Video Transcript
What quantity does a hot-wire ammeter measure? (A)Temperature, (B) frequency, (C) current, (D) all of the answers are correct.
To determine which of these answers is correct, let’s consider each of the components of a hot-wire ammeter. A hot-wire ammeter, also known as a thermal ammeter, is the ammeter that is used inside alternating current circuits. If we were to open up this hot-wire ammeter, we would find that it is made of quite a few strange components. When there is a current across the ammeter, it splits along a parallel path, with one path containing a shunt resistor and the other path containing a variety of noncircuit components. Specifically, everything to the right of these two circles is a noncircuit object.
Let’s go over each of these components, starting with the parts directly connected to the circuit. These are platinum–iridium wires to which a silk string is attached at one end. The other end of the string is attached to a spring, which keeps the silk under tension. The middle of this string is also wrapped around the pulley in the center here. And in the center of this pulley is a needle, the end of which is able to point at different parts of a dial above it. This dial can be seen on the outside of the hot-wire ammeter such that we can read it. We’ll find that since this is an ammeter, the dial will measure in some units of amperes, which are the SI unit of current.
But are we really measuring current here? The name “hot-wire” seems to imply we’re dealing with temperature somehow. And indeed, we are, specifically with the temperature of the platinum–iridium wires. These wires are the hot wires in the name, as they heat up through a process called resistive dissipation. Resistive dissipation is a process that occurs in any circuit that has current through a resistor. As electric charge moves through a resistor, a portion of the electrical energy of the current is converted to thermal energy, causing the resistor to heat up.
Whenever we look at circuit diagrams, we usually assume that the connecting wires have no resistance, meaning that no resistive dissipation occurs in the wires. But real-life wires do have some small amounts of resistance. And these platinum–iridium wires have some resistance that is higher than the other connecting wires, the reason being that we want these wires to heat up. This is because when metal heats up, it expands slightly. A slight expansion of these platinum–iridium wires will cause the attached silk string to relax slightly. Since the string is held under tension by the spring, a relaxation will cause it to be pulled slightly towards the direction of the spring, causing the attached pulley to rotate and subsequently the needle of the dial to move to a different reading.
So there are actually quite a few different things we could be measuring in this hot-wire ammeter. There is current in the platinum–iridium wire, the temperature of that wire, the tension in the silk string, and the rotation that occurs of the pulley. Yet, after all of this, the units that the dial measures in are some form of amperes, which means that the quantity that this hot-wire ammeter measures must be current.
What matters are the final units. It doesn’t matter what steps you took to get there. These steps, in this case, are understanding that a specific current would correspond to a specific temperature in the platinum–iridium wires, causing them to expand, which relaxes the tension in the silk string, causing the pulley and thus needle which reads the dial to rotate a specific amount. Although there are different quantities in the measurement process, each is just a step in eventually measuring the current through the platinum–iridium wires. The correct answer is (C), current.
