Principles of Electricity

The Galvanometer

The Galvanometer, one of the earliest known methods of measuring electronic signals, is also one of the easiest to build. Simply take some electrical wire (14-22 guage) and wrap several times around a standard compass. The more wraps, the better. I suggest about 50 wraps, but it is ok to use fewer wraps if you are using thicker wire.

A galvanometer, an early form of Ammeter, is used for measuring electrical (DC) current. As Direct Current (DC) flows through the coil of wire wrapped around the compass, it developes a magnetic field around the coil. That magnetic field causes the magnet in the compass to deflect from its normal position. The higher the current, the more it deflects. Also, if you reverse the direction of the current, it reverses the poles of the "electromagnet", and hence makes the compass deflect in the opposite direction.

Historically, it was Hans Oersted who, in 1820, discovered that the magnetic needle of a compass could be deflected by current flowing through a wire. It was Johann Schweigger who is attributed to devising the first "true" galvanometer (compass wrapped by coils of wire) later that same year. Andrè-Marie Ampère also played a part in its early development. The term "galvanometer" itself was apparently derived from the name of Italian electricity researcher Luigi Galvani. He was made famous for noticing that he could use electricity to make a frog's legs move.

The modern day "Ammeter" (a device which measures Amps or electrical current) is a later developement of the galvanometer. Instead of the typical compass, it uses a D'Arsonval Movement, which has moving magnets connected to a thin "needle" (think second hand on a clock), which is then spring loaded to return back to its original position. The advantage is that it always has the same tension on it, so a scale can be added and made more accurate way of measuring the amount of flowing current. The disadvantage is that it only deflects in one direction.


Test for electrical current in a battery. Lay the galvanometer down on the table, with the North end of the compass pointing North. Now take the 2 test leads coming from the galvanometer, and connect one to each end of the battery. Write down which way the Galvanometer turns (and how many degrees it turns for extra credit). Now turn the battery around, and connect the leads to the opposite ends of the battery they were connected to before. Again, write down your results. Write down any theories you may have at this time.

Testing a lemon (or a potato - or both!). Put a galvanized (zinc plated) nail in one end, and a copper, brass, or aluminum nail in the other end. Again, take the 2 test leads and connect one to each of the "electrodes" (or terminals). Write down the results, flip the lemon around, and write it down again. Compare with the first experiment, compare your theories and discuss.

Have FUN!

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