The diagram shows a moving-coil galvanometer. Which of the following is the purpose of the labeled component? (A) The component allows the deflection of the galvanometer arm to be measured. (B) The component increases the strength of an induced magnetic field. (C) The component produces a magnetic field. (D) The component provides a restoring force on the galvanometer coil. (E) The component carries current.
Let’s begin by clarifying that the labeled component in question is the scale up here. The provided answer choices give us short descriptions of the purposes of different parts of a moving-coil galvanometer. Let’s start at the bottom and work our way up. Answer choice (E) says the component carries current. The purpose of a moving-coil galvanometer is to detect and indicate electric current. So it is important to provide a conducting path for electrical charge to follow. But the scale is not designed to carry current itself. The current is meant to travel by means of the torsion springs and coils. The diagram does show one end of a torsion spring passing behind of the scale. But in reality, these two components are totally separate. Option (E) then is not very convincing, so let’s cross it out.
Next, (D) says the component provides a restoring force on the galvanometer coil. We often encounter restoring forces when working with springs. Earlier, we mentioned that the torsion springs help carry current in the galvanometer, but that’s only one of their intended purposes. The springs are also there to keep the coils from rotating freely by opposing the torque applied on the coils by the magnetic field. In any case, we want to recognize what the scale is for, and it’s not there to exert any force. So answer choice (D) is incorrect as well.
Moving on to (C), it says the component produces a magnetic field. This is pretty clearly not the purpose of the scale. Looking at the diagram, we can see the poles of a permanent magnet, which creates a magnetic field that points from the north to south pole. There’s no reason that the scale should produce or even affect a magnetic field. So let’s cross out (C) as well.
Option (B), which reads the component increases the strength of an induced magnetic field, isn’t very believable either. A moving-coil galvanometer does need a magnetic field in order to apply a torque on the coils based on the current they carry. But these mechanisms are at work here in the center of the galvanometer where the magnet and coils and iron core are. The coils are attached to this arm, or pointer, in order to indicate a current value on the scale up here. But neither the scale nor the arm is meant to actually affect the magnetic field. Answer choice (B) is incorrect.
When a current is present, the galvanometer arm rotates along with the coils and, as a result, points to some nonzero value on the scale. The scale itself is specifically designed to indicate the value of current that corresponds to the coil’s rotation. This agrees with answer choice (A), which says that the purpose of the component labeled in the diagram is to allow the deflection of the galvanometer arm to be measured. Therefore, option (A) is the correct answer.