Of the different measurement procedures for the electrical parameter we will consider three broad groups: Those who use current and voltage values The compensation Those who provide directly the value 2. Measurement of resistance with voltmeter and ammeter is the procedure seems more sensible given the definition of Ohm, although as will usually be the one that provides less precise results. In it, the item under test is powered by a voltage source, stable and measured as simultaneously as possible and the relevant instruments values of potential difference and intensity. The resulting value of resistance is: R = U / I is important to note that the test values must be such that no unacceptable disruptions or damage to the element whose resistance is measured. The test current should be such that no Joule heating produces inadequate, electro mechanical stress source that can produce shock or injury or if the item is a test coil does not produce current or magnetic field induction can alter the characteristics of ferromagnetic cores or elements affected. The accuracy of this procedure is directly related to the accuracy with which the two electrical parameters measured.
With average quality instruments does not exceed the 0'l% uncertainty, and that once corrected for systematic errors inherent in the procedure. Accuracies between 0.5 and 5% may be common as the use of this procedure is quite restricted and reserved for special measures which can not be done quickly by other procedures. But still interesting to study this process because as has been said, is not ruled out alternative use when you have high quality instrumentation, or when required to be tested under special conditions. 2.1. Short and long links short connection Figure 7.1. Measurement of resistance in connection short and long connection. The two connections shown in Figure 7.1 shows the two possibilities topological connection of the voltmeter and ammeter. In the first, called short connection, the voltmeter is connected directly to the terminals of the element to be measured and subsequently fed to the voltage.
But the meter is placed in a branch that includes not only the intensity of the unknown resistance but also consumed by the voltmeter which provides an indication of more than adequate, and therefore a lower apparent value of the real resistance. This in fact require the use of equipment or voltmeter draws no current, such as electrostatic voltmeters or compensation procedures. Fortunately, in most cases you can waive this requirement without sacrificing too much precision. In the second, called long connection is the meter that is placed directly in the same branch of the element to be measured and thus sharing with it the same intensity, while the voltmeter is fed by the voltage unknown plus the voltage drop of intensity occurs in the amperometric instrument. As in the previous case can be avoided using intensity measurement procedures without voltage drop that today can only be done by a process of compensation. If you want to know more about, visit the section on Instrumentation Amperis.