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 amperes; E = electromotive force in volts; R = resistance in ohms.
From (1) is derived the following:
E = IR (2)
R = E/I (3)
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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.
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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 is31 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.
Fig. 36.--Hydraulic analogy 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 pass 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 quantity 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 pass 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 analogous case above mentioned, and it is only when all such breaks have32 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.
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 glass 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.
When the elements are connected at their terminals by a wire or conductor a chemical action takes place, producing a current which
flows from the copper to the zinc. This device is called a cell, and the combination of two or more of them connected so as to form
a unit is known as a battery. The word battery is frequently used incorrectly for a single cell. That terminal of the element from
which the current flows is called the plus or positive pole, and the terminal of the other element the negative pole.
Cells are said to be primary or secondary according as they generate a current of themselves, or first require to be charged from an
external33 source, storing up a current supply which is afterwards yielded in the reverse direction to that of the charging current.
An electric current is generated mechanically by a dynamo. In either case no electricity is produced, but part of the supply already existing is simply set in motion by creating an electric pressure.
An electric current, according to the third method, is generated directly from heat energy, as will be later explained; the current thus obtained is very feeble.
Fig. 38.--Hydrostatic analogy of fall of potential in an electrical circuit.
Fig. 39.--Showing method of connecting voltmeter to find potential difference between any two points as m and n on an electrical
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