The water stored in a reservoir is conveyed by a pipe to a mill, where it may be converted into mechanical energy. The quantity of water passing depends upon the rate of flow and the time during which it flows. The gallon is a unit of liquid measurement, and the gallon-per-second could be applied to express the quantity flowing. The electrical unit of quantity is called the cou-lumb, and is the amount of electricity of a strength of one ampere passing through a conductor in one second. At one ampere flowing for 15 seconds, or 15 amperes for one second, the quantity of electricity delivered will be 15 coulumbs. An ampere-hour therefore equals 3,600 coulumbs. The capacity of storage batteries is frequently expressed in coulumbs.

If the water in a reservoir is delivered under sufficient pressure to turn a turbine, the energy stored in the water is converted into power depending upon the quantity of the water and the height through which it falls. Similarly, an electric current flowing through a circuit conveys electric energy from a battery or dynamo to some point in the circuit, where it is converted into work by means of a motor, battery furnace or lamp. The motor transforms it into mechanical work; a battery utilizes it for electro-plating or chemical work; the furnace and lamp produce heat and light.

Mechanical energy is expressed in foot-pounds, one foot-pound being the amount of work performed in raising a mass weighing one pound through a distance of one foot. The unit of electrical energy is the joule, which is the work done in sending one coulumb through a difference of pressure or potential of one volt, and equals .7372 foot-pounds approximately. If three cou-lumbs flow through a circuit at a distance of potential of one volt, the expenditure of energy would be three joules.

Power is the rate at which work is done, and the reader must carefully distinguish between the amount of work and the rate or time of doing it.

When electrical work is done at the rate of one joule per second, the power exerted is called a watt, which equals .7372 foot-pounds per second, or 44.23 foot-pounds per minute, approximately. Mechanical energy being expressed in horse-power, which is equal to 33,000 foot-pounds per minute, or 550 foot-pounds per second, one watt equals 1/746 horse-power, and 1,000 watts, or one kilowatt, equals 1.34 horse-power; or roughly, 11/3sepower. If the meanings of these terms are remembered, the estimating of the power of a dynamo or battery will not be difficult. To find the number of watts of power a dynamo supplies, multiply the number of amperes of current by the number of volts at which the current is driven.

Previous mention has been made of the necessity of having a complete or closed electric circuit in order to maintain a steady electric current, such circuits consisting of two essentially distinct parts. In one part, as illustrated by the electric light mains along the streets, the potential constantly decreases as the energy is converted into heat, light, or mechanical power; in the other, illustrated by the dynamo, the electricity is raised from a low to a high potential, to maintain the power available in the circuit. The electrical transmission of energy is somewhat analogous to an endless driving belt running on two pulleys. The driving pulley corresponding to the dynamo gives energy to the belt, corresponding to the wire circuit which conveys it to the driven pulley, corresponding to the electric light or motor. The backward pull of the driven pulley also illustrates the counter electromotive force (C. E. M. F.) of a running motor, the rotation of the armature generating an opposing current to that which excites it.

When by means of an electrical machine such as a dynamo, one form of energy is transformed to another form, there is always some loss. Besides the losses due to mechanical causes, such as friction, there is an electrical loss due to resistance, heat being produced even when this is not the form of electrical energy wanted. The useful energy is consequently less than the amount of energy used up to produce it. The value of an electrical machine for converting one form of energy into another depends, therefore, upon the proportion of useful energy produced to the amount used up in the process, this being known as the "efficiency."

We are all more or less familiar, at least in a general way, with those forms of electrical machines known as the dynamo and motor. The former converts mechanical energy into electrical energy, and the latter converts electrical energy into mechanical energy. Both are forms of electro-magnetic machines; that is, a magnetic held and an electric current are characteristics of both. As they exist to-day, they may be considered as especially efficient machines for the conversion of energy.

Fig. 25.

The causes which operate to produce an electric current from a dynamo will first be considered. The power of attraction and repulsion of some forms of electro-magnets has already been considered. The space surrounding such magnets in which this influence is active is known as the magnetic field, which is made up of magnetic lines of force. These lines of force emanate from the positive or north pole, towards the negative or south pole, as illustrated in Fig. 25, which shows the magnet field of a dynamo. These lines of force never cross each other, but tend to follow the shortest path from the positive to the negative pole, each line repelling every other line, and tending to get as far away as possible.

The operation of a dynamo depends upon the principle that an electric conductor, if moved in a magnetic field so that it cuts a constantly varying number of lines of force, will induce an electric current, provided there is a closed circuit through which it may flow. This current is proportional to the rate at which these lines of force are cut, which in turn depends upon the density of the magnetic field, the area of the conductor and the speed with which the conductor moves. In Fig. 25 this is illustrated in a general way. The space between the north and south poles of an electro-magnet forms the magnetic field filled with the lines of force. The armature is represented by a single coil of wire in four positions, which, as it revolves, successively cuts the lines of force, thus inducing a current. The direction and strength of the current constantly changes as the coil revolves, and this we will now consider. Assuming that the movement begins with the coil in the position 1-2, it will be seen that to reach the position 3-4 it cuts an increasing number of lines, and then a decreasing number in reaching the position 2-1. During this movement the current increases to its maximum and then decreases to nothing. Continuing the movement until the coil again reaches the position 1-2, the current increases and diminishes as before, but is in the reverse direction. To obviate this fluctuation in the strength and direction of the current the armature of a dynamo is made up of many coils of wire, the terminals of which are so arranged that a continuous current in one direction is secured. How this is done will be considered in the next chapter.