# In which direction would the galvanometer's needle deflect when the current enters through the red lead?

Imagine that this is the galvanometer, and we attach the red terminal of a power source to the red terminal of the galvanometer, and the black terminal of the power source to the black one of the galvanometer. Would the needle read a positive value or a negative value?

This is counter-intuitive because I would expect the galvanometer to give a positive reading (needle points right), but if the needle ought to point where the current flows, it would point left.

One may ask why I expect the reading to be positive. Imagine we swap out the galvanometer with a volt-meter. We would expect a positive reading. So If we apply the same analogy, we would expect the reading to be positive for the galvanometer. However, I always hear that the galvanometer's needle actually points in the direction of the flow of current.

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When you buy a nice galvanometer, it should come with a manual that tells you what it does.

However, I always hear that the galvanometer's needle actually points in the direction of the flow of current.

I’ve never heard this. Most galvanometer-based DC ammeters that I’ve used are single-sided anyway. If you swap the positive and negative inputs, the needle goes the “wrong” way against some safety stopper (which might be the wall of the enclosure housing the needle), and you kind of frantically disconnect it before the needle snaps off.

If you have one of these and you want the needle to go the other way, you could just unscrew the red and black plastic housings from the terminals and swap them.

• Is this the same for ammeters? That when the current flows into the red lead the reading is positive? Commented Feb 23, 2022 at 4:37
• Generally, yes. In a multimeter, the difference between the “volts” and “amps” settings is the resistor network between the input terminals and the sensing element (whether galvanometer, FET, or otherwise) but not the direction which current flows through the sensor.
– rob
Commented Feb 23, 2022 at 17:15

The convention often used is that current entering the red terminal will deflect the needle to the right or towards the $$+$$ sign on the dial.
I have not heard of a convention which states that the galvanometer's needle actually points in the direction of the flow of current.