THE ELECTRICAL CIRCUIT The electrical circuit can best be studied by making a comparison between it and a hydraulic problem where the various quantities bear prac tically the same relation to one another. When there is a flow of water through a pipe from a pump or some source of pressure, the stream of water flowing depends upon the pressure and upon the resistance that the pipe offers in the way of friction—it is, in fact, proportional to the pressure divided by the frictional resistance.
297 The same condition exists in an electrical cir cuit, where a current of electricity flows in a wire due to the pressure of a battery or other source of electrical energy; and the value of the current is equal to the pressure acting, divided by the opposition or resistance offered by the circuit to such flow.
When the electrical pressure acting tends to send a current through the circuit always in the same direction, it results in what is termed a direct current; if, however, it is continuously reversing in direction—first in one direction and then in the opposite direction—the current flow ing then will be what is termed an alternating current. The following discussion applies only to direct current, which is the form of current commonly used in houses for lighting, heating, etc.
In the hydraulic problem, the resistance offered to the flow of water, neglecting that due to bends, depends upon the length, the area, and the condition of the inner surface of the pipe. If the length of the pipe is increased, resistance is increased; if the area is increased, resistance is decreased; if the inner surface of the pipe is smooth, it will offer less resistance than it would if rough. There is a somewhat similar condition in the electrical circuit. If the area of a con ductor carrying current remains constant, re sistance will increase directly as the length of the conductor increases; if area increases, re sistance will decrease at the same rate.
Resistance will also depend upon the kind of material that forms the circuit, which corre sponds to a certain extent to condition of sur face of pipe. The resistance, in the case of pipe, is only on the outer surface of the stream in contact with the pipe; while, in the case of the electrical circuit, it is uniform throughout the conductor. The resistance of a circular conduc tor .001 of an inch in diameter (one mil) and one
foot long, is termed the mil-foot resistance of the material. If this value is known, which is 9.52 ohms for copper at a temperature of Centi grade (the freezing point of water), the value of the resistance of a copper conductor of any given dimensions can be calculated by the use of the following equation: Length in feet R = 9.52x Area in circular mils Area in circular mils equals diameter in mils squared. This means that the resistance is found by multiplying the length of the conduc tor by 9.52, and dividing the product by the area of the conductor in circular mils.
Table I gives the diameters, weights, and re sistance of copper wires drawn according to the Brown & Sharpe gauge.
The conductivity of a conductor, or its power of receiving and transmitting current, bears an inverse ratio to its resistance. Copper, on ac count of its commercial application as an elec trical conductor, is taken as having a 100 per cent conductivity; and all other materials compared with it as a standard.
Electrical Units and Definitions It might be well at this point to give the names of the various electrical quantities and their relations.
The unit in which we measure electrical pres sure is the volt, the ordinary dry cell giving ap proximately 1.5 volts.
The unit in which current is measured is the ampere, and the unit of resistance is called the ohm.
The power used in causing a current to flow in a circuit is measured in a unit called the watt; and the number of watts expended in a circuit is equal to electrical pressure at the terminals, multiplied by the current flowing; that is: Power --, Volts X Amperes. A thousand watts is called a kilowatt. There are 746 watts to the horse-power. A watt acting for one hour is called a watt-hour, and is a unit of electrical en ergy. A kilowatt-hour is a thousand watts for one hour, or any number of watts which, multi plied by the time they act in hours, gives 1,000. The kilowatt-hour is the unit used in selling electrical energy. The wattmeter is so con structed that its reading is proportional to the voltage multiplied by the current multiplied by the time in hours multiplied by some quantity (known as constant of wattmeter), which is often unity.