FRICTIONAL ELECTRICITY
Frictional electricity is high potential, current alternating, and of
high voltage but very low amperage. Apart from certain uses in
laboratory and medical practice, it is valueless. In its greater volume
it is akin to the lightning-bolt and is dangerous; but in its smaller
volume it is a comparatively harmless toy. From the glass rod, or the
amber, rubbed on a catskin to the modern static machines is a long jump,
and the period of exploitation covers centuries of interesting
experiments, most of which, however, have been practically useless for
any commercial purpose.
Static or frictional electricity is generated by friction only, without
the aid of magnets, coils of wire, or armatures rotating at high speed.
The simple process of the glass and catskin has been variously modified,
until at last Wimshurst invented and perfected what is known as the
“Wimshurst Influence Machine.” It is self-charging, and does not require
“starting.” It will work all the year round in any climate and
temperature, and is the greatest improvement ever made in static
electric machines.
Apart from its efficiency under all conditions, it is the simplest of
all machines to make, and can easily be constructed by a boy who is
handy with tools, and who can obtain the glass and brass parts necessary
in its construction. The principal parts of an influence machine are the
glass disks, wooden bosses, driving pulleys and crank, glass standards,
brass arms with the spark-balls at the ends, and the base with the
uprights on which these parts are built up and held in position.
A Wimshurst Influence Machine
Obtain a stiff piece of brown paper twenty inches square, and with a
compass describe a circle twenty inches in diameter. Inside of this
circle make another one fourteen inches in diameter, and near the centre
a third circle six inches in diameter. Another circle four inches in
diameter should be drawn inside of the six-inch circle, so that when the
bosses are made fast to the glass plates they can be properly centred.
Also mark sixteen lines radiating from the centre, equal distances
apart, as shown in Fig. 1.
From a dealer in glass purchase two clear, white panes of glass eighteen
inches square. Be careful not to get the green glass, as this is not
nearly so good as the white for static machine construction. If it is
possible to get crystal plate so much the better. The panes should be
thin, or about one-sixteenth of an inch in thickness, and free from
bubbles, wavy places, scratches, or other blemishes.
From these panes cut two disks sixteen inches in diameter with a rotary
cutter, as described in the chapter on Miscellaneous Apparatus, page
294, and rub the edges with a water-stone (see chapter on Formulæ, page
330.)
From flat, thin tin-foil cut thirty-two wedge-shaped pieces four inches
long. They should be one inch and a half wide at one end and
three-quarters of an inch at the other, as shown at Fig. 2 A. Give each
plate of glass two thin coats of shellac on both sides; then lay one on
the paper pattern (Fig. 1) so that the outside edge of the glass will
lie on the largest circle. Place a weight at the middle of the glass to
hold it in place; then make sixteen of the tin-foil sectors fast to the
plate, using shellac as the sticking medium. But first give one side of
each sector a thin coat of shellac, allowing it to dry; then give it
another coat when applying it to the glass. The sectors are to be
symmetrically arranged on the glass, using a line of the pattern as a
centre for each piece (as shown at A in Fig. 1), and the fourteen and
six inch circles as the outer and inner boundaries. Each piece, as it is
applied, should be pressed down upon the glass, so that it will stick
smoothly, without air bubbles or creases. A very good plan is to lay a
piece of soft blotting-paper over the sector and drive it down with a
small squeegee-roller such as is used in photography, taking care,
however, not to shift the sector from its proper position. When all the
sectors are on, the plate should appear as shown in Fig. 2. After the
shellac, which holds the sectors to the glass, is dry, run a brush full
of shellac around the inner and outer extremities of the tin-foil strips
for half or three-quarters of an inch in from the ends. The shellac will
hold the sectors firmly to the glass, and will slightly insulate them as
well, thereby preventing the escape of electricity. Apply the remaining
sectors to the other plate of glass in a similar manner; and as a result
two disks of glass, with the applied strips, will be ready to mount in
the frame.
[Illustration: FIG. 1
FIG. 2
FIG. 3
FIG. 4
FIG. 5
FIG. 6
DETAILS OF WIMSHURST INFLUENCE MACHINE]
A hole three-quarters of an inch in diameter should be made in each
glass plate, so that a three-eighths spindle may pass through them and
into the bosses, so as to keep them in proper line. It is preferable,
however, not to bore these holes if bosses and accurately bushed holes
can be made in the uprights of the frame which support these disks.
At a wood-working mill have two bosses made that will measure four
inches in diameter at the large end, and one inch and a half at the
small one. They should be of such length that when the plates and two
bosses are arranged in line (to appear as shown in A A at Fig. 9) they
will fill the entire space between the uprights B B. Near the small end
a groove is turned in each boss, so that a round leather belt will fit
in it, as shown in Fig. 3.
The base is made from pine, white-wood, cypress, or any other wood that
is soft and easily worked. It is composed of two strips twenty-four
inches long, three inches wide, and one inch and a quarter in thickness,
and two cross-pieces fourteen inches long, three inches wide, and one
inch and a half thick.
These are put together with glue and screws, and at both sides of the
base notches are cut to accommodate the feet of the uprights. The
uprights are seventeen inches high, three inches wide, and one inch and
a half thick. The notch at the foot of each one is cut so that, when
fitted in place, the foot of the upright will rest on a table on a line
with the bottom of the end cross-pieces under each corner. At the foot
of the uprights a piece of sheet rubber should be made fast, with glue
or shellac, so that when in operation the machine will not move about or
slide.
A groove is cut at one side of each upright six inches above the bottom,
as shown at Fig. 4 A. In this groove the driving-wheel axles fit, and
near the top holes are made in the uprights through which the spindles
pass, which in turn support the bosses and glass disks.
At the middle of each cross-piece forming the ends of the base a
one-inch hole, for the glass standard rods, is bored through the wood,
as shown at Fig. 4 B B. After attaching the uprights to the base with
glue and screws, and giving all the wood-work several successive coats
of shellac, the frame will be ready for its mountings.
The driving-wheels are of wood seven-eighths of an inch thick and seven
inches in diameter; they should be turned on a lathe and a groove cut in
the edge so that a round leather belt will fit in it. These wheels are
mounted on a wooden axle that can be made from a round curtain-pole,
with a half-inch hole bored through its entire length. The axle is as
long as the distance between uprights B B in Fig. 9. The wheels are to
be arranged and glued fast to the axle, so that the grooves will line
directly under those in the bosses, as shown in Fig. 9. A half-inch axle
is driven through the hub, and at one end it is threaded and provided
with two washers and nuts; or a square shoulder and one washer and nut
may be used, so that a crank and handle may be held fast. Shellac should
be put on the shaft to make it hold fast in the hub.
The complete apparatus of wheels, axle, hub, and handle is shown at Fig.
5, and in the frame this is so hung that the iron axle rests in the
grooves cut in the uprights. To hold this in place two metal straps, as
shown in Fig. 6, are made and screwed fast to the wood. When finally
adjusted the driving-wheels should rotate freely whenever the crank is
turned. The wooden bosses, Fig. 3, are given two or three coats of
shellac; then they are made fast to the glass disks on the same side to
which the tin-foil sectors are attached. The disks should be placed over
the paper plan, Fig. 1, and so adjusted that the outer line tallies with
the large circle. Acetic glue[4] is then applied to the flat surface of
the boss, and the latter is placed at the middle of the disk to line
with the small circle. Place a weight on the end of the boss to hold it
down, and leave it for ten or twelve hours or until thoroughly dry.
[4] See Formulæ, Chapter xiv., for the recipe of acetic glue.
Both bosses should be set at the same time so that they may dry
together.
If the bosses are turned on a lathe a hole should be made in each one
about half-way through from the small end. This, in turn, should be
bushed or lined with a piece of brass tube, which should fit snugly in
the hole. A little shellac painted on each piece of tube will make it
stick. Pieces of steel rod that will just fit within the tubing are to
be cut long enough to enter the full length of the hole, pass through
the holes made in the top of the uprights, and extend half an inch
beyond, as shown in Fig. 9. The bosses and axles will then appear as
shown in Fig. 7.
Flat places should be filed on each rod where it passes through the wood
upright; a set-screw will then hold it fast and keep it from revolving.
When the hole, or tubing, is oiled so that the boss and disk will
revolve freely on the axle, the disks, bosses, and axles are ready to be
mounted in the frame.
A red fibre washer, such as is used in faucets, should be made fast to
one glass disk at the centre, so as to separate the disks and prevent
them from touching when they are revolving in opposite directions. These
fibre washers can be had from a plumber or purchased at a hardware
store. Shellac or acetic glue will hold the washers in place.
[Illustration: FIG. 7]
[Illustration: FIG. 8]
[Illustration: FIG. 9]
[Illustration: FIG. 10]
Mount one disk by holding the boss with the small end opposite a hole in
one upright, and slip an axle through from the outside of the upright.
Hold the other disk in place, and slip the remaining axle through the
other upright and into the boss. When both plates are in place and
centred, turn the set-screws down on the flattened axles to hold them in
place.
To reduce the friction between the bosses and the uprights it would be
well to place a fibre washer between them. A few drops of oil on these
washers will lubricate them properly, and allow the machine to run
easier. An end view of the apparatus, as so far assembled, will appear
as shown in Fig. 9, A being the disks, bosses, and axles, B B the
uprights supporting them, C the hub, and D D the driving-wheels. The
handle and crank (E) extends out far enough from the side to allow a
free swinging motion without hitting the upright or base. Having
arranged these disks and wheels so as to revolve freely, it will now be
necessary to construct and add the other vital parts and the connecting
links in the chain of the complete working mechanism.
From a supply-house obtain two solid glass rods an inch in diameter and
fifteen inches long. These fit in the holes (B B) bored in the
end-pieces of the base, Fig. 4. Procure two brass balls, two or two and
a half inches in diameter, such as come on brass beds, and two short
pieces of brass tubing, one inch inside diameter, that will fit over the
ends of the rods. These tubings are to be soldered fast to the balls so
that both tubes and balls will remain at the top of the glass rods.
From brass rod three-sixteenths or a quarter of an inch in diameter make
two forks, as shown at Fig. 8, and solder small brass balls at the ends
of the rods. The prongs of the fork are six inches long and the shank
four inches in length. Along the inside of the forks small holes are
bored, and brass wires, or “points,” are soldered fast. These extend out
for half an inch from the rods, and are known as the “comb,” or
collectors. The forks should be so far apart that when mounted with the
glass disks revolving between them the points will not touch or scratch
the tin-foil sectors, and yet be as close to them as possible. A hole
should be bored in the brass balls, and the shank of the fork passed
through and soldered in place, as shown in Fig. 10.
A three-eighth-inch hole is bored directly in the top of each brass ball
to hold the quadrant rods, which extend over the top of the disks.
In the illustration of the complete machine (Fig. 12) the arrangement of
the glass pillars, balls, combs, and quadrant rods is shown. The rods
are three-eighths of an inch in diameter and are loose in the holes at
the top of the balls, so that they can be moved or shifted about,
according as to whether it is a left or a right handed person who may be
turning the crank.
At the upper end of each rod a brass ball is soldered, one being
three-quarters of an inch in diameter, the other two inches. The
projecting ends of the forks should be provided with metal handles or
brass balls, as shown in Fig. 12; these may be slipped over the end or
soldered fast. Obtain two small brass balls with shanks, such as screw
on iron bed-posts, and have the extending ends of the axles that support
the bosses threaded, so that the balls will screw on them. Bore a
quarter-inch hole through each ball, and slip a brass rod through it and
solder it fast. Each end of these rods should be tipped with a bunch of
tinsel or fine copper wires. These are the “neutralizers,” and the ends
are curved so that the brushes of fine wires will just touch the disks
when the latter are revolved, as shown in Fig. 12. The ball holding the
rod is to be screwed fast to the axle; then the axle is pushed back into
the boss and made fast in the head of the upright with the set-screw.
[Illustration: FIG. 12]
The rod-and-ball at the opposite side of the disks is arranged in a
similar manner, but the rod points in an opposite direction to that on
the first side. Cord or leather belts connect the driving-pulleys and
bosses, the belt on one side running up straight over the boss and down
again around the driving-pulley. The belt at the opposite side is
crossed, so that the direction of the boss is reversed; and in this
manner the disks are made to revolve in opposite directions, although
the driving-pulleys are both going in the same direction.
A portion of the sectors are omitted in the illustration (Fig. 12) so
that a better view of the working parts may be had. When the disks are
revolving the accumulated electricity discharges from one ball to the
other, above the plates, in the form of bright blue sparks sufficiently
powerful to puncture cardboard if it is held midway between the balls.
A Large Leyden-jar
When experimenting with this machine it would be well to have one or
more Leyden-jars to accumulate static charges. A large one of
considerable capacity is easily made from a battery jar, tin-foil, brass
rods and chain, and some other small parts.
Obtain a bluestone battery jar, and after heating it to drive all
moisture from the surface, give it a coat of shellac inside and out.
With tin-foil, set with shellac, cover the bottom and inside of the jar
for two-thirds of its height from the bottom, as shown in Fig. 11. Cover
the outside and bottom in a similar manner, and the same height from
the bottom, and provide a cork, or wooden cap, for the top. If a large,
flat cork cannot be had, then make a stopper by cutting two circular
pieces of wood, each half an inch thick, the inner one to fit snugly
within the jar, the other to lap over the edges a quarter of an inch all
around. Fasten these pieces together with glue, as shown at Fig. 13, and
give them several good coats of shellac. Make a small hole at the middle
of this cap and pass a quarter-inch rod through it, leaving six inches
above and below the cap. To the top of the rod solder a brass ball. At
the foot a piece of brass chain is to be made fast, so that several
links of it rest on the tin-foil at the bottom of the jar.
To charge a jar from the Wimshurst machine, stand the jar on a
glass-legged stool, and connect a copper wire between one of the
overhead balls on the machine and the ball at the top of the rod in the
stopper of the jar. Make another wire fast to the other ball at the top
of the machine, and place it under the jar so that the tin-foil on the
bottom touches it. By operating the machine the jar is charged.
To discharge the jar make a [T]-yoke, as shown at Fig. 14, by nailing a
brass rod fast to a wooden handle and soldering brass knobs, or
hammering a lead bullet, on the ends of the rod. Hold one knob against
the top knob of the jar and bring the other near the foil coating at the
outside, when a spark will jump from the foil to the knob with a loud
snap.
A Glass-legged Stool
One of the most useful accessories in playing with frictional
electricity will be a glass-legged stool. A stool with glass feet is
perhaps too expensive for a boy to purchase, but one may be made at
little or no cost from a piece of stout plank, four glass telegraph
line-insulators, and the wooden screw-pins on which they rest when
attached to a pole.
[Illustration: FIG. 11]
[Illustration: FIG. 13]
[Illustration: FIG. 14]
[Illustration: FIG. 15]
[Illustration: FIG. 16]
The general plan of the stool is shown at Fig. 15, and the top measures
twelve by fifteen by two inches. Under each corner a screw-pin is made
fast by boring a hole in the wood and setting the pin in glue. The pins
are cut at the top, as shown in Fig. 16, and when they are set in place
the glass insulators may be screwed on. The wood-work should be given a
few coats of shellac to lend it a good appearance and help to insulate
it.
There are a great many interesting experiments that may be performed
with static or frictional electricity, and these may be looked up in the
text-books on electricity used in school. A word of caution will not be
misplaced. Remember that the current, in large volume, is dangerous. For
example, a series of charged Leyden-jars may contain enough electricity
to give a very severe shock to the nervous system of the person who
chances to discharge it. Its medical use should be under the advice and
supervision of a physician.