ELECTRICAL MACHINES AND THE LEYDEN JAR
The amount of electricity produced by the rubbing of glass or
sealing-wax rods is very small, and experimenters soon felt the need of
apparatus to produce larger quantities. In 1675 the first electrical
machine was made by Otto von Guericke, the inventor of the air-pump.
His machine consisted of a globe of sulphur fixed on a spindle,
and rotated while the hands were pressed against it to provide the
necessary friction. Globes and cylinders of glass soon replaced the
sulphur globe, and the friction was produced by cushions instead of
by the hands. Still later, revolving plates of glass were employed.
These machines worked well enough in a dry atmosphere, but were very
troublesome in wet weather, and they are now almost entirely superseded
by what are known as _influence_ machines.
In order to understand the working of influence machines, it is
necessary to have a clear idea of what is meant by the word influence
as used in an electrical sense. In the previous chapter we saw that a
pith ball was attracted by an electrified body, and that when the ball
touched that body it received a charge of electricity. We now have to
learn that one body can receive a charge from another body without
actual contact, by what is called “influence,” or electro-static
induction. In Fig. 2 is seen a simple arrangement for showing this
influence or induction. A is a glass ball, and BC a piece of metal,
either solid or hollow, made somewhat in the shape of a sausage, and
insulated by means of its glass support. Three pairs of pith balls are
suspended from BC as shown. If A is electrified positively, and brought
near BC, the pith balls at B and C repel one another, showing that
the ends of BC are electrified. No repulsion takes place between the
two pith balls at the middle, indicating that this part of BC is not
electrified. If the charges at B and C are tested they are found to be
of opposite kinds, that at B being negative, and that at C positive.
Thus it appears that the positive charge on A has attracted a negative
charge to B, and repelled a positive one to C. If A is taken away, the
two opposite charges on BC unite and neutralise one another, and BC
is left in its original uncharged condition, while A is found to have
lost none of its own charge. If BC is made in two parts, and if these
are separated while under the influence of A, the two charges cannot
unite when A is removed, but remain each on its own half of BC. In this
experiment A is said to have induced electrification on BC. Induction
will take place across a considerable distance, and it is not stopped
by the interposition of obstacles such as a sheet of glass.
[Illustration: FIG. 2.--Diagram to illustrate Electro-static
Induction.]
We can now understand why an electrified body attracts an unelectrified
body, as in our pith ball experiments. If we bring a positively
charged glass rod near a pith ball, the latter becomes electrified
by induction, the side nearer the rod receiving a negative, and the
farther side a positive charge. One half of the ball is therefore
attracted and the other half repelled, but as the attracted half is the
nearer, the attraction is stronger than the repulsion, and so the ball
moves towards the rod.
[Illustration: FIG. 3.--The Electrophorus.]
Fig. 3 shows an appliance for obtaining strong charges of electricity
by influence or induction. It is called the _electrophorus_, the name
coming from two Greek words, _electron_, amber, and _phero_, I yield
or bear; and it was devised in 1775 by Volta, an Italian professor of
physics. The apparatus consists of a round cake, A, of some resinous
material contained in a metal dish, and a round disc of metal, B, of
slightly smaller diameter, fitted with an insulating handle. A simple
electrophorus may be made by filling with melted sealing-wax the lid
of a round tin, the disc being made of a circular piece of copper or
brass, a little smaller than the lid, fastened to the end of a stick of
sealing-wax. To use the electrophorus, the sealing-wax is electrified
negatively by rubbing it with flannel. The metal disc is then placed
on the sealing-wax, touched for an instant with the finger, and lifted
away. The disc is now found to be electrified positively, and it may be
discharged and the process repeated many times without recharging the
sealing-wax. The charge on the latter is not used up in the process,
but it gradually leaks away, and after a time it has to be renewed.
The theory of the electrophorus is easy to understand from what we
have already learnt about influence. When the disc B is placed on the
charged cake A, the two surfaces are really in contact at only three
or four points, because neither of them is a true plane; so that on
the whole the disc and the cake are like A and BC in Fig. 2, only much
closer together. The negative charge on A acts by induction on the
disc B, attracting a positive charge to the under side, and repelling
a negative charge to the upper side. When the disc is touched, the
negative charge on the upper side escapes, but the positive charge
remains, being as it were held fast by the attraction of the negative
charge on A. If the disc is now raised, the positive charge is no
longer bound on the under side, and it therefore spreads over both
surfaces, remaining there because its escape is cut off by the
insulating handle.
[Illustration: FIG. 4.--Wimshurst Machine.]
We may now try to understand the working of influence machines, which
are really mechanically worked electrophori. There are various types of
such machines, but the one in most general use in this country is that
known as the Wimshurst machine, Fig. 4, and we will therefore confine
ourselves to this. It consists of two circular plates of varnished
glass or of ebonite, placed close together and so geared that they
rotate in opposite directions. On the outer surfaces of the plates
are cemented sectors of metal foil, at equal distances apart. Each
plate has the same number of sectors, so that at any given moment the
sectors on one plate are exactly opposite those on the other. Across
the outer surface of each plate is fixed a rod of metal carrying at
its ends light tinsel brushes, which are adjusted to touch the sectors
as they pass when the plates are rotated. These rods are placed at an
angle to each other of from sixty to ninety degrees, and the brushes
are called neutralizing brushes. The machine is now complete for
generating purposes, but in order to collect the electricity two pairs
of insulated metal combs are provided, one pair at each end of the
horizontal diameter, with the teeth pointing inward towards the plates,
but not touching them. The collecting combs are fitted with adjustable
discharging rods terminating in round knobs.
The principle upon which the machine works will be best understood
by reference to Fig. 5. In this diagram the inner circle represents
the front plate, with neutralizing brushes A and B, and the outer one
represents the back plate, with brushes C and D. The sectors are shown
heavily shaded. E and F are the pairs of combs, and the plates rotate
in the direction of the arrows. Let us suppose one of the sectors at
the top of the back plate to have a slight positive charge. As the
plates rotate this sector will come opposite to a front plate sector
touched by brush A, and it will induce a slight negative charge on the
latter sector, at the same time repelling a positive charge along the
rod to the sector touched by brush B. The two sectors carrying the
induced charges now move on until opposite back plate sectors touched
by brushes C and D, and these back sectors will receive by induction
positive and negative charges respectively. This process continues
as the plates rotate, and finally all the sectors moving towards
comb E will be positively charged, while those approaching comb F
will be negatively charged. The combs collect these charges, and the
discharging rods K and L become highly electrified, K positively and
L negatively, and if they are near enough together sparks will pass
between them.
[Illustration: FIG. 5.--Diagram to illustrate working of a Wimshurst
Machine.]
At the commencement we supposed one of the sectors to have a positive
charge, but it is not necessary to charge a sector specially, for the
machine is self-starting. Why this is the case is not yet thoroughly
understood, but probably the explanation is that at any particular
moment no two places in the atmosphere are in exactly the same
electro-static condition, so that an uneven state of charge exists
permanently amongst the sectors.
The Wimshurst machine provides us with a plentiful supply of
electricity, and the question naturally arises, “Can this electricity
be stored up in any way?” In 1745, long before the days of influence
machines, a certain Bishop of Pomerania, Von Kleist by name, got the
idea that if he could persuade a charge of electricity to go into
a glass bottle he would be able to capture it, because glass was a
non-conductor. So he partly filled a bottle with water, led a wire down
into the water, and while holding the bottle in one hand connected
the wire to a primitive form of electric machine. When he thought he
had got enough electricity he tried to remove his bottle in order
to examine the contents, and in so doing he received a shock which
scared him considerably. He had succeeded in storing electricity in
his bottle. Shortly afterwards the bishop’s experiment was repeated by
Professor Muschenbrock of Leyden, and by his pupil Cuneus, the former
being so startled by the shock that he wrote, “I would not take a
second shock for the kingdom of France.” But in spite of shocks the end
was achieved; it was proved that electricity could be collected and
stored up, and the bottle became known as the Leyden jar. The original
idea was soon improved upon, water being replaced by a coating of
tinfoil, and it was found that better results were obtained by coating
the outside of the bottle as well as the inside.
As now used the Leyden jar consists of a glass jar covered inside and
outside with tinfoil up to about two-thirds of its height. A wooden
lid is fitted, through which passes a brass rod terminating above in a
brass knob, and below in a piece of brass chain long enough to touch
the foil lining. A Leyden jar is charged by holding it in one hand with
its knob presented to the discharging ball of a Wimshurst machine,
and even if the machine is small and feeble the jar will accumulate
electricity until it is very highly charged. It may now be put down on
the table, and if it is clean and quite dry it will hold its charge for
some time. If the outer and inner coatings of the jar are connected by
means of a piece of metal, the electricity will be discharged in the
form of a bright spark. A Leyden jar is usually discharged by means
of discharging tongs, consisting of a jointed brass rod with brass
terminal knobs and glass handles. One knob is placed in contact with
the outer coating of foil, and the other brought near to the knob of
the jar, which of course is connected with the inner coating.
The electrical capacity of even a small Leyden jar is surprisingly
great, and this is due to the mutual attraction between opposite
kinds of electricity. If we stick a piece of tinfoil on the centre of
each face of a pane of glass, and charge one positively and the other
negatively, the two charges attract each other through the glass;
and in fact they hold on to each other so strongly that we can get
very little electricity by touching either piece of foil. This mutual
attraction enables us to charge the two pieces of foil much more
strongly than if they were each on a separate pane, and this is the
secret of the working of the Leyden jar. If the knob of the jar is
held to the positive ball of a Wimshurst, the inside coating receives
a positive charge, which acts inductively on the outside coating,
attracting a negative charge to the inner face of the latter, and
repelling a positive charge to its outer face, and thence away through
the hand. The electricity is entirely confined to the sides of the jar,
the interior having no charge whatever.
Leyden jars are very often fitted to a Wimshurst machine as shown
at A, A, Fig. 4, and arranged so that they can be connected or
disconnected to the collecting combs as desired. When the jars are
disconnected the machine gives a rapid succession of thin sparks, but
when the jars are connected to the combs they accumulate a number of
charges before the discharge takes place, with the result that the
sparks are thicker, but occur at less frequent intervals.
It will have been noticed that the rod of a Leyden jar and the
discharging rods of a Wimshurst machine are made to terminate not in
points, but in rounded knobs or balls. The reason of this is that
electricity rapidly leaks away from points. If we electrify a conductor
shaped like a cone with a sharp point, the density of the electricity
is greatest at that point, and when it becomes sufficiently great the
particles of air near the point become electrified and repelled. Other
particles take their place, and are electrified and repelled in the
same way, and so a constant loss of electricity takes place. This may
be shown in an interesting way by fastening with wax a needle to the
knob of a Wimshurst. If a lighted taper is held to the point of the
needle while the machine is in action, the flame is blown aside by the
streams of repelled air, which form a sort of electric wind.