### Video Transcript

The diagram shows a moving-coil galvanometer. Full-scale deflection of the galvanometer arm occurs when the galvanometer coils carry a current with a magnitude of 150 microamperes. Which of the following must be true of the current ๐ผ that passes from contact ๐ด to contact ๐ต? (A) ๐ผ equals negative 150 microamperes. (B) ๐ผ equals 150 micro amperes. (C) ๐ผ equals zero microamperes. (D) 150 is greater than ๐ผ is greater than zero microamperes. (E) Negative 150 is greater than ๐ผ is greater than zero microamperes.

To begin, letโs recall that the purpose of a moving-coil galvanometer is to detect and indicate electric current. Itโs the scale and arm or pointer up here that tell the user the magnitude and sign or direction of current in its coils. In this question, weโve been told that when the current reaches a magnitude of 150 microamperes, the arm deflects to its maximum angle for reading on the scale. This feature can be seen in the diagram, as the limiting values marked on the scale are negative 150 microamps and positive 150 microamps.

Since the scale is able to indicate an equal magnitude for both positive and negative values, we see that zero lies exactly in the middle. So if the coils had no current in them, the arm would indicate zero microamperes. But we can see that that isnโt the case here because the arm isnโt aligned with the zero mark. This means thereโs a nonzero value of current in the coils, and we should eliminate answer choice (C).

Still the arm is closer to zero than either extreme value, positive or negative 150 microamps. Although those values are clearly marked on the scale, the arm isnโt pointing to either. So we know answer choices (A) and (B) are wrong as well.

The two remaining answer choices basically just suggest that the current in the coils is either positive or negative. Remember that the sign of the current is indicated on the scale with either a plus or minus sign on either side of the zero mark. So these values to the left of the zero mark are negative, all the way down to negative 150 microamps. But clearly the pointer arm isnโt in this region, so the value of current is not negative. And we can eliminate option (E).

Therefore, option (D) is looking pretty good. And just to confirm, we can practice reading the current from the scale. The arm is pointing to the second mark in the positive region of the scale, where there are 10 marks up to 150 microamps. Thus, each mark denotes 15 microamperes, so it looks like the arm is indicating a value of about positive 30 microamperes. This does lie within the range given by answer choice (D), 150 is greater than ๐ผ is greater than zero microamperes. Therefore, (D) is the correct answer.