Military roentgenology / War Department.

Date:: [1944]

Catalogue details

Licence: Public Domain Mark

Credit: Military roentgenology / War Department. Source: Wellcome Collection.

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wave-form of the current. Similar graphical repre- sentations pertain to emf’s. Thus, the wave form of a continuous d-c current is a straight line. (See fig. 12.) The wave forms of alternating and direct- currents are dependent upon mechanical! construc- tion of the dynamo and composite effects of the equipment utilized. The most desirable design of dynamo is one which gives a sine curve type of wave form. (See fig. 14a.) The peak or maximum pomts of the curves represent maximum currents or maximum emf’s. Most current and voltage measuring instruments do not measure maximum values; instead, they measure either effective values or average values. The effective value of an alter- nating-current is equal to the value of direct-current which produces an equal effect, that is, equal heating effect in a wire. The average value of an alternating- current is zero (true sine wave form). If the wave form of a current Is truly a sine wave, then the effective value of the current is equal to 0.707 times the maximum value, and the average value of one- half cycle is equal to 0.64 times the maximum value. The maximum, effective, and average values of a continuous direct-current are equal. b. Knowing one of these values, in the case of a true sine wave, it is possible to compute either of the other two. The maximum value is equal to 1.41 times the effective or 1.57 times the average. The effective is equal to 0.71 times the maximum or 1.11 times the average. [he average is equal to 0.64 times the maximum or 0.90 times the effective. c. Considerable distortion of wave form is likely to prevail in the wave form of circuits having an inductive type of load, such as produced by trans- formers. Because of such loads, in the case of X-ray apparatus, the voltage wave may not coincide either as to time or shape with the amperage wave, in which case there would result distortion of summa- tion (that is, power) wave form. Each X-ray unit might be considered to have its own particular high tension wave form. This explains some of the varia- tions in roentgenographic performances. As much as 30 to 40 percent difference has been found in comparing the X-radiation performance of one unit with that of another, using identical technical factors and correcting for meter variations, etc. 26. ELECTRICAL MEASURING INSTRUMENTS. Instruments which measure electric currents are called ammeters; those measuring voltages are called voltmeters. Each of these is designed to operate with either direct-current or alternating-current. Thus, there are d-c meters and a-c meters. D-c meters measure average values. All a-c meters, unless stated otherwise, measure effective values. The scale may be either linear or nonlinear depending upon the portion utilized. In the case of the nonlinear scale, the deflections are proportional to the square of the current. For this reason, with nonlinear a-c meters, currents producing small deflections cannot be measured as accurately as currents producing ap- proximately full-scale deflections. (See fig. 15.) 27. CHOKE COILS. a. The magnetic field of force which is developed about a conductor carrying an alternating-current, tends to restrain the movement of the electrons. There is actually the paradoxical effect of a force tending to prohibit the very cause of its origin. This force is called a “counter emf.’ The results of a counter emf are essentially that of a resistance. The magnitude of the counter emf depends upon the degree of current flowing and also upon the shape of the conductor and the amount of mag- netizable substance In proximity to it. For instance, a compact arrangement of coils of wire such as found in a solenoid, provides for great concentration of a field of force. If there be inserted into this field of force, a magnetizable substance such as soft iron or silicon steel, the already concentrated field of force will become imtensified—as described in con- nection with electromagnets. If the magnetizable substance (the soft iron core) be made adjustable so that it can be partially or entirely moved into or out from the magnetic field of the solenoid the instrument is described as a ‘“‘choke coil.” (See fig. 9.) b. Choke coils, then, are voltage regulating devices utilizing the principles of self-induction (counter emf) to control the flow of alternating current. With X- “ray equipment they are used particularly in the primary circuit of filament transformers. There, they serve to regulate the voltage in the primary and therefore the voltage im the secondary circuit of the filament transformer. Thus, they control the current and hence the heating of the filament of the X-ray tube. For this reason, they are described as “fila- ment regulators.” 28. TRANSFORMERS. a. General. The physical effects described in connection with choke coils and electromagnets are utilized in the functioning of transformers. (See fig. 16.) The core of a transformer serves thereby to intensify the magnetic fields of the electrical circuits. There is no electrical connection between these two circuits. The design of a trans- former is very much as if an electromagnet connected into one circuit were approximated to an electro- magnet connected into another circuit. The approxi- mation of these two electromagnet arrangements must be such that the field of force produced by the electrons moving in the one circuit will excite move- ment of electrons (in the opposite direction) in the second circuit. The first circuit, that originally containing moving electrons, is called the primary circuit; that, wherein electrons are excited to move because of the field of force due to the current of the primary circuit, is the secondary circuit. b. Construction. Transformers consist of four essen-

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