "grounded."

When a wave strikes the air wire it sends a current through the coherer

to ground (as before it sent a spark across the air gap), and this wave

acts on the filings in its passage through them; in effect, to lower

their resistance, so that the current is increased through the relay

circuit and the relay armature is attracted to its magnet. The relay

makes contact in the usual manner at the platinum points, and in its

turn causes the local circuit, sounder, bell, or pen register to record

the wave (or signal). After each wave the filings are in such state

that to restore them to their former high resistance it is necessary

to give the coherer a smart tap. This is generally accomplished

automatically by means of an arm extending from the sounder lever,

which strikes against the coherer each time the sounder armature is

moved.

[Illustration: FIG. 74.]

Figures 74 and 75 are diagrams of a simple circuit, Fig. 74 being the

transmitting apparatus and Fig. 75 the receiving apparatus.

In Fig. 74 _P P_ and _S S_ are the primary and secondary of a Ruhmkorff

coil, _D_ two brass balls on the discharger, _B_ the battery, _K_ a

key, in place of the usual contact breaker, which is either absent or

screwed down; _V_ a wire leading from one arm of the discharger up into

the air, of a height varying with the results desired; _G_ a ground

plate in connection with the other discharger arm.

The coil condenser is left out of the diagram for sake of clearness;

but, of course, is necessary to the operation of the apparatus.