This chapter provides a concise overview of the nature of electricity, electrical devices, and the basics of x-ray circuitry and principles of operation. It is true that many types of x-ray equipment are automated (Figure 4-1). However, a radiographer is not someone who merely “pushes buttons.” Rather, he or she has an understanding of the principles of x-ray production and has mastered the art of producing quality images with minimal radiation exposure to the patient. To reach this level of mastery, the radiographer must understand the basic elements of the x-ray machine and the steps in the process. Consider a pilot who flies a modern jet. A pilot untrained for that aircraft may be able to get it off the ground and flying, but without some understanding of the jet’s instrumentation he or she is not likely to stay in the air very long. The safety of the passengers aboard that aircraft rests with the training and knowledge of that pilot. Similarly, the radiographer is responsible for the safety of the patient; the radiation dose that patient receives depends on the radiographer’s understanding and safe operation of the x-ray machine. The concepts presented here are important to the radiographer in that they ground his or her practice in a fundamental understanding of what is happening each time he or she operates the x-ray machine. By understanding what happens within the x-ray machine with each selection made at the operating console, the radiographer is able to use the machine with maximum efficiency and minimal radiation exposure to the patient. The knowledgeable radiographer is also able to make adjustments in exposure technique with variations in machines and daily operation.

FIGURE 4-1 **X-ray Machine Control Panel.**
Touch-screen control panel of a typical radiographic unit.

Nature of Electricity

The nature of electricity may be understood through a discussion of electrostatics and electrodynamics. Electrostatics is the study of stationary electric charges and electrodynamics is the study of electric charges in motion. The latter is most often considered as “electricity.” A few fundamental concepts must first be discussed.

Electric charge is a property of matter. The smallest units of charge exist with the electron and the proton. Electrons have one unit of negative charge and protons have one unit of positive charge. Electrical charges are measured in the systeme internationale (SI) unit “coulomb.” One coulomb is equal to the electrical charge of 6.25 × 10¹⁸ electrons. A measure of electrons is used because electricity most often results from their movement. Except in decaying radioactive elements, protons are generally fixed in their position inside the nucleus of the atom. Electrons, on the other hand, are relatively free to move about, depending on the material. Some materials, such as copper and gold, have a very large number of free electrons, making them good conductors of electricity. Glass and plastic, on the other hand, have very few free electrons, making them good insulators. This is discussed in greater detail later in this chapter.

Although an understanding of the laws of electrostatics is not the primary focus of this chapter, it is helpful in understanding the nature of electricity. There are five general principles of electrostatics. They are as follows:

• Like charges repel and unlike charges attract.

• The electrostatic force between two charges is directly proportional to the product of their quantities and inversely proportional to the square of the distance between them (also known as Coulomb’s law).

• Electric charges reside only on the external surface of conductors.

• The concentration of charges on a curved surface of a conductor is greatest where the curvature is greatest.

• Only negative charges (electrons) are free to move in solid conductors.

In electrostatics, electrification of objects occurs when they gain either a net positive or a net negative charge. An object may be electrified in three ways: by friction, by contact, or by induction. The classic physics experiment involving rubbing a rubber rod with fur is an example of electrification by friction. Once charged, the rod can be discharged by placing it in contact with a conductor. This is an example of electrification by contact. Electrification by induction is the process by which an uncharged metallic object experiences a shift of electrons when brought into the electric field of a charged object. Induction occurs as a result of the interaction of the electric fields around two objects that are not in contact with each other. This is very useful in the design of the x-ray tube, as is discussed in Chapter 5.

Electrodynamics describes electrical charges in motion. This movement is associated with “electricity,” and it is the intended meaning for all further discussions of electricity in this text. For electric current to move, an electric potential must exist. Electric potential is the ability to do work because of a separation of charges. If one has an abundance of electrons at one end of a wire and an abundance of positive charges at the other end (separation of charges), electrons will flow from abundance to deficiency.

Theory to Practice 4-1

By design, the x-ray tube creates a separation of charges and the exposure factors the radiographer selects on the control panel determine the number of electrons that will flow and the magnitude of their attraction to the positive side

Critical Concept 4-1

Nature of Electricity

The smallest units of charge rest with the proton and the electron. However, only electrons are free to move in solid conductors. Therefore “electricity” is most often associated with the flow of electrons.

Electric Potential, Current, and Resistance

Electric potential, current, and resistance are expressions of different phenomena surrounding electricity. Electric potential is the ability to do work because of a separation of charges. Current is an expression of the flow of electrons in a conductor. Finally, resistance is that property of an element in a circuit that resists or impedes the flow of electricity. It should be noted that there is nothing magical about the production of x-rays; it is simply the manipulation of electricity. Of course significant engineering and technological knowledge is required to design and manufacture the equipment, but, when viewed at its most basic level, x-ray production is simply the manipulation of electricity. In fact, the units of measure for electric potential (the volt) and current (the ampere) are the factors selected on the operating console of the x-ray machine to produce x-rays. They are expressed in thousands and thousandths, respectively, but they are electrical terms and are not exclusive to radiology.

Critical Concept 4-2

Expressions of Electrical Phenomena

Electric potential, current, and resistance are expressions of different phenomena surrounding electricity. Electric potential is the ability to do work because of a separation of charges. Current is an expression of the flow of electrons in a conductor. Resistance is that property of an element in a circuit that resists or impedes the flow of electricity.

Electric potential is measured in volts, named for the Italian physicist Volta who invented the battery. A volt may be defined as “the potential difference that will maintain a current of 1 ampere in a circuit with a resistance of 1 ohm” (amperes and ohms are discussed next). It is the expression of the difference in electric potential between two points. The volt is also equal to the amount of work in joules that can be done per unit of charge. (Refer to Chapter 1 for a review of the definition and calculation of work.) A volt is the ratio of joules to coulombs (volt = joules/coulombs). For example, a battery that uses 6 joules of energy to move 1 coulomb of charge is a 6-volt battery.

Again, one of the exposure factors selected on the control panel of the x-ray machine is kilovoltage peak (kVp). The role of kVp within the machine and in image production is discussed later in this text. For now, note that the radiographer is literally selecting the thousands

Make the Imaging Connection 4-1

Chapters 9 and 10

Kilovoltage peak influences many areas of imaging. Among other things, it determines how the beam penetrates the body part, controls contrast in the film image, and influences contrast in the digital image.

of volts that will be applied to the x-ray tube to produce x-rays. An understanding of this unit of measure and the concepts presented here are vital to the competent and safe operation of the x-ray machine.

Current is measured in amperes, named for André-Marie Ampere, a French physicist who made significant contributions to the study of electrodynamics. The ampere may be defined as “1 coulomb flowing by a given point in 1 second.” Reflecting its relationship to the definition of volt (discussed previously), it may also be defined as “the amount of current flowing with an electric potential of 1 volt in a circuit with a resistance of 1 ohm.” For electric current to flow, there must be a potential difference between two electrodes and a suitable medium through which it can travel. With regard to potential difference, electrons flow from abundance to deficiency and will continue to do so as long as that difference exists. Electricity behaves differently depending on the medium through which it travels. Suitable media are conductors and those resisting electric current flow are insulators. Both types of media are important to the production of x-rays. Two in particular, vacuums and metallic conductors, are of particular usefulness in x-ray production. In a vacuum tube, electrons tend to jump the gap between oppositely charged electrodes. This is part of the environment that exists inside an x-ray tube. With metallic conductors, electrons from the conductor’s atoms will move out of the valence shell to a higher energy level just beyond, called the conduction band, where they are free to drift along the external surface of the conductor (refer to Chapter 2 for a discussion of atomic structure). Copper is particularly useful as a conductor and is commonly used as such in electronic devices. Other metals with this characteristic are used extensively in x-ray machine and x-ray tube design.

The two types of current, direct current (DC) and alternating current (AC), are also important to x-ray production and should be understood before moving on. DC is a type of current that flows in only one direction. A battery is a good example: It has a positive and a negative electrode, and, when placed in an electrical circuit, electrons flow from the negative terminal to the positive terminal (current flows in the opposite direction, a topic clarified later). AC is current that changes direction in cycles as the electric potential of the source changes (the negative and positive “terminals,” if you will, alternate). In the United

Critical Concept 4-3