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Help Wanted:
1. Mistake correction
2. Drawing illustrations in vector
In electricity and electronics, a wide variety of meters are used to measure everything imaginable. Normally in electrical play you don’t even need meters of any kind. However if you choose to purchase meters for your own understanding, for experiments, or for designing and testing your own equipment, then this section will give you some basic information on their use and help you understand their limitations. Typically, if you used meters, you’d be using either a voltmeter, an ammeter, an ohmmeter, or a combination of these three called a multimeter (also called a volt-ohm-milliammeter or a VOM). All these meters can be purchased for a few dollars from any electronics supply house.
A voltmeter is a meter that measures – strangely enough – volts. This is a high resistance instrument and is set for either DC or AC and placed in parallel (see C2 P17 F4) across the circuit device you’re measuring. Here most of the current will flow through the lowest resistance of the pair. Therefore the resistance of the meter (normally about 2.5 megaohms) should be much higher than the circuit device it’s measuring. This is really no problem with circuits you’ll normally encounter, like in a TENS unit or a relaxicisor. However, you’ll notice immediately that such a meter will not work for static electricity or high-voltage devices like violet wands (and may be damaged If you try it). Static electricity and high-voltage equipment require very expensive and specialized voltmeters.
An ammeter is a meter that measures current in amps. This is a low-resistance meter and is placed in series (see C2 P17 F5) with the device that you’re measuring the current for. Here you want the ammeter’s resistance to be lower than the resistance of the device you’re measuring. You should have an approximate idea of the current that you want to measure. Too high a current through an ammeter can burn it out – sometimes dramatically. If you don’t know, start working with the largest current setting that the meter will measure and work down. Also the ammeters that you normally get from an electronics supply house will only mea- sure DC current You’ll need special ammeters if you want to read AC current.
Ohmmeters measure resistance in ohms. Inside an ohmmeter is a small battery (DC) that sends a small current through the device you’re trying to measure the resistance of. This tells you right away that you really don’t want to measure the resistance on any device in a circuit that’s already powered, or you could burn out the meter or the circuit.
The nice thing is that all three of the above meters are often combined in one called a multi meter or a Volt-Ohm-Milliammeter (or VOM). You can get these from many different stores quite cheaply. I find it’s a good investment if you know how to use them and you’re going to be testing or building circuits. For the best system, consider investing in an oscilloscope and a spectrum analyzer as well.
Now, one important thing about using any meters. If you are measuring voltages or currents coming directly from a battery (DC) or from a wall outlet or regular transformer (60 Hz (Hertz) AC) then the meters will do a good job giving you reasonably good results. But if that signal has any different shape or characteristic – like with pulsed DC from a TENS unit – all bets are off. You’ll get a value, but that value will be as if it were measuring a battery or an outlet. This is because meters are slow and can’t follow the rapid changes in high frequency AC or even the signal from a TENS unit. You’ll either have to get special meters, or you’ll have to know how to interpret the value you read on the meter to the signal that you’re measuring. This can be done, but you have to understand the mathematics of the signal.