Home made battery tester/galvanometer

A galvanometer is a device for measuring the flow of an electric current. (From Galvani, a professor of physiology at Bologna, and meter, measure.)

In this day and age, electricity has found its way into our lives in the most unexpected areas. Nobody could have imagined how useful computers and cellphones would become, but how far can it go? I thought electric toothbrushes were pushing the limit once, but then I saw a computer controlled toilet seat…..

Without a doubt, at least a basic understanding of electrical principles will remove from anybody’s environment, a lot of potential confusion and mystery. It is common to see people taking advantage of others’ ignorance of the subject for their own purposes, such as selling machines that “point in the direction of gold”.  It may be possible one day but sadly, not yet.

This project demonstrates some important principles of electric currents and magnetic fields, and how they interact. At the center of it is a coil of wire connected to the terminals on the front. A magnet is held in front of the coil on thin nylon fishing line so it can turn freely. A piece of wire wrapped around the magnet serves as a pointer and a place to attach the nylon line that it pivots on. An electric current through the coil produces a magnetic field which will cause the magnet to swing to the left or right, depending on the direction of the current. The nylon line is tied at the bottom to a paper clip and tied at the top (stretched fairly tight) to a plastic clip that can be turned to set the meter at zero.

Here are some pictures before the sides were screwed on.

And here is a picture with more details

The coil details are not very important. Thicker or thinner wire, more or fewer turns of wire, a different sized magnet, will only change the sensitivity of it. The one above, connected to a 1.5 volt battery makes the needle slam hard across to the side. It’s quite sensitive.

The last thing I added were two resistors so it could operate as a voltmeter and battery checker. A 47 ohm gives a full scale reading on a good 1.5 volt battery. A second resistor, 300 ohms, gives a near full scale reading on a good 9 volt battery. (The battery being measured in the first picture is flat.) It’s wired like this:

Various things will become apparent while using this; that it’s very sensitive to nearby magnets, even the Earth’s magnetic field, which can be observed simply by rotating it. It also wobbles around a lot since it has no damping of its movement. They are shortcomings of this kind of meter, but they are also opportunities to observe and learn about natural phenomena at work.

Update Aug 21, 2011

Some changes were made in the galvanometer construction during this summer’s classes. It didn’t really appear necessary to tie the fishing line onto a paper clip. This was originally done to keep some tension on the line, but it was found that the line itself was stretchy enough to maintain sufficient tension. It was simply passed through a hole in the bottom and anchored under a screw head like this:

(Four small rubber feet were stuck on the bottom of this one, so the screw head had plenty of clearance.)

Also, the zero adjustment was made to look a little nicer by using a brass fitting from a sliding door. A piece of wood was hammered into the center and drilled out so the line could be passed through it. It was anchored by pushing a small brass nail into the hole.

Jack doing final assembly of his galvanometer

Finally the movement was damped slightly by sticking a small piece of card onto the side of the magnet to create wind resistance. It was positioned close to but not touching the underside of the scale.

On checking the final calibration it turned out to be quite different to the original. This was possibly due to the coil being a larger diameter and the magnet being a different type. It did however work out to be quite convenient at +/- 3 volts and +/- 20 volts through the two resistors, and a current of +/- 50 ma directly through the coil.

Jack checking galvanometer calibration




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