CDS (II) 2022 General Knowledge · Question 27 of 120

Correct — A, Ammeter. The name carries the answer: ampere plus meter, the instrument that reads current in amperes. Two things follow from what it is for, and both are worth more than the name. First, an ammeter is always wired in series with the element whose current you want, because the whole current has to pass through the instrument to be counted. Second, it is deliberately built with very low resistance, so that inserting it adds almost nothing to the circuit and therefore barely disturbs the current it is measuring — an ideal ammeter would have zero resistance. The classical instrument behind it is the moving-coil galvanometer, a coil in a magnetic field that turns against a spring by an amount proportional to the current through it; connect a small resistance, called a shunt, in parallel with that galvanometer and you have an ammeter that can read large currents.

• (b)Motor — Not a measuring instrument at all. A motor converts electrical energy into mechanical rotation, which is the reverse of what the generator in option (d) does.
• (c)Voltmeter — The right family, the wrong quantity. A voltmeter measures potential difference in volts, is connected in parallel across the two points concerned, and is built with very high resistance so that it draws almost no current away from the circuit. Everything about it is the mirror image of the ammeter, which is why the pair is examined together so often.
• (d)Generator — A generator produces electric current rather than measuring it, converting mechanical energy into electrical energy through Faraday's law of electromagnetic induction.

Electric current is the rate at which charge flows past a point, one ampere being one coulomb per second. The ampere is one of the seven base units of the International System, and since the 2019 redefinition it is fixed by giving the elementary charge the exact value 1.602176634 x 10 to the power minus nineteen coulomb. Current in a circuit is read with an ammeter, potential difference with a voltmeter, and resistance either with an ohmmeter or by dividing one reading by the other, which is Ohm's law used as a measurement.

This is a one-mark recall item, and the only reason to spend time on it is that the ammeter and the voltmeter are examined as a contrasting pair far more often than either is examined alone. Fix the contrast once and a whole family of questions collapses. The ammeter goes in series and has low resistance; the voltmeter goes in parallel and has high resistance. The reason in each case is the same principle — a measuring instrument should disturb what it measures as little as possible. Put an ammeter in parallel across a component and its low resistance short-circuits that component, which is how laboratory ammeters get destroyed. Options (b) and (d) are in the list to check that a student separates measuring devices from energy-converting ones; the motor and the generator are each other's inverse.

• An ammeter measures current in amperes, is connected in series, and has very low resistance; an ideal one would have zero resistance.
• A voltmeter measures potential difference in volts, is connected in parallel, and has very high resistance; an ideal one would have infinite resistance.
• Both are built around a moving-coil galvanometer — a low-resistance shunt in parallel turns it into an ammeter, a high resistance in series turns it into a voltmeter.
• One ampere is one coulomb of charge passing a point per second; the ampere is a base unit of the International System.
• A motor turns electrical energy into mechanical energy; a generator does the reverse, using electromagnetic induction.

• Connecting the ammeter in parallel and the voltmeter in series — the reversal that ruins school laboratory readings.
• Assuming an ammeter has high resistance because it is a precision instrument; the opposite is true.
• Mixing up the motor and the generator, which run on the same hardware in opposite directions.

As a device-to-quantity match, or as a statements item on how an ammeter and a voltmeter are connected and what resistance each should have.

• practice — not a real PYQ

  A voltmeter is connected in a circuit in which of the following ways, and with what resistance?

  • (a)In series, with low resistance
  • (b)In parallel, with high resistance
  • (c)In series, with high resistance
  • (d)In parallel, with low resistance

  Answer(b) In parallel, with high resistance — so that it draws almost no current away from the branch whose potential difference it is reading.

• practice — not a real PYQ

  A moving-coil galvanometer is converted into an ammeter by connecting

  • (a)a high resistance in series
  • (b)a low resistance in parallel
  • (c)a capacitor in series
  • (d)a high resistance in parallel

  Answer(b) a low resistance in parallel — the shunt carries most of the current and keeps the total resistance of the instrument small.