Video Transcript
Can a hot-wire ammeter measure a direct current?
To determine this, let’s first consider a typical direct current circuit with a hot-wire ammeter in it. If we take a look inside this hot-wire ammeter, we would find quite a few components inside, with everything to the right of this yellow dotted line no longer a circuit diagram. As these objects are not typically in circuits, we have platinum–iridium wires, a pulley, a spring, a silk string, and a dial which read out current, usually in some form of amperes. And on the actual circuit side of things, there’s a shunt resistor here.
These are all the components that make a hot-wire ammeter able to measure current, with the hot-wire part of hot-wire ammeter referring to the platinum–iridium wires here. And we’ll be looking at these wires specifically to help determine if a hot-wire ammeter can measure a direct current, because in order for all of the components of a hot-wire ammeter to work and measure a current, the platinum–iridium wires must be able to change the temperature. This is because as these wires heat up, they expand slightly, causing the silk string to be pulled towards the spring, which causes the pulley to rotate moving the arm on the dial and giving a measurement of current.
All of this starts with the platinum–iridium wires being able to change their temperature. And the way they do this is by having a current pass through them. When current passes through any medium which has a resistance, an effect called resistive dissipation occurs, which causes some of the electrical energy in the medium to be converted to thermal energy, which means basically when there is current in the wire that heats up, note that the reason we specify platinum–iridium wires here is that we’re assuming they have some resistance to allow resistive dissipation to occur. This is in contrast to the normal wires that we have for circuit diagrams, which we assume to have no resistance, meaning that no resistive dissipation can occur in them.
So in order for a hot-wire ammeter to be able to measure a direct current, all that has to happen is there a current passes through the platinum–iridium wire, a process which will definitely have to occur in a direct current circuit. So yes, a hot wire ammeter can measure a direct current.
