Question

Fert Berfel is a very naive individual who sets out to take current readings with an ammeter as one would take voltage readings: by simply touching the probes to two locations. The circuit involved is very simple. A red wire goes from the + terminal of a battery to one terminal of a light bulb. A black wire goes from the other terminal of the bulb back to the – terminal of the battery. First he touches the probes to opposite ends of the red wire. What reading does he get? Now he touches the two terminals of the bulb. What reading does he get? Assuming he now goes and buys a new ammeter to replace the one he destroyed, how should he read the current in the red wire?

Answer 1

While measuring current, one should keep the measuring instrument must be kept in series in the circuit.

In the above mentioned case, the method is for measuring VOLTAGE.

If the device is set into CURRENT mode and is used to measure VOLTAGE, the device will get damaged.

This is because, the AMMETER is having ZERO INTERNAL RESISTANCE, and when it is connected in parellel with any circuit, the entire current will be bypassed through this device and the curent will be much higher than what it could actually withstand.

In both cases mentioned above, the AMMETER will show infinite reading, because the resistance will be very low and the curent is given by the OHMs law, I = V/R.

(In actual case the device won't be able to show reading as infinite, but the device itself will get damaged when the connection is made as mentioned above.)

If he need to measure curecur, the two terminals of the AMMETER should be connected in series with the circuit, this can be done by cutting the Red or Black wire and connecting this wire on the ends of this cut wire. It should be carefully noted that the positive terminal of the AMMETER should be connected with the wire coming from the positive terminal of the battery only. Otherwise the device will show wrong reading(current flow will be in the opposite direction, if connected otherwise, which leads to incorrect reading as negative value.)