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Handbook Electromagnetism Electromagnetism Handbook Electromagnetism 27 MOTION Of DIRECTION When tube heaters or filaments are oper- cLec - Of CURRENT ated in series, the current is the same throughout the entire circuit. The re- sistance of all tube filaments must then be made the same if each is to have the same voltage drop across its terminals. The re- sistance of a tube heater or filament should never be measured when cold because the resistance will be only a fraction of the re- sistance present when the tube functions at proper or filament + © heater temperature. Figure 6. The can be calculated resistance satisfac- (A) shows the magnetic lines of force produced torily by using the current and voltage rat- around a conductor carrying an electric current. ings given in the tube tables. It also indicates the difference between the motion of electrons and the flow of current. (B) indicates how the effectiveness of the field may be increased Electromagnetism by winding the conductor into a coil. Everyone is familiar with the common current, magnetomotive force or magnetic bar or horseshoe magnet. The magnetic field voltage. The unit of magnetomotive force which surrounds it allows the magnet to (m.m.f.) is the gilbert. The reluctance of a attract nails, washers, or other pieces of magnetic circuit can be thought of as the iron to it. A peculiarity of an electric cur- resistance of the magnetic path. The re- rent, hence of electrons in motion in gen- lation between the three is exactly the saine eral, is that a magnetic field is set up in the as that between current, voltage and resist- vicinity of the conductor of the current for ance (Ohm's law). as long a period of time as the current is The magnetic flux depends upon the ma- flowing. A field set up by an electric cur- terial, cross section and length of the mag- rent is called an electromagnetic field to dis- netic circuit, and it varies directly as the tinguish it from the permanent field sur- current flowing The reluctance rounding in the circuit. the bar magnet. is dependent upon the length, cross section, Magnetic Flux. The field, or magnetic permeability and air gap, if any, of the mag- lines of force, set up in the vicinity of the netic circuit. conductor extend outwardly from the con- In the electrical circuit, the current ductor in a plane at right angles to its di- would equal the voltage divided by the re- rection. It is these lines of magnetic force sistance, and so it is in the magnetic circuit. that make up the magnetic flu.r. The strength of this flux in the vicinity of a Magnetic Flux (I) _ simple conductor is proportional to the magnetomotive force (m.m.f.) strength of the current. However, if the conductor is wound into a coil the flux for reluctance (r) each turn of wire becomes additive to that Permeability. Permeability describes the of the others and the flux becomes propor- difference in the magnetic properties of tional to the number of turns as well as to any magnetic substance as compared with the current flow. Since the flux is linearly the magnetic properties of air. Iron, for proportional to both the current and the example, has a permeability of around number of turns, the magnetizing effect of 2000 times that of air, which means that a coil may be described as a function of a given amount of magnetizing effect pro- the ampere -turns of that coil; the mag- duced in an iron core by n current flowing netizing effect of a coil is proportional to through a coil of wire will produce 2000 the product of the current strength and the times the flux density that the same mag- number of turns in the coil. netizing effect would produce in air. The The magnetic flux increases or decreases permeabilitics of different iron alloys vary in direct proportion to the change in the quite widely and permeabilities up to 100,- current. The ratio of the change in flux 000 can be obtained. to the change in current has a constant Core Saturation. Permeability is sim- value known as the inductance of n coil. ilar to electric conductivity. There is, how- Electromagnetic Effects. In drawing ever, one important difference: the perme- an analogy of voltage, current and resist- ability of iron is not independent of the ance in terms of magnetic phenomena, magnetic current (flux) flowing through it, magnetic flux might he termed magnetic although electrical conductivity is usually www.americanradiohistory.com.
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