What Do We Use to Measure Current?

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A direct current is measured with a coil and field magnets, but alternating currents need to be measured with an oscilloscope. Learn about moving coil galvanometers with help from a science teacher in this free video on physical science.

Part of the Video Series: Physical & Life Science
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Video Transcript

Hi, I'm Steve Jones and I'm going to explain what we use to measure current. Now current we'll describe is direct current for the moment. For direct current, life is quite easy and these instruments are being with us for hundreds of years, or at least a couple hundred years. Basically it's a little bit like an electric motor. We have a coil and we have field magnets. Now here we have a magnetic field going from north to south and of course we have a field produced by the current going through this coil. So the bigger the current, the stronger the force on the coil. If we make the turning of the coil depend on a spring, that is we put a spring to stop it turning and if we put a pointer on the coil what will happen as the coil turns more, the spring gets tighter and it slows down. So the bigger the current, the further over it goes until it reaches it's maximum. And therefore the meter has a range, you put a scale at the top here, at the end of the pointer and that enables you to measure current. But that works for direct current. Sometimes we want to measure alternating current, that is current that isn't going in one direction, but is going backwards and forwards. If you try and do it with this, you'll get nowhere because of course one minute is going one way, the next minute is going another way and in fact it'll just sit happily on zero and do nothing. So what we have to do is either rectify the current before we allow it to go through the loop or we have to have a totally different device. The most useful device is this one which is called the oscilloscope. An oscilloscope is basically something like a TV screen. So here's your fluorescent screen, we'll call it TV screen for short and what it has is two sets of plates. A set of X plates and a set of Y plates. The difference being the X plates make a spot move left and right and the Y plates make it move up and down. And what you can do is you apply a voltage to these plates and the voltage is a special one which looks like this called a saw tooth that makes the spot on the screen to go across and then fly back and it flies back so quickly you don't see it. So it just looks as though the spot is going from left to right across the screen. The X plates then you can apply your alternating current on the X plates and it'll go up and down, you don't notice that when it gets to this end it shoots back and does it again so in fact this enables us to display these kinds of signal where you've got alternating current. This doesn't measure current, it actually measures voltage but of course the two are linked through a relationship with resistance and therefore you can also use it to measure current. So current is usually measured by something like this, this is called a moving coil galvanometer or moving coil meter and this is the tried and trusted way of measuring current.

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