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.
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.
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.