FRICTIONAL FUN
SOME AMUSING ELECTRICAL EXPERIMENTS[1]
ELECTRICITY may be said to have a finger in most pies. From its
manifestations it would appear to be an all-pervading force, and modern
research tends always to substantiate this conclusion. Thus two coins--a
cent and a quarter--cannot be brought into contact without producing
their quotum of electrical force, and in like manner the principles of
its working may often be demonstrated by the simplest apparatus.
There are two states which this subtle force assumes, called Static and
Dynamic. The former word indicates a condition of rest, and is applied
to electricity when stored in suitable magazines ready for discharge at
a favorable opportunity, just as water may be held in lofty reservoirs,
ready to pour down and perform work--whether useful or otherwise--if the
pipes be opened. Dynamic, on the other hand, indicates a state of
motion; so that Dynamic Electricity means that it is flowing along
conductors from one place, where there is a large quantity of the force,
to another less powerful, in the same way as--again using the comparison
with water--the latter pours along connecting pipes from a high level to
a lower. Phenomena of static electricity are invariably produced by
friction, and some experiments of this class will be first described.
The chief element of success in friction electrical experiments is a
warm and dry atmosphere. The operations should therefore be conducted in
a well-ventilated room where the fire has been burning some time, whilst
all apparatus may with advantage have stood warming for some time
before.
1. Rub a 6-inch square of brown paper with a warm silk handkerchief,
then place flat against the wall or marble fireplace. It should cleave
tightly to either of the latter surfaces.
2. Again electrify the brown paper and hold it above any willing
person’s head of hair. The locks will fly up towards the paper and
appear like bristles. You may then make sure of a safe retreat, and
utter side remarks about “wire.”
3. Sprint smartly after the cat, which, like the other apparatus, should
have been warming itself on the hearthrug, and having effected a
capture, stroke the fur backwards. If the hand be not moist,
considerable sparking should occur and be easily visible in a darkened
room.
4. By combing dry hair quickly with a warm ebonite comb, and then
applying this to the knuckles, distinct sparks may often be obtained.
5. Cut a piece of paper into small confetti, and place near it a stick
of sealing-wax or ebonite rod, which has been rubbed with flannel. The
paper atoms are attracted to the rod through a distance of one inch or
more, and then, contact having once occurred, they immediately drop off.
6. In the previous experiment it was found that the paper atoms having
once touched the rod, dropped off directly. This behavior was caused by
a sequence of happenings.
First the negatively excited sealing-wax caused positive charges to
possess the upper surface of the paper--the equal negative charges being
driven downwards--so that, the two kinds of electric energy having a
mutual affinity, the paper sprang towards the wax. Immediately they
touched, however, the positive charges of the paper were neutralized by
the large excess of negative energy from the sealing-wax, and then,
since electricities of like sign exercise mutual repulsion, the negative
paper was driven from the negative wax.
Now this repulsion of similarly charged particles is demonstrable by a
straightforward experiment, whose simplicity of description is in nowise
proportionate to its entertainment. In truth, it consists merely of
blowing a soap-bubble and catching it upon an excited stick of glass or
wax. The bubble becomes electrified, its myriads of particles repel one
another, and the beautiful globe expands in size, possibly to breaking
point.
7. Scratch any design or word upon a small sheet of indiarubber--such as
is used in the manufacture of mats--and then dust over the surface a
thoroughly-shaken mixture of red-lead and sulphur. When all excess of
the powder has been blown away, the design should appear in grains of
red-lead, showing to excellent effect on the gray rubber ground.
8. By the following method a spark of sufficient intensity to light
house gas may be obtained from brown paper. Support a salver on four
well-dried inverted tumblers, placing a rubber ring between the tray and
each glass, and twist a copper wire round a spoon which rests on the
tray. The other end of this wire must be held--insulated from the
fingers by a piece of indiarubber or by the ordinary covering of the
wire--just over the gas-jet, whilst about ¹⁄₈-inch away is held another
piece in metallic connection with the hand; that is, in reality, _via_
the operator’s body with the earth.
Now, if the gas be turned on slightly and a piece of warm brown paper,
electrified by stroking vigorously with a dry brush, be placed on the
salver, a spark should pass across the ¹⁄₈-inch gap and ignite the
escaping gas. If the spark does not pass at once, approach the wire
extremities slightly closer together.
[Illustration: FIG. 1.--Raising paper by touching wooden lath with
electrified sealing-wax.]
9. The following experiment owes its origin to Gray--an early worker in
the field of static electricity: Erect two dry tumblers with a piece of
waxed cardboard between, after the fashion shown in Fig. 1, and lay a
lath on the top glass. Beneath one end of the lath spread small pieces
of paper, over an ebonite or wood surface supported on another inverted
glass. Then if a stick of sealing-wax, which has been vigorously rubbed
with flannel, be approached to the other end of the lath, the particles
of paper will fly backwards and forwards between the lath and the
ebonite surface--on which they would normally rest.
10. A simple trick, but one nevertheless causing considerable amusement
to spectators, may be arranged in the following manner: Bore a hole
carefully at the middle of a wooden meat skewer (A, Fig. 2), of such
size as to take a bone knitting-needle tightly (B, Fig. 2).
Sharpen the blunt end of the skewer and mount it so as to turn easily
between bearings erected on inverted wine-glasses. These may consist of
two wood blocks (C, Fig. 2), in each of which a shallow conical hole is
made to receive the points of the skewer. On these stand weights (E,
Fig. 2) to keep wood-bearings firm. When this is in position and swings
easily, adjust the needle until it balances exactly. It should be at
right angles to the skewer, and may be fixed in position, if necessary,
with glue or sealing-wax.
[Illustration: FIG. 2.--The balanced disc.]
Now cut two discs of white paper (D, Fig. 2) of a diameter about 1 inch
less than the knitting-needle’s length, and, having printed across one
the word “DONE” in large type, glue them--centers at the point where the
needle and skewer intersect--on either side of this cross. Erect the
apparatus as shown, so that the needle stands vertical, with the blank
paper disc facing the audience, and then invite the latter to see if
they can effect a movement without either touching or blowing the disc
or creating any disturbance whatever. The conditions should preclude any
person, except the canny operator, from attaining success. He, however,
innocently takes a postcard (previously warmed and dried), tears it
sharply in half, and presents one ragged edge to the projecting bone
needle. The cardboard became electrified by tearing, and should exercise
quite enough attraction on the needle to upset its nice balance,
whereupon the other disc becomes visible, together with its notice
“DONE.” The audience may take the ambiguous meaning according to their
pleasure.
Few further experiments can be performed without the use of special
apparatus. So far, we have dealt only with the most minute quantities of
electricity, and if more striking effects are to be produced, our stores
of energy must be increased.
Fortunately instruments of a fairly wide application may be rudely
constructed at home, although, naturally, results must not be looked for
comparable to those obtained with perfected apparatus. However, brief
directions now follow for the manufacture of such simple instruments,
and so, as commencement, let us appropriate two 3-lb. jam pots from
which Leyden jars are to be evolved.
[Illustration: FIG. 3.--Leyden jar.]
LEYDEN JARS
Procure as much metal foil as possible, either by raiding the nursery or
by purchasing some tinfoil-wrapped butter scotch at the confectioner’s,
and gum it round the jam jars so as to extend about half-way up the
outside (A, Fig. 3).
Next solder stiff brass wires (8 inches long) to two round pieces of
metal, so cut as to pass the jar mouths readily. To the other end of one
brass rod solder another disc of metal, tin--or, better, brass--nicely
smoothed along its two edges by sandpaper. The other brass rod may be
looped at its end and have tinfoil wrapped round until a respectable
knob be formed, or else have a brass bedstead globe soldered on.
[Illustration: FIG. 4.--The electroscope.]
These brass standards are then stood inside, with their metal bases
resting on the bottom of the jars and surrounded by lead shot to the
same height as that of the tinfoil, without. The finished article should
appear something like the sketch shown as Fig. 3.
GOLD LEAF ELECTROSCOPE
[Illustration: FIG. 5.--Electroscope parts.]
Obtain a square glass pickle or preserve jar, to the mouth of which has
been fitted a large cork bung (A, Fig. 4). Next take a 7-inch length of
¹⁄₈-inch brass wire (B, Fig. 4) (similar to that used for the Leyden
jars above), and flatten one end by hammering (F, Fig. 5), after having
bent it triangular-shape, as in (C, Fig. 4).
A 2¹⁄₂ inch length of cycle valve-tubing (A, Fig. 5) is to be slipped
over the brass rod B (Fig. 5), and then strips of gummed brown paper, 2
inches wide, lapped round and round the rubber-tubing until a paper
cylinder some ¹⁄₄-inch across the outside is formed (C, Fig. 5). Anyway,
this paper cylinder must be a good fit for a hole bored through the
center of cork (D, Fig. 5), into which, moreover, it must be glued when
dry. The stopper, paper, &c., must then be allowed to soak in hot candle
wax for an hour or so, wiped clean and put aside to cool. Excess of
grease must be particularly removed--with paraffin if necessary--from
the plain end of the metal rod, to which we must next solder a brass
knob (D, Fig. 4) or, as an alternative, tinfoil may be twisted round a
loop in the wire until a ball is formed, as in the case of the Leyden
jar.
The next operation is to cut two strips of Dutch metal--or, better, gold
leaf--size ³⁄₄-inch by 2 inches, and gum them on either side of the
flattened brass rod triangle (E, Figs. 4 and 5). This is best managed by
laying the brass, after being lightly gummed, upon the Dutch metal
strips in the correct position, and when fixed trimming off the corners
of the leaves close to the sides of the triangle--not, of course, at the
base, or no strips will remain to hang down. Dutch metal and beaten gold
are both employed in gilding, and should be obtainable in small
quantities at any picture-framer’s shop.
If the cork be now fixed in the mouth of the jar and the brass rod
adjusted so that the gold leaves hang free of the interior, our
electroscope is practically complete. However, a few pieces of calcium
chloride, or pumice stone soaked in strong sulphuric acid, may be
included at the bottom of the jar (F, Fig. 4), in order to absorb
moisture; and lastly, as a brass ball is not always the most
satisfactory terminal for this instrument, one other small accessory may
be made. This consists of a 2-inch circle of tin, or, preferably, brass
(G, Fig. 4), exactly like that fitted to one of the Leyden jars, and to
the under side of which is soldered a ring of springy brass (H, Fig. 4)
about ³⁄₄-inch deep, so made as to fit securely on the brass ball
terminal. Thus the metal table is adaptable to the brass knob, whenever
such an arrangement is required.
[Illustration: FIG. 6.--The electrophorus.]
ELECTROPHORUS
Our Leyden jars have been constructed to store electrical energy, and
the gold leaf electroscope to indicate its presence. But we do not yet
possess the means of producing this energy in any considerable quantity.
To make an instrument for this purpose proceed as follows: Clean the
inner surface of a circular tin lid--diameter 6 inches or 7 inches, and
about ⁵⁄₈-inch deep--(A, Fig. 6), and in the center stand upright on its
head a brass screw, whose point has been filed down until it _nearly_
reaches to the level of the upper edge of the lid (B, Fig. 6). Then
carefully pour in melted lead or zinc to a depth of ¹⁄₈-inch. This
process is not absolutely necessary, but is advisable if the metal be
available and the extra trouble is not distasteful.
When this metal has thoroughly cooled, melt up sufficient rough resin to
just overflow the tin, pour it in and allow to solidify properly without
the least disturbance (D, Fig. 6).
For the upper part of the electrophorus, take a disc of tin or brass,
absolutely flat, and of the same diameter as the resin surface (E, Fig.
6). In the center of its upper side solder a short piece of brass tube
(F, Fig. 6), the correct internal width to fit tightly on a 12-inch
length of polished wood rod, cut, say, from a walking-stick (G, Fig. 6).
The drawing shows the general arrangement of this electrophorus,
wherewith, by the help of a piece of fur, electricity may be generated.
A small cavity must be made in the resin, just above the screw (H, Fig.
6), so that the point of the latter may be clearly seen.
[Illustration: FIG. 7.--The discharger.]
DISCHARGER
One last piece of apparatus--the simplest in construction--remains to be
made. Bend a piece of stout brass wire into a semicircle (A, Fig. 7);
then, having flattened it at the middle, pass through a hole at the end
of a wooden handle (B, Fig. 7), and fix in position by means of a tiny
glued wedge (C, Fig. 7). Next twist the rod at both ends into loops, and
wrap tinfoil round so as to form metal knobs (D, Fig. 7). Brass balls
may be used instead. This arrangement is now complete, and we may
commence experimenting.
11. Twist a stout piece of copper wire surmounted by a brass or tinfoil
ball round the outside coating of a Leyden jar (A, Fig. 8), so that the
two knobs are about 3 inches apart. Suspend a pith ball (B, Fig. 8),
made from dried wood pith, by silk thread (C, Fig. 8) from some support
above, so that it hangs normally midway between the two knobs.
Next excite the resin surface of the electrophorus by rubbing with a
dry rabbit’s fur, muff, or flannel; and, holding the cover of this
instrument by the handle’s extreme end, place it upon the resin. Remove
immediately and bring near to the knob, which connects with the interior
of the Leyden condenser, at the same time touching its outer tinfoil
surface with the finger. A spark should pass between the electrophorus
cover and the Leyden jar, whereupon, if the former and finger be
removed, and the pith ball allowed to drop into position, this will
oscillate violently to and fro between the knobs.
[Illustration: FIG. 8.--Oscillating pith ball.]
[Illustration: FIG. 9.--Sparks from discharger and Leyden jar.]
12. Construct a Leyden jar with knob terminal similar to that previously
described, but cover part way up the outside with gold paint (A, Fig.
9), instead of tinfoil. Charge this condenser by means of the
electrophorus cover, as described in experiment 11, and if sufficient
induction does not take place with one contact, replace the metal disc
on the resin and pass a spark to the Leyden condenser knob several
times, taking care to touch the gold paint coating with the finger on
each occasion. If now the semicircular discharger, held by the wood
handle, be brought so that one knob touches that of the Leyden condenser
whilst the other moves over the gold-paint surface, long series of
sparks, differing greatly in appearance from any previously produced,
will pass between the gilt and the discharger.
13. Excite a stick of sealing-wax by rubbing with flannel, and bring it
against a pith ball suspended by silk. The sealing-wax was negatively
charged, and the pith ball is now in a like condition. To prove this,
approach the flannel which excited the wax, and had consequently
acquired a positive charge; inasmuch as the pith ball is immediately
attracted and we know that positively excited bodies have an affinity
for those negatively affected, the pith ball must be of the latter
character.
If, further, a glass rod be excited by rubbing with silk and brought
near to the negative pith ball, the latter will again be attracted,
showing the glass to be positively charged.
On the other hand, the silk rubber repels a pith ball which has been in
contact with excited sealing-wax, in the same way as it will repel the
latter substance itself if suspended freely, thus indicating that the
silk is similarly, _i.e._ negatively charged.
The phenomenon of positive and negative electricity may be somewhat
differently demonstrated by means of the electroscope. Touch the
terminal of this instrument with rubbed sealing-wax. The gold leaves
diverge. Moreover, they may be thrown farther asunder by bringing the
electrophorus resin near, so that evidently the charge of this
instrument is also negative. Perhaps as a diversion it may be noted that
resin is one of the chief ingredients of sealing-wax. The electroscope
leaves remain apart even when the electrified body has been removed, but
they may be made to collapse by touching the knob with the finger or
bringing an excited glass rod near. The former action “earths” (_i.e._
connects, _via_ the person’s body, with the house walls and the earth)
the gold leaves and allows the electrical charge to escape, whilst the
latter operation counteracts the negative charge by virtue of the
positively excited glass.
14. If the electrophorus and electroscope be placed as shown in Fig. 10,
the gold leaves being already held asunder by a negative charge from
sealing-wax, and the cover of the first instrument be lifted, whilst a
strip of cardboard bridges from its upper surface to the disc terminal
of the electroscope (A, Fig. 10), the instrument’s leaves will fall
together again. From this behavior it may be gathered that the charge on
the upper surface of the electrophorus cover is positive, or opposite in
character to that induced by rubbing on the resin cake.
[Illustration: FIG. 10.--An experiment with the electrophorus and
electroscope.]
15. Stand a charged Leyden jar and the electroscope close together on
the table. They must not be so near, however, that the gold leaves are
affected by the condenser’s presence.
If, now, a few pieces of sulphur, spread upon the Leyden jar’s disc
terminal, be ignited so as to burn freely, the gold leaves will
immediately diverge slightly, indicating that part of the electric
charge, which is being rapidly dissipated by way of the flame into the
atmosphere, has traced a path to the electroscope.
[Illustration: FIG. 11.--The swinging mannikin.]
16. An amusing variation of experiment 11, wherein a pith ball was
rendered restless between the two terminals of a Leyden condenser, is
constituted by carefully modeling a little seated figure out of pith and
sealing-wax. The mannikin is threaded on a silk strand, so that the legs
hang down as though seated on a swing, and is then suspended by tying
the two silk ends to a support above. Fig. 11 indicates the
arrangements.
Two Leyden jars are next required, of opposite influences. To effect
this, charge one by the ordinary method of touching its terminal several
times with an electrophorus cover, being careful on each occasion to
“earth” the outer surface, whilst the other condenser is charged by
presenting its _outer_ surface to the electrophorus cover, and each time
touching the knob terminal with the finger, _i.e._ “earthing” it.
By this method, whilst the charge at the knob of one Leyden jar is
positive, that at the terminal of the other is negative. Place these two
instruments at equal distances on either side of the “swing” support,
and at such an elevation that the pith figure may just touch the brass
knobs.
Then, if the swing be started going, it should continue to oscillate for
a considerable time. As the pith figure approaches, say, the positive
knob, it acquires a negative charge and is thereby attracted. Directly
it touches, however, this negative state is counteracted, a positive
charge supersedes it, and repulsion between pith figure and positive
knob ensues. The pith doll then swings over towards the negative knob
and--being positively influenced--is of course attracted until contact
takes place. Then negative supersedes positive in the pith figure,
repulsion again results, and a similar cycle of changes is repeated.
Thus the figure continues to oscillate until the charges of the jars
have been exhausted or have leaked away.
17. In the same manner as the alternate attraction and repulsion of a
light body has been used to maintain oscillation, so similarly an
electrostatic motor may be constructed embodying the same principles.
Cut out a 16-pointed star of cardboard, 8-inch diameter, and, having
glued a ¹⁄₄-inch slice of cork in the middle of each side (A, Fig. 12),
pass a knitting-needle through the star’s center at right angles (B,
Fig. 12). Both ends of this spindle rest in conical cavities (C, Fig.
12), which may be punched or drilled in small brass discs to act as
bearings, and the apparatus is mounted as shown.
[Illustration: FIG. 12.--Electrostatic motor.]
It is important that the cardboard should be perfectly dry, and with
this end in view it may well have previously received a saturation in
paraffin wax. When set hard, stick a brass pin bent at right angles into
each point of the star, and set Leyden jars of different charges beneath
any two diametrically opposite points. The motor, being given a start,
should continue to revolve for some time owing to the succeeding
attraction and repulsion of the metal pins.
The following description is of a small machine which may be fairly
easily constructed, and when in good working order will give far better
results than the electrophorus. Fig. 13 shows the reference letters and
general arrangement. Two cork bungs (A, Fig. 13), previously saturated
with paraffin wax or shellac varnish, are fixed into the ends of a
cylindrical lamp chimney, and through a hole in the exact center of each
passes a wooden rod (B, Fig. 13), which must be glued in place. Each end
is supported in a wooden bearing (C, Fig. 13), black lead being used as
a lubricant, and to the protruding one a small handle (D, Fig. 13) is
fitted, wherewith the glass may be revolved. Next a long pad of wash
leather (E, Fig. 13), stuffed with horse-hair, is fixed so as to press
gently against the cylinder, whilst a silk flap (F, Fig. 13) about 2
inches wide hangs over the top surface of the glass. A brass chain (G,
Fig. 13) is also fixed to the wash-leather cushion and, in the ordinary
way, connects to “earth” either direct or through the operator. The
wash-leather cushion is well black-leaded where it touches the glass.
[Illustration: FIG. 13.--Frictional machine (more useful than the
electrophorus for producing electric charges).]
On the opposite side of the cylinder a metal comb (H, Fig. 13) is
supported, the teeth being ¹⁄₁₆-inch away from the glass, whilst the
back of the comb is soldered to a brass knob (I, Fig. 13) fixed on a
wood pedestal. Any noticeable edges at the base of the brass globe
should be filed off until only round surfaces are presented.
Care should be taken to dry the machine thoroughly, and if then the
handle be turned regularly, as indicated by the arrow, the brass knob
should rapidly become charged to such an extent that sparks may be drawn
with the knuckles, possibly with discomforting results!
If the brass ball possess any sharp edges or, alternatively, if a length
of wire girdle it so that one end projects radially about one inch, then
the electricity will leak from these extremities in the form of a glow
discharge as fast as it is developed. This frequently creates such a
disturbance in the surrounding air as to distinctly blow a candle flame
when held at the discharging point.
In 1752 Faraday invented an arrangement of attraction and repulsion by
which three bells were kept ringing. To the electrical machine conductor
was attached a metal support from the ends of which hung two bell domes
on brass chains, whilst from the center another was supported by a silk
strand and connected by a brass chain to earth. Between the bells small
brass clappers were hung by silk threads, as shown in Fig. 14.
The action in working was for the machine to charge its conductor and
hence the two outer bell domes, which then attracted their brass
clappers until contact took place. This caused repulsion, so that the
clappers swung over to the center dome, where discharge to earth ensued
and a condition obtained when the cycle of events might be repeated.
[Illustration: FIG. 14.--Faraday’s bell chimes.]
Having thus arrived, _via_ the simplest phenomena, at a piece of
mechanism which will work, our experiments in the field of static
electricity must conclude; but rather, let it be noted, on account of a
dearth of apparatus than from any scarcity of phenomena to be studied.
If therefore the reader foresees an opportunity of pursuing the subject
further, such for example as the use of a well-equipped laboratory, let
him not hesitate to snatch the advantage. To mention two facts
demonstrable by further experiment: (1) Frictional electricity is
produced in excessively small quantities at a very high pressure. For
this reason sparks in air may be produced frictionally of such a length
as cannot be obtained direct from batteries. (2) Friction itself is not
a necessary element in the production of electrical energy, but is
adopted in order to bring every portion of the rubbed surfaces into
perfect contact. Thus far and no further. We must say “Au revoir,” and
splash down a full stop.
[1] For the experiments in this and the following chapter I am
indebted to the assistance rendered by Mr. Stanley S. Barnard.--C. H.
B.