When _l_ reaches the position _u_, a part of its now strong positive charge pa.s.ses to _CD_, thus increasing the positive charge upon this inductor.

In the position _v_ the remainder of the positive charge on _l_ pa.s.ses 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 k.n.o.bs _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 gla.s.s 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 s.p.a.ced--radially if on a plate, and circ.u.mferentially 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.

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 princ.i.p.al 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]

[Ill.u.s.tration: FIG. 34.--The Wimshurst Electric Machine.]

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 s.p.a.ce 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 ma.s.s 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 acc.u.mulated 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, depending 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.

[Ill.u.s.tration: FIG. 35.--a.n.a.logy 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 quant.i.ty that pa.s.ses 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.

=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 quant.i.ty 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 32 Fahr., 14.4521 grams in ma.s.s, 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 amperes; E = electromotive force in volts; R = resistance in ohms.

From (1) is derived the following:

E = IR (2)

R = E/I (3)

From (1) it is seen that the flow of the current is proportional to the voltage and inversely proportional to the resistance; the latter depends upon the material, length and diameter of the conductor.

Since the current will always flow along the path of least resistance; it must be so guarded that there will be no leakage. Hence, to prevent leakage, wires are _insulated_, that is, covered by wrapping them with cotton or silk thread or other insulating material. If the insulation be not effective, the current may leak, and so return to the source without doing its work. This is known as a _short circuit_.

The conductor which receives the current from the source is called the _lead_, and the one by which it flows back, the _return_.

When wires are used for both lead and return, it is called a _metallic circuit_: when the ground is used for the return, it is called a _ground circuit_. An electric current is said to be:

1. _Direct_, when it is of unvarying direction; 2. _Alternating_, when it flows rapidly to and fro in opposite directions; 3. _Primary_, when it comes directly from the source; 4. _Secondary_, when the voltage and amperage of a primary current have been changed by an _induction coil_; 5. _Low tension_, when its voltage is low; 6. _High tension_, when its voltage is high.

A high tension current is capable of forcing its way against considerable resistance, whereas, a low tension current must have its path made easy.

=Production of the Electric Current.=--To produce a steady flow of water in a pipe two conditions are necessary. There must first be available a hydraulic pressure, or, as it is technically called, a "_head_" of water produced by a pump, or a difference of level or otherwise.

In addition to the pressure there must also be a suitable path or channel provided for the water to flow through, or there will be no flow, however great the "head," until something breaks down under the strain. In the case just cited, although there is full pressure in the water in the pipe, there is no current of water as long as the tap remains closed. The opening of the tap completes the necessary _path_ (the greater part of which was already in existence) and the water flows.

[Ill.u.s.tration: FIG. 36.--Hydraulic a.n.a.logy of the electric current. If, say 10 gallons of water flow in every second into a system of vessels and pipes of any shape, whether simple or more complicated as shown in the figure, and 10 gallons flow out again per second, it is evident that through every cross section of any vessel or pipe of the system 10 gallons of water pa.s.s every second. This follows from the fact that water is an uncompressible liquid and must be practically of the same density throughout the system. The water moves slowly where the section is large and quickly where it is small, and thus the quant.i.ty of water that flows through any part of the system is independent of the cross section of that part. The same condition holds good for the electric current; if in a closed circuit a constant current circulates, the same amount of electricity will pa.s.s every cross section per second. Hence the following law: _The magnitude of a constant current in any circuit is equal in all parts of the circuit._]

For the production of a steady electric current two very similar conditions are necessary. There must be a steadily maintained electric pressure, known under different aspects as "electromotive force,"

"potential difference," or "voltage." This alone, however, is not sufficient. In addition, a suitable conducting path is necessary. Any break in this path occupied by unsuitable material acts like the closed tap in the a.n.a.logous case above mentioned, and it is only when all such breaks have been properly bridged by suitable material, that is, by conductors, that the effects which denote the flow of the current will begin to be manifested.

The necessary electromotive force or voltage required to cause the current to flow may be obtained:

1. Chemically; 2. Mechanically; 3. Thermally.

In the first method, two dissimilar metals such as copper and zinc called _elements_, are immersed in an exciting fluid or _electrolyte_.

[Ill.u.s.tration: FIG. 37.--Volta's "Crown of Cups." The metallic elements C and Z each consisted of two metals, the plate C being of copper and the plate Z of zinc. They were placed, as shown, in the gla.s.s vessels, which contained salt water and ordinary water or lye. Into each vessel, except the two end ones, the copper end of one arc and the zinc end of the next were introduced, the series, however long, ending with copper dipping into the terminal vessel at one end and zinc into that at the other. The arrangement is almost exactly that of a modern one-fluid primary battery.]