Re: What Are The Instruments Used To Measure Small Amounts Of Electricity

I take it that you mean you want to build batteries out of fruit and see which fruit makes the best batteries.

For those who have never done this before, you can make a battery out of a suitable fruit or vegetable (lemons and potatoes are commonly used) by sticking two pieces of different metal in it and connecting wires to the pieces of metal. For example, you can stick a penny and a nickel in a lemon and it will produce electricity. This leads to a lot of good experiments, because the performance of your battery depends not only on what fruit you use, but what two metals you use, how big the pieces of metal are, the temperature, and how far apart you put the pieces of metal in the fruit.

Now, you don't get a very strong battery here, so, as you already realize, you need some fairly sensitive instruments. There are two basic parameters that you will probably want to measure: the voltage and the current produced by your battery.

Voltage (measured in "volts") tells you how hard your battery is pushing on the electrons in the wire. You measure voltage with an instrument called a voltmeter. I'd expect you to see voltages of around one volt (abbreviated "1 V") in these experiments, so be sure you find a meter than can accurately measure voltages that low. You will need to measure these voltages accurately to the nearest tenth of a volt, and hundredths of a volt would probably be better. This means you should probably get a digital voltmeter (which displays a numeric value) rather than an analog voltmeter (which has a moving needle that you have to interpret). Digital meters are also much easier for beginners to use, and they are just about all you can buy these days, anyway!

Current (measured in "amperes" or "amps") tells you how fast electrons are moving through the wire. You should realize that it is possible to have a high voltage but still have a low current if your circuit has a high resistance to electrical current. This is similar to the situation where you have a kink in a garden hose: there is still a lot of pressure in the hose (on the high-pressure side of the kink, of course), but not much flow. In electrical circuits, voltage behaves like water pressure in a hose, and current corresponds to the flow rate of the water.

You measure current with an ammeter, or when you want to measure small currents, as in this case, a milliammeter (a milliamp is a thousandth of an amp). It's a little harder to predict just how much current you will need to measure, but it should be much less than 100 milliamps (abbreviated "100 mA"), perhaps even less than 1 mA. Again, a digital meter will be easier to use and more precise than an old-fashioned analog meter.

If you need to buy new equipment for your experiments, you should buy an instrument called a digital multimeter. It is a digital meter that can measure voltage, electrical resistance, and (usually) current. Make sure you get a model that can measure current (i.e. milliamps) since that is important to your experiments. Suitable meters are available at Radio Shack and similar electronics dealers. While you are there, I'd suggest getting a 1,000 Ohm resistor (i.e. 1K Ohm, because K = kilo = 1,000), and maybe a few other values, so you can also experiment with Ohm's law and the effect of various loads on your batteries.

Two of the characteristics you will want to measure for your batteries are open-circuit voltage and short-circuit current. Open-circuit voltage is measured simply by measuring the voltage across the terminals of a battery that doesn't have anything connected to it. Since a voltmeter has a very high resistance, it will not affect your battery much, and you measure open-circuit resistance by connecting the voltmeter probes directly to the battery terminals. If you get a negative reading, swap the two leads. Note on using voltmeters: since your voltmeter is actually measuring the difference in (electrical) potential energy between two points in your circuit, you always measure voltage by connecting the leads of the meter to two different points in your circuit, for example, two different ends of an electrical component (like your battery).

Short-circuit current is measured by using a piece of wire to "short-circuit" the two battery terminals. This tells you the maximum amount of current that you can get out of your battery. Since an ammeter has nearly zero electrical resistance, it acts a lot like a piece of wire, so you can measure short-circuit current the same way you do open-circuit voltage: just connect the two leads of your meter to the two terminals of your battery. Note on measuring current: current is a measure of the number of electrons passing one spot in a given period of time, so to measure current you cut a wire in your circuit and connect the two leads of the milliammeter to the two cut ends of the wire. Or you can replace the wire with the meter and its leads.

WARNING!! If you try to measure short-circuit current of a larger battery or power supply by shorting it with your ammeter, either the battery, or your meter, or both, might explode! Don't try it! You can get away with it here because a fruit battery is too weak to generate dangerous currents. Something as simple as a common "NiCad" rechargeable battery can explode when it is shorted, though.

Question: which of the physical parameters of your battery (type of fruit, types of metal, size of metal pieces, spacing of metal pieces, temperature) affect the open-circuit voltage? Which affect the short-circuit current?

Another experiment you might want to try is to connect different resistors to the terminals of your battery and measure the voltage and current. Does it obey Ohm's law? Remember, you measure voltage between the two ends of an electrical component, but you measure current by inserting the meter at a single point in the circuit.

Need more ideas? If power in Watts is calculated by multiplying voltage (volts) by current (amps), how much power is being produced by your battery for each resistor? Which one of your batteries produces the most power? This is important because in many (but not all) situations, the amount of power you can get from a battery is more important that its open-circuit voltage or its short-circuit current. How does the power vary if you change the resistance?

There are quite a few Web sites that will be helpful to you. For example, Jughead's Information Station has information on basic electronics, including explanations of voltage, current, resistance, and Ohm's law, and a lot more.

You might also try Tomi Engdahl's Electronics Basics.

You can also read more about batteries in general.

The Money Pile is a very similar experiment you might enjoy reading about.

Putting two pieces of metal in a piece of fruit actually makes a voltaic cell. A battery is two or more cells connected together. I've called the fruit cell a "battery" simply because that's the word most people use when they mean "voltaic cell". For example, a 1.5 volt D-cell "battery" isn't really a battery, it's a single cell.