A Cylinder Electric Machine
The electrophorus described in the last chapter is capable of furnishing
sufficient electricity for many interesting experiments, but for the
purpose of procuring larger supplies of electricity, a static electric
machine is necessary.
An electric machine is composed of two parts, one for producing the
electricity by the friction of two surfaces rubbing against each other,
and the other an arrangement for collecting the electricity thus formed.
The earliest form of electric machine consisted of a ball of sulphur
fixed upon a spindle which could be rotated by means of a crank. When
the dry hands were pressed against the sulphur by a person standing on a
cake of resin, which insulated him, sparks could be drawn from his body.
Later a leather cushion was substituted for the hands, and a glass
cylinder for the ball of sulphur, so that the frictional electric
machine now consists of a cylinder or a disk of glass mounted upon a
horizontal axis capable of being turned by a handle. A leather cushion,
stuffed with horsehair and covered with a powdered amalgam of zinc or
tin, presses against one side of the cylinder. A "prime" conductor in
the shape of an elongated cylinder presents a row of fine metal spikes,
like the teeth of a rake, to the opposite side. A flap of silk attached
to the leather cushion passes over the cylinder and covers the upper
half.
[Illustration: Fig. 29.—Front View of a Cylinder Electric Machine.]
When the handle of the machine is turned, the friction produced between
the leather cushion and the glass generates a supply of positive
electricity on the glass, which is collected, as the cylinder revolves,
by the row of sharp points, and transferred to the prime conductor.
The first thing required in the construction of an electric machine is a
large glass bottle having a capacity of from two to four quarts.
The insulating power of glass varies considerably. Common green glass
(not white glass colored green by copper, but glass such as the
telegraph insulators are made from) generally insulates the best. Some
sorts of white glass, the Bohemian especially, are good insulators, but
this quality will not usually be found in ordinary bottles.
[Illustration: Fig. 30.—Method of Finding the Center of a Circle.]
Select a smooth bottle which has no lettering embossed upon it, and
stand it upon a piece of white paper. Trace on the paper a line around
the circumference of the bottle so that the circle thus formed is of the
same size as the bottom of the bottle. Lay a carpenter’s square on the
circle so that the point _C_ just touches the circumference. Draw a line
from _A_ to _B_ where the sides of the square cut the circumference. The
point in the middle of this line is the center of the circle.
Place the paper on the bottom of the bottle so that the circle coincides
with the circumference, and mark the center of the bottle.
The bottle must now be drilled. This is accomplished with a small
three-cornered file, the end of which has been broken off so as to form
a ragged cutting edge. The file is set in a brace and used like an
ordinary drill. During the boring process the drill must be frequently
lubricated with a mixture of gum camphor and turpentine. The drilling,
which will require almost an hour before the glass is pierced, if the
bottle is a thick one, should be performed slowly and carefully, so as
to avoid all danger of cracking the glass. The hole, when finished,
should be from one-quarter to three-eighths of an inch in diameter.
After the hole has been bored, fit a wooden plug into the neck of the
bottle and cement it there with a mixture composed of one-half a pound
of resin, five ounces of beeswax, one-quarter of an ounce of plaster of
Paris, and three-quarters of an ounce of red ocher, melted together over
a moderately warm stove. Dip the plug in the molten cement and force it
into the neck of the bottle. When the cement dries it will be impossible
to remove it.
The sizes of bottles vary, so that it is quite impossible to give
dimensions which must be closely followed in constructing the machine.
Those in the text are approximate. The drawings have been made to scale
so as to show the proportions the parts bear to each other.
A heavy wooden base will be required to mount the machine on. Two
uprights are mounted on the base to support the axis of the bottle.
Through one of these bore a hole of the same diameter as the wooden plug
fitted in the neck of the bottle. The end of the wooden plug projecting
through the upright is notched and fitted with a crank so that the
bottle may be revolved. The handle of the crank is an ordinary spool
having one flange cut off and mounted with a screw and a washer.
[Illustration: Fig. 31.—The "Rubber."]
The machine is now ready for the "rubber" and "prime conductor." The
rubber is a piece of wood one inch square and from six to eight inches
long. A piece of undressed leather is tacked on as shown in the
illustration and stuffed with horsehair. The wood is shellacked and
covered with tin-foil previous to tacking on the leather. A strip of
wood, two inches wide and one-half an inch thick, is fastened to the
back of the rubber. The strip should be just long enough so that when
the lower end rests on the base the rubber is level with the axis of the
bottle. The lower end may be fastened to the base by means of a small
brass hinge. Two rubber bands stretch from hooks between the rubber and
the base so as to pull the former tightly against the bottle. The
illustration shows a method of mounting the rubber on a foot-piece held
to the base with a thumb-nut so that it may be slid back and forth and
the pressure varied at will.
The prime conductor is formed from a piece of curtain-pole two inches in
diameter and eight inches long. The ends are rounded with a rasp and
then smoothed with sandpaper. The whole surface is then shellacked and
covered with a layer of tinfoil. The heads of a number of dressmaker’s
pins are cut off, and the pins forced into the side of the prime
conductor with a pair of pincers. They should form a row like the teeth
of a rake about three-eighths of an inch apart. A hole is bored in the
center of the under side of the prime conductor to receive a glass rod
one-half inch in diameter. A second hole of the same size is bored in
the base in such a position that when the glass rod is in place, the
teeth on the prime conductor are on a level with the axis of the bottle,
and their points about 3-32 of an inch away from the glass. The glass
rod must be used in order to insulate the prime conductor and prevent
the escape of the electricity. It is secured with some of the cement
described on page 33. A piece of water-gauge glass may be used in place
of a glass rod.
[Illustration: Fig. 32.—The Prime Conductor or Collector.]
A strip of oiled silk, or in its place a strip of silk which has been
shellacked, eight or nine inches wide, and long enough to reach half-way
around the bottle, is tacked to the rubber so that the silk covers the
upper half of the cylinder and comes over to within one-quarter of an
inch of the steel points.
The machine is now complete, and when the handle is turned rapidly, you
will be able to draw sparks from the prime conductor. The sparks will
probably be very short, about one-half of an inch long. These can be
increased, however, to three inches, if the glass is of the right
quality, by treating the rubber with amalgam.
The amalgam is formed by melting one ounce of tin and adding to it one
ounce of zinc in small bits. As soon as the zinc has also melted add to
the mixture two ounces of mercury which has been previously warmed. Be
careful not to inhale any of the vapor during this operation. Pour the
mixture into a vessel of cold water, which will reduce the metal to
small grains. Pour off the water and grind the amalgam to a powder by
pounding the grains with a hammer.
The leather rubber should be _thinly_ smeared with lard and the powdered
amalgam rubbed on it.
In order to obtain the greatest effect from an electric machine, it must
be carefully freed from dust and particles of amalgam adhering to the
glass, and the insulating column rubbed with a warm woolen cloth. The
best results are obtained by placing the machine near a stove or
radiator where it is warm.
[Illustration: Fig. 33.—The Complete Cylinder Electric Machine.]
A Wimshurst Machine
The Wimshurst Machine consists of two varnished glass plates revolving
in opposite directions. On the outside of each of these plates are
cemented a number of tinfoil "sectors," arranged radially. Two
conductors at right angles to each other extend obliquely across the
plates, one at the back and the other at the front. These conductors
each terminate in brushes of tinsel which electrically excite the
"sectors" as the plates revolve. The electricity is collected by a set
of "collectors" arranged in a somewhat similar manner to the collector
on the cylinder electric machine.
*The Glass Plates* are each eighteen inches in diameter. Purchase two
panes of clear glass twenty inches square from a glass dealer. The white
glass is far preferable to the green glass and will make the best
electric machine. The plates should be of the thickness known as "single
light" and should be perfectly free from wavy places, bubbles, or other
imperfections.
[Illustration: Fig. 34.—Paper Pattern for laying out the Plates.]
The work is first laid out on a piece of stiff paper twenty inches
square as a pattern. Describe a circle four inches in diameter. Using
the same center, draw other circles, making them respectively eight,
sixteen, and eighteen inches in diameter. Then mark sixteen radial
lines, from the center, making them equal distances apart, as shown in
Figure 34.
[Illustration: Fig. 35.—Plate with Sectors in Position, and a Pattern
for the Sectors.]
Lay one of the glass panes over the pattern and cut out a glass circle
eighteen inches in diameter, or perhaps you may be able to have a
glazier do the cutting for you and so save considerable trouble and
possible breakage. Two such plates should be made.
The Sectors are cut from heavy flat tinfoil according to the pattern
shown in Figure 35. They should be made one inch and one-half wide at
the wide end and three-quarters of an inch at the other end. They are
each four inches long. Thirty-two such sectors are required. The easiest
way to make them is to cut out a pattern from heavy cardboard to serve
as a guide.
Clean and dry both of the glass plates very carefully and then give them
each two thin coats of white shellac. After they have been dried, lay
one of the plates on the paper pattern so that the outside of the plate
will coincide with the largest circle on the paper.
Then place a weight in the center of the plate so that it will not move,
and stick sixteen of the tinfoil sectors on the plate with thick
shellac. The sectors are arranged symmetrically on the plate, using the
eight-inch and sixteen-inch circles and the radial lines as guides. Both
plates should be treated in this manner. Each sector should be carefully
pressed down on the glass, so that it will stick smoothly without
air-bubbles or creases. When all the sectors are in place the plates
will appear like that shown in Figure 35.
*The Bosses* will have to be turned out at a wood-working mill or at
some place where they have a turning-lathe. The bosses are four inches
in diameter at the large end and one inch and one-half at the other. A
groove is turned near the small end of each to accommodate a round
leather belt.
A hole should be made in each boss about half-way through from the small
end. These holes should be bushed with a piece of brass tubing having an
inside diameter of one-half inch. The tubing should go into the hole
very snugly and be a "driven fit."
[Illustration: Fig. 36.—A Side View of one of the Bosses, showing the
Brass Bushing used.]
The bosses should both be given a coat of shellac, and after this is
dry, fastened to the glass plates on the same side to which the tinfoil
sectors are attached. The best plan is to lay the disks on the paper
pattern and adjust them until the outer edge coincides with the largest
circle.
Then apply some _bichromate glue_ to the flat surface of one of the
bosses and place the latter in the center of the plate in line with the
smallest circle.
Place a weight on the boss to hold it down firmly against the plate and
leave it over night, or for ten or twelve hours, until thoroughly dry.
The glue is prepared by placing some high-grade glue in a tin cup and
covering it with cold water. Allow it to stand until the glue absorbs
all the water it will and becomes soft. Then pour the water off and add
enough _glacial acetic acid_ to cover the glue.
Heat the mixture until it is reduced to a liquid, stirring it until it
is perfectly smooth. Add a teaspoonful of powdered bichromate of potash
to the glue.
The glue must now be kept in the dark, for sunlight will "set" the glue
so that it becomes insoluble.
The Frame of the machine is composed of two strips twenty-five inches
long, three inches wide, and an inch and one-half in thickness, and two
cross-pieces of the same thickness and width fifteen inches long.
[Illustration: Fig. 37.—The Frame.]
Notches are cut at both sides of the base to admit the feet of the
uprights.
*The Uprights* are seventeen inches long, three inches wide, and one and
one-half inches thick.
[Illustration: Fig. 38.—The Upright.]
The notch at the foot is cut the same width as the thickness of the long
members of the frame and is arranged so that when fitted in place, the
foot of the upright will rest on the table in line with the bottom of
the cross-pieces.
*The Driving-Wheels* are turned out of wood on a lathe. They are seven
inches in diameter and seven-eighths of an inch thick. A groove should
be turned in the edge to carry a small round leather belt. The wheels
are mounted on a wooden axle made from a round curtain-pole. They are
glued to the axle and arranged so that the grooves will fall directly
underneath the pulleys turned in the bosses.
[Illustration: Fig. 39.—The Driving-Wheels and Axle.]
The ends of the axle pass through the uprights, five inches above the
bottom.
The front end of the axle is fitted with a crank and a handle.
[Illustration: Fig. 40—The Boss and Axle. For sake of clearness, the
Plate is not shown.]
The plates are mounted on short iron axles passing through the top of
the upright into the brass bushings. One end of each of the axles is
filed flat where it passes through the wood upright so that it may be
firmly held by a set-screw and prevented from revolving.
Fasten a small fiber washer to the center of one glass disk so that it
will separate the plates and prevent them from touching when revolving.
The collectors, quadrant rods, etc., are mounted on glass rods one inch
in diameter. The bottoms of the rods fit in holes (_H H_) bored in the
cross-pieces of the base, Figure 37. The upper ends are each fitted with
a brass ball two inches in diameter. The balls are mounted on the rods
by soldering a piece of brass tubing to the ball and slipping it over
the rod. The rods should be of the proper length to bring the center of
the balls on a line with the center of the plates.
[Illustration: Fig. 41—Showing how the Ball, Comb, etc., are mounted on
the Glass Rod.]
Make two forks as shown in Figure 42 out of brass rod, three-sixteenths
of an inch in diameter and solder brass balls at the ends. The forks are
eleven inches long.
A number of small holes must be bored in the "prongs" and pins made by
cutting ordinary dressmakers’ pins in half and soldering them in place.
These pins, mounted on the forks, form the combs or collectors.
Bore a horizontal hole through each of the brass rods on the top of the
glass rods and pass the shanks of the forks through and solder them in
place.
One of the shanks may be provided with a discharge ball at the end as
shown by _D B_ in Figure 44. The other is provided with a hard rubber
handle made from a piece of rod. Bore a three-eighths hole directly in
the top of each brass ball to receive the quadrant rods forming the
spark-gap.
[Illustration: Fig. 42.—A Comb or Collector.]
The quadrant rods extend over the top of the plates and are
three-eighths of an inch in diameter. They are loose in the tops of the
balls so that they may be moved about or removed entirely.
A small brass ball three-quarters of an inch in diameter should be
soldered to the top of one of the quadrant rods and a similar ball two
inches in diameter to the other.
[Illustration: Fig. 43.—Showing how the Tinsel Brushes are arranged on
the "Neutralizer" Rods.]
Two large brass balls, two inches in diameter, are fitted over the ends
of the axles, which project through the uprights. Bore a
one-quarter-inch hole through each ball at right angles to the axle and
slip a one-quarter-inch brass rod through and solder it fast.
[Illustration: Fig. 44.—The Complete Wimshurst Electric Machine. B B B
B, _Brushes_. C C, _Combs_. D B, _Discharge Ball_. I I, _Glass Rods_. H,
_Handle_. Q Q, _Quadrant Rods_. S S S S S, _Sectors_. S G, _Spark-Gap_.
P P, _Driving-Wheels_. For the sake of clearness, several of the sectors
are not shown.]
The ends of the rods should be tipped with a bunch of tinsel or fine
copper wires and be curved so that the brushes so formed will just touch
the sectors on the disks when the latter are revolved.
These are the neutralizers and are arranged in the approximate positions
shown in Figure 44.
The driving-wheels are connected to the bosses by means of small round
leather belts. The belt at the rear of the machine is crossed in order
to make the plates revolve in opposite directions.
If the machine has been properly built it is now ready for operation. It
may be necessary to charge the machine the first time that it is used by
touching several of the sectors with the charged cover of an
electrophorus. Then if the handle is turned the accumulated electricity
should discharge across the spark-gap at the top of the machine in the
form of bright blue sparks.
Experiments with an Electric Machine
Many interesting experiments can be performed with an electric machine.
The number is almost unlimited. A few of the most instructive ones are
described below. Others can be found in almost any text book on physics.
*The Leyden jar* consists of a glass jar coated with tinfoil part way up
on both the outside and inside. Through the wooden stopper passes a
brass rod or a heavy copper wire which connects with the inner coating
of tinfoil by means of a small brass chain. The upper and outside end of
the rod usually terminates in a brass ball or knob.
It is a very simple matter to make a good Leyden jar.
[Illustration: Fig. 45.—The Leyden Jar.]
The jar must be thoroughly cleaned and dried before coating. The inside
is then given a thorough brushing over with shellac or varnish. Before
it is dry, carefully insert the tin-foil and press it smoothly against
the glass. The outside of the jar is treated and coated in the same
manner. The inside and outside of the bottom are also coated by cutting
the tinfoil in circular pieces and shellacking them on.
In order to charge the Leyden jar, grasp it in the hand near the bottom
and hold the knob against the prime conductor while turning the handle
of the machine.
[Illustration: Fig. 46.—A Wooden Mortar for Igniting Gunpowder.]
*Igniting gunpowder.* Bore a hole one-half inch in diameter and one inch
deep in a block of hardwood. Pass two small brass wires through holes in
the sides, letting the ends of the wires be about one-eighth of an inch
apart. Pour a little gunpowder in loosely over the wires. Tie a piece of
thoroughly moistened cotton twine, three inches long, to one of the
wires and attach it to the outside coating of a charged Leyden jar.
Connect the knob of the jar to the other wire. The gunpowder will
immediately explode. Keep the face and hands away from the gunpowder
when performing this experiment.
[Illustration: Fig. 47.—An Electric Umbrella.]
*Electric Umbrella.* The repulsion of similarly electrified bodies which
was illustrated by the action of the pith ball electroscope may be
better illustrated by pasting some narrow streamers of tissue paper
about one-eighth of an inch wide and four inches long to a small cork
covered with tinfoil. The cork is mounted on the upper end of a stiff
copper wire supported in a bottle. When the wire is connected to the
prime conductor and the machine set in motion, the strips will spread
out like an umbrella.
*Lightning Board.* A pane of glass is thoroughly cleaned and then given
a coat of shellac or varnish. Before the varnish is dry, press on a
piece of tinfoil large enough to cover one side of the glass and rub it
down smoothly.
[Illustration: Fig. 48.—A Lightning Board.]
After the shellac or varnish is dry, cut the tinfoil up into innumerable
little squares with a sharp knife and ruler, leaving two solid strips of
tinfoil at the ends of the glass pane.
The pane is mounted by cementing it in a slot in the cork of a bottle.
Connect one of the tinfoil strips to the prime conductor and the other
to the earth or the body. When the machine is turned, innumerable little
sparks will pass between the tinfoil squares and give an appearance very
similar to that of lightning.
[Illustration: Fig. 49.—An Electric Dance.]
*The Electrical Dance.* A number of little balls of cork or pith are
enclosed in a cylinder of glass about two and one-half or three inches
high formed by cutting off the top of a lamp chimney. The top and bottom
of the cylinder are closed by two circular pieces of sheet brass or
copper. The top disk is connected to the prime conductor while the
bottom one is connected to the rubber. When the machine is set in
motion, the little balls will dance up and down. Bits of feather or
paper cut to represent figures of men and women may be used as well as
pith or cork balls.
*The Electric Whirl.* The whirl consists of an S shaped piece of brass
wire, pointed at both ends and supported on a needle by a little conical
depression made in the center with a punch.
[Illustration: Fig. 50.—An Electric Whirl.]
The needle is stuck in a cork in the top of a bottle and connected with
the prime conductor of the electric machine. When the latter is set in
motion, the whirl will commence to revolve at a high rate of speed.
*Lichtenberg’s Figures* can be produced by charging a Leyden jar by
connecting the knob or inside coating with the prime conductor and
holding the outside coating in the hand.
Then trace a small circle on the electrophorus bed with the knob.
Charge a second Leyden jar by connecting the outside coating with the
prime conductor.
The inside coating should be connected to the rubber by means of a wire
fastened to the knob. The same result may be obtained by connecting the
outside coating with the prime conductor and touching the knob with the
hand.
Then trace a cross on the electrophorus bed with the knob, making the
cross inside of the circle.
[Illustration: Fig. 51.—Lichtenberg’s Figures.]
Shake a mixture of red lead and sulphur through a muslin bag from a
height of several inches over the electrophorus.
The red lead will accumulate around the cross and the sulphur around the
circle.