It is easy to understand how electricity behaves and what it does if you get the right idea of it at the start. In the first place, if you will think of electricity as being a fluid like water its fundamental actions will be greatly simplified. Both water and electricity may be at rest or in motion. When at rest, under certain conditions, either one will develop pressure, and this pressure when released will cause them to flow through their respective conductors and thus produce a current.

Electricity at Rest and in Motion.--Any wire or a conductor of any kind can be charged with electricity, but a Leyden jar, or other condenser, is generally used to hold an electric charge because it has a much larger capacitance, as its capacity is called, than a wire. As a simple analogue of a condenser, suppose you have a tank of water raised above a second tank and that these are connected together by means of a pipe with a valve in it, as shown at A in Fig. 28.Fig. 28.--Water Analogue for Electric Pressure.

Now if you fill the upper tank with water and the valve is turned off, no water can flow into the lower tank but there is a difference of pressure between them, and the moment you turn the valve on a current of water will flow through the pipe. In very much the same way when you have a condenser charged with electricity the latter will be under pressure, that is, a difference of potential will be set up, for one of the sheets of metal will be charged positively and the other one, which is insulated from it, will be charged negatively, as shown at B. On closing the switch the opposite charges rush together and form a current which flows to and fro between the metal plates. [Footnote: Strictly speaking it is the difference of potential that sets up the electromotive force.]

The Electric Current and Its Circuit.--Just as water flowing through a pipe has quantity and pressure back of it and the pipe offers friction to it which tends to hold back the water, so, likewise, does electricity flowing in a circuit have: (1) quantity, or current strength, or just current, as it is called for short, or amperage, and (2) pressure, or potential difference, or electromotive force, or voltage, as it is variously called, and the wire, or circuit, in which the current is flowing has (3) resistance which tends to hold back the current.

A definite relation exists between the current and its electromotive force and also between the current, electromotive force and the resistance of the circuit; and if you will get this relationship clearly in your mind you will have a very good insight into how direct and alternating currents act. To keep a quantity of water flowing in a loop of pipe, which we will call the circuit, pressure must be applied to it and this may be done by a rotary pump as shown at A in Fig. 29; in the same way, to keep a quantity of electricity flowing in a loop of wire, or circuit, a battery, or other means for generating electric pressure must be used, as shown at B.Fig. 29.--Water Analogues for Direct and Alternating Currents.

If you have a closed pipe connected with a piston pump, as at C, as the piston moves to and fro the water in the pipe will move first one way and then the other. So also when an alternating current generator is connected to a wire circuit, as at D, the current will flow first in one direction and then in the other, and this is what is called an alternating current.

Current and the Ampere.--The amount of water flowing in a closed pipe is the same at all parts of it and this is also true of an electric current, in that there is exactly the same quantity of electricity at one point of the circuit as there is at any other.

The amount of electricity, or current, flowing in a circuit in a second is measured by a unit called the ampere, [Footnote: For definition of ampere see Appendix.] and it is expressed by the symbol I. [Footnote: This is because the letter C is used for the symbol of capacitance] Just to give you an idea of the quantity of current an ampere is we will say that a dry cell when fresh gives a current of about 20 amperes. To measure the current in amperes an instrument called an ammeter is used, as shown at A in Fig. 30, and this is always connected in series with the line, as shown at B.Fig. 30.--How the Ammeter and Voltmeter are Used.

Electromotive Force and the Volt.--When you have a pipe filled with water or a circuit charged with electricity and you want to make them flow you must use a pump in the first case and a battery or a dynamo in the second case. It is the battery or dynamo that sets up the electric pressure as the circuit itself is always charged with electricity.

The more cells you connect together in series the greater will be the electric pressure developed and the more current it will move along just as the amount of water flowing in a pipe can be increased by increasing the pressure of the pump. The unit of electromotive force is the volt, and this is the electric pressure which will force a current of 1 ampere through a resistance of 1 ohm; it is expressed by the symbol E. A fresh dry cell will deliver a current of about 1.5 volts. To measure the pressure of a current in volts an instrument called a voltmeter is used, as shown at C in Fig. 30, and this is always connected across the circuit, as shown at D.

Resistance and the Ohm.--Just as a water pipe offers a certain amount of resistance to the flow of water through it, so a circuit opposes the flow of electricity in it and this is called resistance. Further, in the same way that a small pipe will not allow a large amount of water to flow through it, so, too, a thin wire limits the flow of the current in it.

If you connect a resistance coil in a circuit it acts in the same way as partly closing the valve in a pipe, as shown at A and B in Fig. 31. The resistance of a circuit is measured by a unit called the ohm, and it is expressed by the symbol R. A No. 10, Brown and Sharpe gauge soft copper wire, 1,000 feet long, has a resistance of about 1 ohm. To measure the resistance of a circuit an apparatus called a resistance bridge is used. The resistance of a circuit can, however, be easily calculated, as the following shows.Fig. 31.--Water Valve Analogue of Electric Resistance. A- a valve limits the flow of water. B- a resistance limits the flow of current.

What Ohm's Law Is.--If, now, (1) you know what the current flowing in a circuit is in amperes, and the electromotive force, or pressure, is in volts, you can then easily find what the resistance is in ohms of the circuit in which the current is flowing by this formula:

That is, if you divide the current in amperes by the electromotive force in volts the quotient will give you the resistance in ohms.

Or (2) if you know what the electromotive force of the current is in volts and the resistance of the circuit is in ohms then you can find what the current flowing in the circuit is in amperes, thus:

That is, by dividing the resistance of the circuit in ohms, by the electromotive force of the current you will get the amperes flowing in the circuit.

Finally (3) if you know what the resistance of the circuit is in ohms and the current is in amperes then you can find what the electromotive force is in volts since:

That is, if you multiply the resistance of the circuit in ohms by the current in amperes the result will give you the electromotive force in volts.

From this you will see that if you know the value of any two of the constants you can find the value of the unknown constant by a simple arithmetical process. This relation between these three constants is known as Ohm's Law and as they are very important you should memorize them.

What the Watt and Kilowatt Are.--Just as horsepower or H.P., is the unit of work that steam has done or can do, so the watt is the unit of work that an electric current has done or can do. To find the watts a current develops you need only to multiply the amperes by the volts. There are 746 watts to 1 horsepower, and 1,000 watts are equal to 1 kilowatt.

Electromagnetic Induction.--To show that a current of electricity sets up a magnetic field around it you have only to hold a compass over a wire whose ends are connected with a battery when the needle will swing at right angles to the length of the wire. By winding an insulated wire into a coil and connecting the ends of the latter with a battery you will find, if you test it with a compass, that the coil is magnetic.

This is due to the fact that the energy of an electric current flowing in the wire is partly changed into magnetic lines of force which rotate at right angles about it as shown at A in Fig. 32. The magnetic field produced by the current flowing in the coil is precisely the same as that set up by a permanent steel magnet. Conversely, when a magnetic line of force is set up a part of its energy goes to make up electric currents which whirl about in a like manner, as shown at B.(A) and (B) Fig. 32.--How an Electric Current is Changed into Magnetic Lines of Force and These into an Electric Current. (C) and (D) Fig. 32.--How an Electric Current Sets up a Magnetic Field.

Self-induction or Inductance.--When a current is made to flow in a coil of wire the magnetic lines of force produced are concentrated, as at C, just as a lens concentrates rays of light, and this forms an intense magnetic field, as it is called. Now if a bar of soft iron is brought close to one end of the coil of wire, or, better still, if it is pushed into the coil, it will be magnetized by electromagnetic induction, see D, and it will remain a magnet until the current is cut off.

Mutual Induction.--When two loops of wire, or better, two coils of wire, are placed close together the electromagnetic induction between them is reactive, that is, when a current is made to flow through one of the coils closed magnetic lines of force are set up and when these cut the other loop or turns of wire of the other coil, they in turn produce electric currents in it.

It is the mutual induction that takes place between two coils of wire which makes it possible to transform low voltage currents from a battery or a 110 volt source of current into high pressure currents, or high potential currents, as they are called, by means of a spark coil or a transformer, as well as to step up and step down the potential of the high frequency currents that are set up in sending and receiving oscillation transformers. Soft iron cores are not used in oscillation inductance coils and oscillation transformers for the reason that the frequency of the current is so high the iron would not have time to magnetize and demagnetize and so would not help along the mutual induction to any appreciable extent.

High-Frequency Currents.--High frequency currents, or electric oscillations as they are called, are currents of electricity that surge to and fro in a circuit a million times, more or less, per second. Currents of such high frequencies will oscillate, that is, surge to and fro, in an open circuit, such as an aerial wire system, as well as in a closed circuit.

Now there is only one method by which currents of high frequency, or radio-frequency, as they are termed, can be set up by spark transmitters, and this is by discharging a charged condenser through a circuit having a small resistance. To charge a condenser a spark coil or a transformer is used and the ends of the secondary coil, which delivers the high potential alternating current, are connected with the condenser. To discharge the condenser automatically a spark, or an arc, or the flow of electrons in a vacuum tube, is employed.

Constants of an Oscillation Circuit.--An oscillation circuit, as pointed out before, is one in which high frequency currents surge or oscillate. Now the number of times a high frequency current will surge forth and back in a circuit depends upon three factors of the latter and these are called the constants of the circuit, namely: (1) its capacitance, (2) its inductance and (3) its resistance.

What Capacitance Is.--The word capacitance means the electrostatic capacity of a condenser or a circuit. The capacitance of a condenser or a circuit is the quantity of electricity which will raise its pressure, or potential, to a given amount. The capacitance of a condenser or a circuit depends on its size and form and the voltage of the current that is charging it.

The capacitance of a condenser or a circuit is directly proportional to the quantity of electricity that will keep the charge at a given potential. The farad, whose symbol isM, is the unit of capacitance and a condenser or a circuit to have a capacitance of one farad must be of such size that one coulomb, which is the unit of electrical quantity, will raise its charge to a potential of one volt. Since the farad is far too large for practical purposes a millionth of a farad, or microfarad, whose symbol is mfd., is used.

What Inductance Is.--Under the sub-caption of Self-induction and Inductance in the beginning of this chapter it was shown that it was the inductance of a coil that makes a current flowing through it produce a strong magnetic field, and here, as one of the constants of an oscillation circuit, it makes a high-frequency current act as though it possessed inertia.

Inertia is that property of a material body that requires time and energy to set in motion, or stop. Inductance is that property of an oscillation circuit that makes an electric current take time to start and time to stop. Because of the inductance, when a current flows through a circuit it causes the electric energy to be absorbed and changes a large part of it into magnetic lines of force. Where high frequency currents surge in a circuit the inductance of it becomes a powerful factor. The practical unit of inductance is the henry and it is represented by the symbol L.

What Resistance Is.--The resistance of a circuit to high-frequency currents is different from that for low voltage direct or alternating currents, as the former do not sink into the conductor to nearly so great an extent; in fact, they stick practically to the surface of it, and hence their flow is opposed to a very much greater extent. The resistance of a circuit to high frequency currents is generally found in the spark gap, arc gap, or the space between the electrodes of a vacuum tube. The unit of resistance is, as stated, the ohm, and its symbol is R.

The Effect of Capacitance, Inductance and Resistance on Electric Oscillations.--If an oscillation circuit in which high frequency currents surge has a large resistance, it will so oppose the flow of the currents that they will be damped out and reach zero gradually, as shown at A in Fig. 33. But if the resistance of the circuit is small, and in wireless circuits it is usually so small as to be negligible, the currents will oscillate, until their energy is damped out by radiation and other losses, as shown at B.Fig. 33.--The Effect of Resistance on the Discharge of an Electric Current.

As the capacitance and the inductance of the circuit, which may be made of any value, that is amount, you wish, determines the time period, that is, the length of time for a current to make one complete oscillation, it must be clear that by varying the values of the condenser and the inductance coil you can make the high frequency current oscillate as fast or as slow as you wish within certain limits. Where the electric oscillations that are set up are very fast, the waves sent out by the aerial will be short, and, conversely, where the oscillations are slow the waves emitted will be long.