Why does the multimeter show a negative reading?

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Multimeters are quite basic instruments. It seems like a routine action: you hook up your multimeter to two points and get quite a surprise: it’s reading negative! What does this mean? Is there something wrong? Do you need to take action? Read on more to find out. 

On a side note: think about buying a new multimeter.

Why does the multimeter show a negative reading?

A negative reading on a multimeter could mean two things. One (probably the most common) is that it is being measured the wrong way. You flipped the probes. The other is that there is a series of batteries and one of them is depleted. 


The reversal: A simple mistake


If you’re a newbie or you don’t use your multimeter frequently, it’s not unfathomable for you to mix up the probe. Remember, the red colored probe is for the positive end and the black colored probe is for the negative end. If they are switched, the multimeter will display a reversed (negative) number. You don’t need a PHD in physics to see why this is. 

Remember, voltage is the difference in potential between two points. One usually is higher and so the difference between the higher and the lower end is shown as positive voltage. If you mix-up the two, the number will display the reverse. 

This is no big deal, although with some other things paying attention to where the red and black probes go is very important. 


 A series with a depleted battery

The second case happens when you have a series of batteries. One is nearly depleted while others are not yet drained. The series will have a lot of remaining capacity and as the empty battery is discharged by the forced current from the other batteries, it will get a negative voltage. There will be a chance of leakage so it’s recommended that you take out and trash the old batteries. 

The situation is bad if you have a multi celled battery. Most modern electronics have a low battery cutoff. For example, if you have an electronic screwdriver, stop using it if it slows down instead of forcing it. Go charge it and return to the job. This might be somewhat inconvenient but this will prevent you from potentially having to replace the battery. Take care of  your power tools as the parts can be quite expensive. 


Be careful in regards to red/black


repairman with yellow digital multimeter

As a side note, if you reversed the red and black probes it’s no big deal right now. If you were trying to jump start a car your experience might be different! Do your best to memorize the colors. Raemember: red positive, black negative. Jump starting with the wrong colors could lead to a fire or even an exploding battery. Imagine going out of your way to be a good samaritan to help a stranger with her car, only to wreck your own. Be careful. 


Seeing a negative reading on a multimeter is quite a surprise. Fortunately, diagnosing the problem is quite easy. You either have reversed the probes or have a depleted battery.


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