style="font-size:15px;"> In front of E is a revolving glass plate carrying discs l, m, n, o, p and q, called carriers.24To 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 the reservoirs A and B stands at different heights. As long as this difference of level is maintained, water from B will flow through the pipe R to A. If by means of a pump P the level
in B be kept constant, flow through R will also be maintained. Here, by means of the work expended on the pump, the level in the reservoir is kept constant; and in the corresponding case of the electric current, by the conversion of chemical energy a constant difference of potential is maintained.
The pressure under which a current flows is measured in volts and the quantity that passes in amperes. The resistance with which the current meets in flowing along a conductor is measured in ohms.
Ques. What is a volt?
Ans. A volt is that electromotive force (E. M. F.) which produces a current of one ampere against a resistance of one ohm.29
Ques. What is an ampere?
Ans. An ampere is the current produced by an E. M. F. of one volt in a circuit having a resistance of one ohm. It is that quantity of electricity which will deposit .005084 grain of copper per second.
Ques. What is an ohm?
Ans. An ohm is equal to the resistance offered to an unvarying electric current by a column of mercury at 32deg Fahr., 14.4521 grams in mass, of a constant cross sectional area, and of the length of 106.3 centimeters.
Ohm's Law.--In a given circuit, the amount of current in amperes is equal to the E. M. F. in volts divided by the resistance in ohms;
that is:
current = pressure / resistance = volts / ohms
expressed as a formula:
I = E/R (1)
in which
I = current strength in
