Yes, it’s true. You can pick up a fairly nice digital multimeter that doesn’t even cost that much. Most of these can measure electric current, electric potential difference (voltage) and resistance. But why get something that you could build yourself? Wouldn’t that be more fun?
For the first device, I’m going to build an ammeter and voltmeter using a galvanometer (example picture above). This is basically a coil of wire that can rotate near a magnet. When there is an electric current passing through the galvanometer, there is a magnetic field in the coil that interacts with the other magnet to cause it to rotate. This makes the needle move. In short, the deflection of the needle is proportional the the electric current — so this measures electric current.
However, there are two problems. First, notice that there are no units on the scale. I just goes from -5 to +5. Second, if you run even a fairly small electric current through the galvanometer, the needle will max out past the 5 unit mark. So, if we want to use this to measure electric current or electric potential difference, we will need to make some modifications.
Finding the Maximum Current and Resistance of a Galvanometer
Before I build an ammeter, I need to know some properties of this particular Galvanometer. If I pass electrical current through the meter, what does each tick mark represent? What is the maximum current that gets the meter to the 5 mark?
Well, there’s really only one option here. I’m going to run electric current through the galvanometer AND through a known ammeter. That way I can measure the actual current to compare to the Galvanometer reading. Of course there is one small problem — the maximum current through the meter is super tiny.
Don’t worry, I can still measure the current through the galvanometer. Here’s the setup I used.
OK, it’s not fully plugged in and running but I wanted to show you the stuff. On the very far right is a variable power supply. You might think I could get tiny current…