Question

An n-p-n transistor has three leads A, B and C. Connecting B and C by moist fingers, A to the positive lead of an ammeter, and C to the negative lead of the ammeter, one finds large deflection. Then, A, B and C refer respectively to:
(A) Emitter, Base and Collector
(B) Base, Emitter and Collector
(C) Base, Collector and Emitter
(D) Collector, Emitter and Base

Answer 1

Hint: The positive and the negative ends of the ammeter are connected to the positive and the negative ends of a device to measure the current through it. Also, out of the three terminals of a transistor, the emitter has the most number of charge carriers and so it will allow large current.

Complete step by step solution:
Now, according to the question, A lead of the transistor is connected to the positive lead of the ammeter and the ammeter gives a deflection. We know that in an n-p-n transistor, the emitter and the collector are negatively doped, while the base is positively doped. So, in this type of transistor the electric field is from the base to the other terminals. Thus, the lead A must be the base of the transistor.
Hence, the options A and D are incorrect.
Now, the lead C is connected to the negative lead of the ammeter. Also, the leads B and C are connected by moist fingers. So, some conduction is there in between these two terminals. Therefore, both the emitter and the collector are connected to the negative end of the terminal. But the moist fingers are less conductive than the lead. So the lead B is not directly connected to the negative end.
We are having a large deflection in the ammeter. This means that there is a huge amount of current from the base to the other terminal. This is possible only when the terminal adjacent to the base terminal has a high number of charge carriers. As the emitter is most highly doped, so the lead C must correspond to the emitter.
Now the only terminal left for the third lead B is the collector. So, the lead B must be the collector.
Thus, A is the Base, B is the collector, and C is the emitter.
Hence, the correct answer is option C.

Note:
If instead of connecting the leads B and C by moist fingers, we had connected them through a conductor as good as lead, then both the leads B and C would have been directly connected to the negative end of the ammeter. So, we would not be able to differentiate the leads B and C in that case.