Section 1: Freeing a stuck needle
- Tools:
- Something long and thin, such as the blade of a pocket knife
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| Figure 1 shows an old galvanometer used in the Faraday's Law experiment. Note that the needle is pinned to the far-left side of the meter face, meaning it has become stuck behind one of the end stops (click each photo for a larger image). |
Figure 1: Needle not visible |
Figure 1a shows the needle in this galvanometer is resting against the white end stop.
- Old-school note: each of these galvanometers has a hand-written serial number!
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Figure 1a: Needle against end stop |
| Access the innards of the meter by turning the meter over and unscrewing (by hand) the four rubber feet at the bottom (Figure 2). Note that the top two screws have been removed in the photo. |
Figure 2: Remove the rubber feet from the meter bottom |
| Once the rubber feet have been unscrewed, the bottom plate will pop out. Note that a thin felt pad sits on the inside of the meter (Figure 3). |
Figure 3: The bottom has been removed |
| Figure 4 shows the two (white) end stops inside the meter. The galvanometer needle will sometimes get past these end stops. The photo shows that the needle is stuck on the right-side end stop. |
Figure 4: The needle stuck on the right-side end stop |
| Insert the long blade of a pocket knife between the meter face and glass and gently move the needle around the end-stop (Figure 5). |
Figure 5: The needle is moved using a pocket knife |
| Once the needle has been set free, reattach the bottom plate (remember that the felt piece goes inside the meter) using the four rubber feet. Figure 6 shows that the needle is once again visible. |
Figure 6: The galvanometer needle is visible again |
Figure 7 shows that the needle is no longer parallel to the meter face. The needle is thin and can be easily bent. If it does not move freely after being released from the end stop, press on the needle with the pocket knife blade to try to flatten it and keep it from rubbing against the meter face or glass. |
Figure 7: The needle moves freely but is no longer flat |
Section 2: Repairing a mangled needle
- Tools:
- Something long and thin, such as the blade of a pocket knife
- Small/medium flat-head screwdriver
- Small pliers: needle-nose and something with a flat surface
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The needle in the galvanometer in Figure 8 was freed, but you can see that it is severely bent. The guts of the meter need to be removed to straighten and flatten the needle. |
Figure 8: A severely bent needle |
Turn the meter upside down and remove the bottom plate by unscrewing the four rubber feet (Figures 2 and 3, above). The meter face, needle and magnets are all attached to a metal bracket inside the meter. Remove the two slot-head screws and split-ring lock washers shown in Figure 9.
- Pro tip: Use a pair of needle-nose pliers to help remove the screws and washers.
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Figure 9: Remove the screws holding in the meter guts |
The circled area in Figure 10 shows the zero-calibration 'peg' resting between the forked-base of the needle. Make sure the peg is within the fork tines when reassembling. |
Figure 10: The needle alignment apparatus |
Once the screws and lock washers have been removed, pull the guts out and place them upside-down on the bench (Figure 11). Note that it is not necessary to disconnect the wires from the meter. |
Figure 11: That needle has seen better days |
Carefully flatten and straighten the bent needle using the small pliers (Figure 12). The needle material is very thin and should be handled carefully. |
Figure 12: Repairing the bent needle |
Figure 13 shows the finished repair. It's not perfect but it's better than it was. |
Figure 13: Close enough for government work |
Reassemble the meter, making sure the needle zero-calibration apparatus is put back together correctly, as shown in Figure 10. Turn the zero-calibration screw using the slot-head screwdriver (Figure 14) if the needle is not vertical at rest. Note that you may not be able to set the needle vertical. The meter will still be functional even if it is offset.
- Pro tip: You will find it helpful to hold the screw and lock washer with a pair of needle-nose pliers when reattaching the guts of the galvanometer.
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Figure 14: Close enough for government work |
