![]() In regular, “alligator” style clips, both halves of the jaw are electrically common to each other, usually joined at the hinge point. ![]() Special connecting clips called Kelvin clips are made to facilitate this kind of connection across a subject resistance: This method of measurement which avoids errors caused by wire resistance is called the Kelvin, or 4-wire method. Measurement accuracy may be improved even further if the voltmeter’s current is kept to a minimum, either by using a high-quality (low full-scale current) movement and/or a potentiometric (null-balance) system. Thus, those long lengths of wire connecting the voltmeter across the subject resistance will drop insignificant amounts of voltage, resulting in a voltmeter indication that is very nearly the same as if it were connected directly across the subject resistance:Īny voltage dropped across the main current-carrying wires will not be measured by the voltmeter, and so do not factor into the resistance calculation at all. However, upon closer inspection it is seen that nothing is lost at all, because the voltmeter’s wires carry miniscule current. Our goal, though, was to measure this subject resistance from a distance, so our voltmeter must be located somewhere near the ammeter, connected across the subject resistance by another pair of wires containing resistance:Īt first, it appears that we have lost any advantage of measuring resistance this way, because the voltmeter now has to measure voltage through a long pair of (resistive) wires, introducing stray resistance back into the measuring circuit again. Because we’re only measuring voltage dropped across the subject resistance (and not the wires’ resistances), though, the calculated resistance is indicative of the subject component’s resistance (R subject) alone. Thus, we should be able to determine the resistance of the subject component if we measure the current going through it and the voltage dropped across it:Ĭurrent is the same at all points in the circuit, because it is a series loop. We know from Ohm’s Law that resistance is equal to voltage divided by current (R = E/I). Usually, wire resistance is very small (only a few ohms per hundreds of feet, depending primarily on the gauge (size) of the wire), but if the connecting wires are very long, and/or the component to be measured has a very low resistance anyway, the measurement error introduced by wire resistance will be substantial.Īn ingenious method of measuring the subject resistance in a situation like this involves the use of both an ammeter and a voltmeter. Such a scenario would be problematic because an ohmmeter measures all resistance in the circuit loop, which includes the resistance of the wires (R wire) connecting the ohmmeter to the component being measured (R subject): ![]() If you do not allow these cookies, you will experience reduced relevant content.Suppose we wished to measure the resistance of some component located a significant distance away from our ohmmeter. They do not store directly personal information, but are based on uniquely identifying your browser and internet device. They may be used by Analog Devices to build a profile of your interests and show you relevant content on our site. Targeting Cookies: These cookies may be set through our site by Analog Devices and our service providers. If you do not allow these cookies we will not know when you have visited our site, and will not be able to monitor its performance. All information these cookies collect is aggregated and therefore anonymous. They help us to know which pages are the most and least popular and see how visitors move around the site. Performance Cookies: These cookies allow us to count visits and traffic sources so we can measure and improve the performance of our site. If you do not allow these cookies then some or all of these services may not function properly. They may be set by us or by third party providers whose services we have added to our pages. Functional Cookies: These cookies enable the website to provide enhanced functionality and personalization. These cookies do not store any personally identifiable information. You can set your browser to block or alert you about these cookies, but some parts of the site will not then work. They are usually only set in response to actions made by you which amount to a request for services, such as setting your privacy preferences, logging in or filling in forms. Strictly Necessary Cookies: (Always Active) These cookies are necessary for the website to function and cannot be switched off in our systems. After we finish updating our website, you will be able to set your cookie preferences. Analog Devices is in the process of updating our website.
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