Figs. 30 and 31.--The Leyden jar and discharger. Its discovery is attributed to the attempt of Musschenbrock and his pupil Cuneus to collect the supposed electric "fluid" in a bottle half filled with water. The bottle was held in the hand and was provided with a nail to lead the "fluid" down through the cork to the water from the electric machine. The invention of the Leyden jar is also claimed by Kleist, Bishop of Pomerania.
Condensers; Leyden Jar.--A condenser is an apparatus for condensing a large quantity of electricity on a comparatively small surface. The form may vary considerably, but in all cases it consists essentially of two insulated conductors, separated by an insulator and the working depends on the action of induction.
A form of condenser generally used in making experiments on static electricity is the Leyden jar, so named from the town23 of Ley-den where it was invented. It consists of a glass jar coated inside and out to a certain height with tinfoil, having a brass rod terminat-ing in a knob passed through a wooden stopper, and connected to the inner coat by a loose chain, as shown in fig. 30.
The jar may be charged by repeatedly touching the knob with the charged plate of the electrophorus or by connecting the inner coating to one knob of an electrical machine and the outer coating to the other knob.
The discharge of a condenser is effected by connecting the plates having an opposite charge. This may be done by use of a wire or a
discharger, as shown in fig. 31; the connection is made between the outer coat and the knob.
When the knob of the discharger is sufficiently close to the knob of the jar, a bright spark will be observed between the knobs. This discharge occurs whenever the difference of potential between the coats is great enough to overcome the resistance of the air between the knobs.
Let a charged jar be placed on a glass plate so as to insulate the outer coat. Let the knob be touched with the finger. No appreciable discharge will be noticed. Let the outer coat be in turn touched with the finger. Again no appreciable discharge will appear. But if the inner and outer coatings be connected with the discharger, a powerful spark will pass.
Electric Machines.--Various machines have been devised for producing electric charges such as have been described. The ordinary
"static" or electric machine, is nothing but a continuously acting electrophorus.
Fig. 32 represents the so-called Toepler-Holtz machine. Upon the back of the stationary plate E, are pasted paper sectors, beneath which are strips of tinfoil AB and CD called inductors.
In front of E is a revolving glass plate carrying discs l, m, n, o, p and q, called carriers.24
To the inductors AB and CD are fastened metal arms t and u, which bring B and C into electrical contact with the discs l, m, n, o, p and q, when these discs pass beneath the tinsel brushes carried by t and u.
A stationary metallic rod rs carries at its ends stationary brushes as well as sharp pointed metallic combs.
The two knobs R and S have their capacity increased by the Leyden jars L and L''.
Fig. 32.--The Toepler-Holtz electric machine.
Fig. 33.--Principle of Toepler-Holtz electric machine.
Action of the Toepler-Holtz Machine.--The action of the machine described above is best understood from the diagram of fig. 33. Suppose that a small + charge is originally placed on the inductor CD. Induction takes place in the metallic system consisting of the discs l and o and the rod rs, l becoming negatively charged and o positively charged. As the plate carrying l, m, n, o, p, q rotates in the direction of the arrow the negative charge on l is carried over to the position m, where a part of it passes over to the inductor AB, thus charging it negatively.
When l reaches the position n the remainder of its charge, being repelled by the negative electricity which is now on AB, passes over
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into the Leyden jar L.25
When l reaches the position o it again becomes charged by induction, this time positively, and more strongly than at first, since now
the negative charge on AB, as well as the positive charge on CD, is acting inductively upon the rod rs.
When l reaches the position u, a part of its now strong positive charge passes to CD, thus increasing the positive charge upon this inductor.
In the position v the remainder of the positive charge on l passes over to L''. This completes the cycle for l. Thus as the rotation continues AB and CD acquire stronger and stronger charges, the inductive action upon rs becomes more and more intense, and positive and negative charges are continuously imparted to L'' and L until a discharge takes place between the knobs R and S.
There is usually sufficient charge on one of the inductors to start the machine, but in damp weather it will often be found necessary
to apply a charge to one of the inductors by means of the ebonite or glass rod before the machine will work.
The Wimshurst Machine.--The essential parts of an ordinary Wimshurst machine, as shown in fig. 34, are two insulating plates or drums. On each plate are fixed a large number of strips of conducting material, which are equal in size and are equally spaced--radi-ally if on a plate, and circumferentially if on a drum. The plates, or drums, are made to rotate in opposite directions. The capacity of the inductors therefore varies from a maximum when each strip on one plate is facing a strip on the other, to a minimum when the conducting strips on each plate are facing blank or insulating portions of the other plate.26
There are three pairs of contact brushes, the members of two of the pairs being at opposite ends of diametrical conducting rods placed at right angles to one another; the third pair are insulated from one another and form the principal collectors, the one giving positive and the other negative electricity.
The plates are revolving in opposite directions; thus if there be a charge on one of the conducting segments of one plate and an opposite charge on one of the conducting segments on the other plate near it, their potential will be raised as the rotation of the plates separates them.[2]
Fig. 34.--The Wimshurst Electric Machine.
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CHAPTER III
THE ELECTRIC CURRENT
The ordinary statement that an electric current is flowing along a wire is only a conventional way of expressing the fact that the wire and the space around the wire are in a different state from that in which they are when no electric current is said to be flowing.
In order to make laymen understand the action of this so called current, it is generally compared with the flow of water.
In comparing hydraulics and electricity, it must be borne in mind, however, that there is really no such thing as an "electric fluid," and that water in pipes has mass and weight, while electricity has none. It should be noted, however, that electricity is conveniently spoken of as having weight in explaining some of the ways in which it manifests itself.
All electrical machines and batteries are merely instruments for moving electricity from one place to another, or for causing electricity, when accumulated in one place, to do work in returning to its former level of distribution.
The head or pressure in a standpipe is what causes water to move through the pipes which offer resistance to the flow.
Similarly, the conductors, along which the electric current is said to flow, offer more or less resistance to the flow, depending28 on
the material. Copper wire is generally used as it offers little resistance.
The current must have pressure to overcome the resistance of the conductor and flow along its surface. This pressure is called voltage caused by what is known as difference of potential between the source and terminal.
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Fig. 35.--Analogy of the flow of water to the electric current. The water in