Measurement of resistance with voltmeter and ammeter is the procedure that seems most sensible to taking into account the definition of ohm, but as we shall see it is often that provides less precise results. In it, the test element is powered by a continuous and stable voltage source and potential and intensity difference values are measured more simultaneously possible and with the relevant instruments. The resulting value of resistance is: R = U/I is important to point out that the assay values must be such that they do not produce unacceptable disturbance or damage to the element whose resistance is measured. The intensity of test must be such that it does not produce heating by Joule effect inadequate, mechanical stress of electrodynamic origin liable to cause shock or damage or that if the test item is a coil does not produce intensities or inductions of magnetic field that may alter the characteristics of cores or elements affected ferromagnetic. The accuracy of this procedure is directly linked to the accuracy with which the two electrical parameters are measured. With instruments of medium quality 0 is not exceeded? l % of uncertainty, and that once corrected the systematic errors inherent in the procedure.

Did precisions between 0? 5 and 5% can be usual therefore the use of this procedure is quite restricted and reserved for special measurements that cannot be carried quickly by other procedures.Nevertheless remains interesting study of this procedure because as you said, its alternative use is not disposable when instrumentation of high quality are available, or when the test to be carried out in special conditions. 2.1 Connections short and long short long connection connection Figure 7.1. Measurement of short connection and long connection resistances. The two connections shown in Figure 7.1 shows the two topological connection of the voltmeter and ammeter options. In the first, so-called short connection, the voltmeter is connected directly to the bomas of the element to be measured and subsequently fed to the corresponding voltage.

However the ammeter is placed in a branch that collects not only the intensity of the unknown resistance but also the consumed by the volt meter which provides a top to the proper indication, and therefore a resistance less than the real apparent value. This fact would require the use of instruments or systems voltimetricos which does not consume power such as electrostatic voltmeters or compensation procedures in rigor. Fortunately, in the majority of cases cannot ignore this demand without excessive loss of precision. In the second, so-called connection long, is the ammeter that is positioned directly in the same branch of the element to be measured and therefore sharing with this same intensity, while the meter is powered by the unknown tension increased in the voltage drop that intensity produces in the Amperimetrico.como in the above instrument cannot ignore the use of measurement procedures of intensities without voltage drop which today can only be by compensation procedures. If you want to know more about measurement of resistances with ohmmeter, visit Amperis instrumentation ohmeters. Original author and source of the article.