With an accuracy in some percent apply ohmmeters with direct counting to the approached measurement of. They carry out measurement on the basis of the dependence mentioned above between and tension and directly show by means of the crossed-coil instrument (R=U/values According to other way at a known tension measure current, and graduate a scale directly in ohms. of this type build in in (mnogopredelna devices for an of current and tension.
If Rx is not enough (within 1 Ohms — 10 mky), transitional resistance are comparable with the measured resistance and bring a considerable error in measurements. In this case a little more difficult Thomson Bridge which is also easy-to-work.
Active resistance of copper and aluminum wires of small diameter (to 10 mm) with a frequency of alternating current of 50 Hz slightly exceeds statistical (for them k a little more unit), but it is significantly more than it in steel wires with big magnetic permeability of ma.
Accuracy of ohmmeters at a linear scale is characterized by a error in relation to a measurement limit. At a (hyperbolic) scale of an error of the device are characterized. also given error, %, but in relation to length of the scale expressed in millimeters, t. ; γ = (∆ l/lshk) 10
The phenomenon of superficial effect can physically be explained (according to the offer V. F. Mitkevich as follows. Section S with alternating current of i simply So collected from n of hollow cylinders with an identical area of cross section can imagine the cylindrical conductor. Let's assume that current of each of i=i/n cylinders creates round the cylinder on one magnetic line. As a result the external layer of the conductor will be linked to the magnetic line only of the current, and everyone the subsequent to the direction to an axis – to the and other external lines. The greatest number of power lines surrounded a core of the conductor. As a variation magnetic field, in hollow cylinders will be induced different.. . and they will have various inductive resistance: the greatest – the internal cylinder, the smallest – external. It leads to that density of alternating current in wire section not the constant – in a core minimum and gradually increases to external layers.
the electric measuring mechanism, at U condition = Const of the indication will be defined the measured Rx resistance. Therefore, the scale can be an otgraduirovana in terms of resistance.
As a result of radial replacement of alternating current from inside layers of a wire in external the useful section of a wire to this current as if decreases, and its resistance increases. Respectively also losses of energy increase by heating of a wire. With high frequencies of alternating current electrons are forced out from the conductor even outside – the wire radiates part of the energy in the form of an orange blue luminescence. For this reason powerful C of modern electric furnaces are carried out by hollow cables, and VL – stalealyuminevy wires; the external conductor layer of the last is made of aluminum, internal – in the form of a steel cable for giving to a wire of mechanical durability.
Electronic ohmmeters (subgroup E are widely used for measurement of active resistance in the range of 10E-4 - 10E12 of Ohms at an of resistance of resistors, isolation, contacts, superficial and volume resistance and in other cases.
As in practice of measurements this way calculation of is often made on the approached formula (it is necessary to know, what scheme should be chosen in order that the error was minimum.
Active, or resistive, resistance the chain element in which there is an irreversible process of transformation of electric energy into the thermal possesses. Active resistance is the parameter of a resistive element in a chain of alternating current. Resistance of the same occasion to alternating current (... the self-induction can neglect) slightly more, than to a direct current, i.e. Ra> Rst that is caused by the phenomenon of superficial effect. Conditionally active resistance (as well as it is designated by letters R, r, and on on electric circuits of replacement the resistive element is represented in the form of the extended rectangle.